Talk:Twilight/Archive 2

"Twighlight"?

We could use a better source for this alternate spelling than "Webster's Online Dictionary", which appears to be an affiliate-heavy amateur site whose only example of usage is a typo in a single Amazon entry. None of the reputable dictionary sites seem to support "twighlight" as an alternate spelling, and I can't find any significant usage online. --McGeddon (talk) 12:51, 5 February 2010 (UTC)

The best I can find is this, which isn't really saying much. Set Sail For The Seven Seas 326° 36' 0" NET 21:46, 5 February 2010 (UTC)

The editor who added it says on their talk page that it's the "preferred spelling" in Scotland, so I've clarified the article to say that for now. Google News backs it up with only a handful of uses in Scottish newspapers, though, and I'm struggling to find anything online discussing the spelling, so I'm not sure about "preferred". --McGeddon (talk) 19:17, 7 February 2010 (UTC)

I've had another look and came up with these two, but still nothing significant. However, I would be willing to give Hanii Puppy the benefit of the doubt, but some sort of verification would be best. One way to go about it could be by asking Scottish Wikipedians or those on the Scottish Wikipedia. Set Sail For The Seven Seas 1° 10' 15" NET 00:04, 9 February 2010 (UTC)

Images

I questioned the proliferation of sunset images on this page a while ago and stopped short of removing them all as there wasn't a clear consensus for it at the time. I still don't understand what relevance the Lyme Regis image has here, other than filling a space with some colour and silhouettes. I noticed this edit and reversion today and have to say the removal rationale, however inaccurate, had my sympathy. Assuming the Campechano image showed the sun above the horizon is completely forgivable, the sky is so bright. Nautical twilight should be illustrated with at least a visible star in a dusk sky and ideally a barely-discernible horizon. And the Lyme Regis one confuses the subject even more, as it was taken around 400 miles longitudinally (?) south of any of the cities mentioned directly adjacent to it. Both images should go, even if replacements images aren't yet available.
Really, do we want descriptive illustrations here, or just pretty filler? --mikaul^talk 08:01, 20 September 2009 (UTC)

Well, if the pictures do need changing, then there's no lack of alternative pictures at Category:Twilight over on commons.   Set Sail For The Seven Seas  203° 29' 30" NET   13:33, 20 September 2009 (UTC)

I removed the Lyme Regis photo because:

• It didn't support the discussion in the Length section where it appeared, and
• It was taken before sunset.

Using info from the desciption page:

http://www.spectralcalc.com/solar_calculator/solar_position.php

Date and time: 2-10-2008 16:36:00

Timezone offset: 0 in February; is +1 at this writing in September for DST

Time of sunset: 17:18:34

Latitude: 50.72264

Longitude: -2.937889

Solar Altitude: 5.199

As for the other photos:

The Campeche photo appears in the Nautical Twilight section, which it surely does not illustrate. The reason given for a previous deletion was that it wasn't taken during twilight. I restored it because it appears to me that it was. Its description page doesn't give time and date info to confirm that. I would have no objection to removing the Campeche photo for the reason that it doesn't support the text in the section where it appears.

The image in the Civil Twilight section supports the discussion there, though my eye is drawn first to all the city lights.

The lead image is a lead image.

-Ac44ck (talk) 16:49, 20 September 2009 (UTC)

I've replaced the Campeche photo appears in the Nautical Twilight section, since the description says, "Sunset at the bay of Campeche, Mexico" clearly stating sunset. Plus, this is definitely not Nautical Twilight.   Set Sail For The Seven Seas  282° 26' 0" NET   18:49, 20 September 2009 (UTC)

Good stuff, nice to see some positive action! I've joined the boldfest and replaced the nautical twilight one with one that clearly shows visible stars and horizon and looks, er, nautical :) --mikaul^talk 19:49, 20 September 2009 (UTC)

It's definitely a nice picture, but it's way too bright to be nautical twilight. Actually, I thought that my picture was too bright, but this one is even brighter. The picture should really follow the description in that section. Mind you it is a nice picture.   Set Sail For The Seven Seas  342° 45' 45" NET   22:51, 20 September 2009 (UTC)

It's a little overexposed, I agree, but it's the precise moment required. I can upload a version with corrceted exposure a bit later. mikaul^talk 02:36, 21 September 2009 (UTC)

Darker version uploaded. The original was irretrievably "blown" in the brightest areas but at least this looks as if it might actually have been taken at twilight. I guess it's not helpful re the "definition" discussion above, but this is, I think, technically within the definition of nautical twilight in that it shows stars and horizon clearly visible. --mikaul^talk 03:48, 21 September 2009 (UTC)

You see, the first thing is that the shade of blue for the sky is too light, I would think that the nautical twilight sky would have the shade of blue in this picture; the second thing is that the orange part is not seen in nautical twilight, only civil twilight; but most importantly of all, those aren't stars in the picture, they are bits of dirt. :O   Set Sail For The Seven Seas  268° 15' 45" NET   17:53, 21 September 2009 (UTC)

Sorry, but the evidence is against you there. I won't argue that a better image can't be found but this one is only inadequate in terms of photographic technique. There's enough overexposure to make the sky bleach out to a white that, fair enough, probably wasn't perceptible as such at the time of capture. However there must still have been a relatively bright area in the sky for it to register this way. That notwithstanding, who are we to say the sky ought to be a certain shade of blue, given the myriad atmospheric possibilities at any one point in time? I calculate the solar altitude to have been -11.8° at the time of capture, and dirt on a DSLR sensor appears as black, not white spots; if you look at the image at full size they're clearly stars. Can we agree that this is good enough for now? --mikaul^talk 20:27, 21 September 2009 (UTC)

I don't mean to badger on about this, but I do still question the reliability of the timestamp. Assuming it's correct, then I'll agree that it is nautical twilight. Another thing is that orange sky. It looks more like the golden hour than nautical twilight. Everything appears to point to it not being nautical twilight. From what I know, an orange sky suggests that the sun has only just set due to its nature, since after this the sky goes light blue as the sun lowers and it is still civil twilight due to the amount of light remaining. Mind you, I could still be completely wrong. Also, at the right end of the horizon there appears to be what looks like the setting sun. One final point, if this is too bright due to overexposure then wouldn't it be misleading, even if it was taken during nautical twilight? After all, the time that the photo was taken is not the most important thing here, it's the light level. Mind you, it is loads better than the first one that was there! Hope this helps.   Set Sail For The Seven Seas  254° 39' 15" NET   16:58, 22 September 2009 (UTC)

By the way, were you trying to find a photo based on its timestamp so as to confirm that the photo was indeed taken during nautical twilight? If so, it does explain the potential suitability of this photo. Hope this helps.   Set Sail For The Seven Seas  217° 2' 45" NET   14:28, 23 September 2009 (UTC)

I managed to find this picture of Frontenac Castle during nautical twilight. I calculated the solar altitude to be 6.54° below the horizon, so it is only just nautical twilight, but nautical twilight nevertheless. I still question whether they were stars in the previous picture, I guess you could have been seeing stars! :D Hope this helps.   Set Sail For The Seven Seas  287° 47' 0" NET   19:11, 23 September 2009 (UTC)

I just checked the solar elevation for this picture and this is what I got.

Latitude: 41° 16', Longitude: 1° 46', Date: 30^th November 2007, Time Zone: CET.

Time	Solar Elevation
20:19	-32.11
19:25	-22.02
18:27	-11.46

This shows that the picture was not taken during civil twilight; in the highest case it is nautical twilight. So it may need to be changed, unless the times are wrong are something. Hope this helps.   Set Sail For The Seven Seas  297° 4' 45" NET   19:48, 23 September 2009 (UTC)

<outdent>Forgive me, but I think some photography 101 is needed here. Last point first: the data you're reading there isn't EXIF data from the original capture, it's metadata recorded when the composite file was generated in Photoshop. This happens by default when two or more images are combined to make a panoramic view. I took the shot myself, so I can vouch for it being recorded at less than 5 minutes after sunset, approximately 12 months prior to that date, a fact you're going to have to take my word for.
I can't vouch for the Acapulco image but I would similarly ask you to accept an Occam's razor approach; the only way this could have been shot during conditions other than nautical twilight is if the photographer found a likely image, pasted in stars and a moon, altered the EXIF data (having used a solar positioning calculator first, of course) and... well really, there's no reason to doubt the veracity of this image. I found it on the same page you found the other one, at Category:Twilight. The simplest explanation is the one I gave you before; an overexposed sky is possible in almost any circumstances, especially after sunset when the rest of the image is relatively dark, entirely dependant on your orientation to the sun. Judging by the brightest area on the Acapulco shot, the camera was pointed directly at the brightest part of the sky which would almost certainly have been where the sun went down, ie patently not off to one side (near the orange clouds, not sky) as you infer.
Your ill-informed nit-picking has now led you to replace this image with one of a castle that just happens to have been taken at (barely) the correct time of day, showing no stars, no visible horizon and all in all about as useful an illustration as one taken indoors. I think I preferred the pretty sunsets. mikaul^talk 21:26, 23 September 2009 (UTC)

Well, I guess you do have a point. I'll edit the article accordingly, but maybe the overexposure should be undone at some point so that it doesn't appear lighter than the civil twilight photo. Hope this helps.   Set Sail For The Seven Seas  248° 27' 30" NET   16:33, 24 September 2009 (UTC)

Well, like I said, it's not ideal by any means but it does have those two descriptive points going for it. I doubt there was ever any hope of undoing that overexposure even before it was first uploaded, but then it's really not an easy thing to illustrate and I don't hold much hope of finding a better one just lying around. You've actually got me so involved in it I'll probably get out and shoot it myself over the holidays... cheers, mikaul^talk 21:12, 24 September 2009 (UTC)

I guess I could try undoing that overexposure, I'm sure it could be done with photoshop. After all, never underestimate the power of photoshop! Hope this helps.   Set Sail For The Seven Seas  343° 12' 0" NET   22:52, 24 September 2009 (UTC)

Not even Photoshop can create something out of nothing. There is absolutely nothing there to undo... overexposure in this case has meant the irretrievable obliteration of all detail. The best you could hope for is a uniform grey which would look artificial and very unappealing. Just hoping to save you hours of fruitless effort... --mikaul^talk 07:00, 25 September 2009 (UTC)

Well, I say I could try undoing that overexposure, but it's more like trying out the different photoshop gadgets for about 10 minutes and then giving up because it ends up worse, I guess one could call it optimism! Anyway, if you do go getting pictures of nautical twilight then I suggest you get them when the sun at about 9° below the horizon so that it is properly nautical twilight and not bordering civil twilight or astronomical twilight. Hope this helps.   Set Sail For The Seven Seas  358° 30' 0" NET   23:54, 25 September 2009 (UTC)

Given the few minutes involved, I'll likely shoot several versions and pick the best from a illustrative point of view. If it checks out with regard to solar positioning, that'll be the one. All I really need is for the rain to stop... --mikaul^talk 00:06, 26 September 2009 (UTC)

Well, it isn't too difficult to calculate when nautical twilight is occuring, since the links at the bottom of the twilight article give various calculators for that. So I guess it is just waiting for the rain to stop! Hope this helps.   Set Sail For The Seven Seas  6° 31' 30" NET   00:26, 26 September 2009 (UTC)

The picture showing astronomical twilight at Prague Castle actually appears to be taken during nautical twilight. During astronomical twilight, the sky is essentially pitch black. In the picture, there is a very distinct amount of light in the sky. Basically, a telescope is needed to make the distinction between astronomical twilight and pure night. DAK4Blizzard (talk) 08:21, 14 April 2010 (UTC)

Definitions table

The table of definitions with numerical values is helpful, but slightly ambiguous. It needs clarity at the limits. For example, when the sun is exactly 6° below the horizon: is it civil twilight or nautical twilight? Ac44ck (talk) 20:36, 4 October 2009 (UTC)

Strictly speaking it is neither, as it is civil dawn or civil dusk at that precise instant as when the sun is 5.99...999° (not 5.99999...) below the horizon it is civil twilight and when the sun is 6.00...001° below the horizon it is nautical twilight. However, since this is civil dawn and civil dusk, it would make more sense to use civil twilight instead of nautical twilight if one had to be chosen. Hope this helps.   Set Sail For The Seven Seas  232° 42' 15" NET   15:30, 11 October 2009 (UTC)

Thanks for the distinction between 5.99...999° and "5.99999...". I think we're on the same page there. So this is analogous to "zero" being neither positive nor negative? Shall we have separate lines in the table at the boundary values for sunrise/sunset and civil/nautical/astronomical dawn/dusk? -Ac44ck (talk) 16:47, 11 October 2009 (UTC)

A temporal analogy: midnight is neither morning nor evening. But midnight is the first instant of a new day, though that is another class of temporal subdivision in the realm of whole-day groupings. In the context of this article does "day" start with or after sunrise? - Ac44ck (talk) 16:57, 11 October 2009 (UTC)

I prefer to think of it like that so that there's no overlap and disambiguity. After all, sunrise, sunset, dawn and dusk are indeed similar to midday and midnight being boundary points. Either way though, the sun being exactly 6° below the horizon is definitely not nautical twilight. However, I'm fine with the table as it is at the moment. If a change does happen though, I would think the best way would not be to add an extra line to the table, but to have a note underneath the table stating this and more importantly linking to the sunrise, sunset, dawn and dusk articles, which should have these definitions. Hope this helps.   Set Sail For The Seven Seas  268° 38' 45" NET   17:54, 14 October 2009 (UTC)

I understand the desire to have no overlap in definitions, but I consider sunrise to be the first instant of "day", as midnight is the first instant of a new calendar day; and calendar week if it is the first instant of a Sunday; and the first instant of a new year if it is also the first instant of January 1; etc. Back to the math analogy: zero isn't positive or negative, but it is the lowest non-negative value. The first non-twilight instant may be both sunrise and the first instant of "day". - Ac44ck (talk) 02:27, 17 October 2009 (UTC)

I am fine with the table as it is, after all these boundary points are infinitesimals and it shouldn't make too much of a difference if they are included with the various twilights. It depends on whether dawn and dusk are considered part of twilight and whether sunrise and sunset are considered part of daytime. Hope this helps.   Set Sail For The Seven Seas  323° 40' 45" NET   21:34, 19 October 2009 (UTC)

I agree with Thesevenseas. The ambiguity is pretty insignificant. The sun is not exactly 6 or 12 degrees below the horizon for very long (a few seconds at most, unless you're very far north/south). Even at the poles, where the sun's apparent movement is parallel to the horizon, the amount of time that the sun is exactly 6 or 12 degrees below the horizon is pretty short. And that is because of the change in solar declination as the Earth continues its revolution around the sun at its 23.44° tilt. DAK4Blizzard (talk) 08:21, 14 April 2010 (UTC)

The Twilight Saga

Most people around the world already know about the hit series from Stephenie Meyer called The Twilight Saga. This saga consists of 4 books: Twilight, New Moon, Eclipse, and Breaking Dawn. She was also working on a fifth or 1.5th book called Midnight Sun but after someone leaked it on the Internet she stopped and refuses to continue writing it. The saga became movies and now is a hit around the world. The movies that came out were Twilight (2008), New Moon (2009), and Eclipse is coming out June 30th, 2010. Word is that Breaking Dawn will be 2 movies because of it's length. If you read or saw Twilight you probably fell in love with the amazing characters such as Edward Cullen or Jacob Black. The books and movies have been great and I hope you read and see them!

Here is the official website's link... world of twilight

Neccagirl (talk) 14:49, 6 March 2010 (UTC)

Hi, thanks for your comment, but I think you actually want this page. Hope this helps. Set Sail For The Seven Seas 224° 56' 30" NET 14:59, 6 March 2010 (UTC)

LOL. --77.109.223.37 (talk) 00:04, 14 June 2010 (UTC)

Twilight Cause

There are two twilights to consider,there is is the daily twilight due to daily rotation and then there is polar twilight which has its origins in the specific way our planet moves through its orbital circumference. The primary cause for the daily twilight and its duration is strictly a matter of daily rotation and latitudinal speeds of a round Earth with a maximum rotational speed of 1037.5 miles per hour at the equator,reducing to 520 miles per hour at 60 degrees latitudes North and South and reducing further to zero at the geographical poles.As the rapid equatorial speed translates into a rapid transition from daylight to darkness,the longer twilight period at 60 degree latitudes can then be accounted for by the slower latitudinal speed of 520 miles per hour with the single polar twilight strictly a consequence of the orbital motion of the Earth as orbital twilight occurs at the orbital point of the equinox. As the Earth does not keep the same face to the Sun while it moves along its orbital circumference,the geographical poles turn through the circle of illumination hence the orbital twilight at the equinox.The idea of the angle of the Sun's descent to the horizon is so crude that it cannot be commented on Oriel36 (talk) 17:07, 9 May 2010 (UTC)

Actually, the cause of twilight is the presence of an atmosphere that reflects sunlight to the surface allowing indirect illumination of the ground by the sun. The duration of twilight is indeed connected to the latitudinal speed of the earth, however this is not its cause since dealing with a rotating object calls for the use of angular velocity rather than tangential velocity. This can easily be seen in noticing that the daily path of the sun has the same speed and path length at the same time anywhere on the earth. The duration of twilight is also connected to the angle of the Sun's descent to the horizon, but again this is not its cause. The actual cause for the varying length of twilight at different latitudes is due to the daily change in apparent solar elevation. Since the solar elevation throughout a day follows a sine curve, the rate of change of solar elevation throughout a day will follow a cosine curve. The amplitude of the sine curve representing the solar elevation will be equal to the colatitude (90° at the equator and 0° at the poles) and therefore the amplitude of the cosine curve representing the rate of change of solar elevation will also be equal to the colatitude using standard calculus. Since the cosine curve representing the rate of change of solar elevation will have higher peaks and troughs for lower latitudes than higher latitudes, the lower latitudes will have faster and shorter twilight than the higher latitudes. Finally, there can only be one form of twilight since there is only one cause of twilight. Polar Twilight is related more to the Polar Night than to Twilight. Hope this helps. Set Sail For The Seven Seas 340° 5' 45" NET 22:40, 14 June 2010 (UTC)

The reasons you give for variations in twilight length at different latitudes are incorrect and geocentric,a person with common sense accepts that twilight lengths correlate with rotational speeds at different latitudes as locations pass through the circle of illumination at slower speeds hence longer twilights away from the equator and towards the geographical poles.What your reasoning does show is a profound intellectual destitution which has neither the confidence or the clarity of purpose to express a normal planetary fact that the equatorial speed of the Earth is 1037.5 miles per hour and the Earth turns its full equatorial circumference in 24 hours.Oriel36 (talk) 12:00, 15 June 2010 (UTC)

Firstly, you needn't be so rude when replying to comments. This is a place for civilised discussion, not heated arguments.

You misunderstand what I am saying. Rotational speeds indeed correlate with twilight lengths but this not the cause. It is angular speed that we must deal with here not tangential speed, however both of these are indeed connected. It is the rate of change of solar elevation or the speed of the up/down movement of the sun that causes this. In other words, the faster the sun changes elevation values, the faster it will get through elevations giving twilight. Hope this helps. Set Sail For The Seven Seas 196° 31' 15" NET 13:06, 15 June 2010 (UTC)

It seems like this discussion has already happened before here and the same point are coming up again. The main point I will make about this is that we are dealing with angular speed not tangential speed. However, you do have a point since rotational speeds indeed correlate with twilight lengths, but varying rotational speeds is not the cause of varying twilight lengths.

My intention here is not to argue against you since you do have a valid point, but to discuss this further. Hope this helps. Set Sail For The Seven Seas 200° 27' 44" NET 13:21, 15 June 2010 (UTC)

The fact that the Earth rotates determines why we experience the day/night cycle,the fact that the Earth is round and rotating makes it certain that twilight lengths vary with latitude so that when everything is reduced to planetary dynamics and dimensions,the cause becomes apparent - slower speeds mean that a location passes through the circle of illumination more slowly than at the equator hence longer twilights..The additional polar twilight at the equinoxes is important as it is a window or a beacon into the orbital behavior of the Earth as the polar coordinates pass through the circle of illumination at the equinoxes.There should be no difficulty with rotational speeds correlating with twilight lengths but all other explanations insist in combining daily and orbital components into a single explanation whereas it needs two separate dynamics to explain two separate twilights.Thanks for being civil and I modified my other response to reflect that civility.

By polar twilight, are you referring to the phenomenon that occurs at and above 72° latitude where the sun remains within 0° and -18° for a whole day? If so, then the two can be merged into one. However, the daily and orbital components are indeed independent, since one deals with the longitude of the overhead sun and the other deals with the latitude. Basically, as the latitude increases, the difference between daily solar elevation extremities (ie. the difference between the solar midday solar elevation and the solar midnight solar elevation) decreases. As the latitude gets high enough, the difference will reduce to 18° and the sun will remain within 0° and -18° for some day of the year, giving 24hr twilight (or polar twilight as you call it). Hope this helps. Set Sail For The Seven Seas 201° 26' 15" NET 13:25, 17 June 2010 (UTC)

Polar twilight is an orbital event which occurs at the equinoxes where 6 months of darkness separates 6 months of daylight at the polar coordinates,as daily rotation is absent or residual at the North and South poles,the actual cause of the passage of the polar coordinates through the circle of illumination thereby switching from 6 months of either daylight or darkness is due to the way our planet turns through 360 degrees as it moves along its orbital circumference.In other words,if daily rotation was absent,all locations on the planet would experience a single daylight/darkness cycle due solely to the orbital motion of the Earth.This is why the polar coordinates act like a beacon or a window into the orbital behavior of the Earth.I simply cannot support geocentric views and please do not expect me to entertain them when planetary dynamics are needed to explain two distinct effects. 1 - latitudinal twilight variations due to a round and rotating Earth and 2 - polar twilight which divides 6 months of darkness from 6 months of daylight at the equinoxes arising strictly from the orbital motion of the Earth.Oriel36 (talk) 13:47, 17 June 2010 (UTC)

Fair enough, in that case then do you think of the dynamics in terms of the Solar Terminator? If so, one can think of twilight as a "grey" band separating day from night. However this would be one continuous band and therefore surely just one form of twilight. Also, daily rotation does still have effects at the poles since the rotation of the earth will change the apparent direction of the sun. Hope this helps. Set Sail For The Seven Seas 249° 57' 0" NET 16:39, 18 June 2010 (UTC)

You really have to be kidding with that 'solar terminator' motion !,for goodness sake,I have tried to maintain a level of conversation which does not descend below a certain intellectual level and if you cannot comprehend why the polar coordinates pass through the circle of illumination at the equinoxes reflecting the orbital behavior of the Earth and a twilight which is sandwiched between 6 months of either daylight or darkness then I couldn't be expected to continue with this.Take a broom handle representing both daily rotation and 'tilt' and walk around a central object while keeping the broom handle pointing in the same direction as you walk/orbit the central object/Sun.You will notice immediately that you have to walk backwards at one point and then forwards in order to maintain the broom/tilt orientation signifying that single daylight/darkness cycle is a result of the orbital motion of the Earth.I am absolutely horrified that another human being would put something like that see-sawing terminator in front of me when I have gone to great lengths to explain why it takes two motions to explain daily twilight and polar twilight,the former due to daily rotation and the latter a product of the orbital motion of the Earth. Oriel36 (talk) 21:59, 18 June 2010 (UTC)

Astronomical

There is a definite contradiction between the text and the picture of the Prague Castle. The article says that during astronomical twilight the sky is almost dark, but this is clearly not even close the case in this picture. I am having a hard time making the distinction between the various types of twilight. --77.109.223.37 (talk) 00:04, 14 June 2010 (UTC)

There are only two twilights to consider and each has a separate cause,the normal twilight where the Earth transits through the circle of illumination on a daily basis with shorter twilights towards the Equator due to faster latitudinal speeds with a maximum equatorial speed of 1037.5 miles per hour.The cause of the polar twilight at the polar coordinates where these coordinates transit through the circle of illumination at the equinoxes thereby separating 6 months of darkness from 6 months of daylight,or visa versa, is a property of the orbital motion of the Earth.We exist in an era of profound intellectual destitution where something so simple as the orbital characteristic is ignored even though it is crucial for explaining both the seasons and the variations in the length of natural noon cycles when allied with daily rotation92.251.255.11 (talk) 20:26, 14 June 2010 (UTC)

@77.109.223.37 (talk) - The picture was chosen due to the date, time and latitude that was given with the picture, since it indicated that it was taken during astronomical twilight.

@92.251.255.11 (talk) - See above.

Hope this helps. Set Sail For The Seven Seas 341° 5' 30" NET 22:44, 14 June 2010 (UTC)

Military use of nautical twilight

The current text claims "This is partially due to tactics dating back to the French and Indian War (part of the Seven Year's War of 1756-1763), when combatants on both sides would use BMNT and EENT to launch attacks". What is the source for this claim, as the earliest references to the term "nautical twilight" that I know date to the late 1930s? AstroLynx (talk) 08:17, 29 June 2010 (UTC)

Brightness of Civil Twilight for "Normal Outdoor Activities"

I have always been perturbed by statements like "During this period [civil twilight] there is enough light from the Sun that artificial sources of light are not needed to carry on [normal] outdoor activities.". First of all, what are 'normal' outdoor activities? Second, at the risk of looking like original research, this needs to be questioned. If we consider the common sports listed at http://www.americanelectriclighting.com/sportslighting/LightLevels/Lux.asp to be 'normal' outdoor activities, the lux levels listed on that site occur naturally outdoors right around sunset or sunrise (under clear skies), rather than at the dark limit of civil twilight. (See http://www.burle.com/cgi-bin/byteserver.pl/pdf/Electro_Optics.pdf under Sources of Radiation in the table of Contents.) The illumination corresponding to the dark limit of civil twilight under clear skies is 3 lux, or roughly that of a candle at a distance of 58 centimeters (23 inches). This is considerably darker than the illumination standards used for many sports, reasonably representative of 'normal' outdoor activities. Something is wrong here. (Remember, dark limit of civil twilight = candlelight!)

I have changed the wording slightly in this statement in the article.

- User: Nightvid (unregistered) —Preceding unsigned comment added by 130.184.13.73 (talk) 03:11, 6 April 2008 (UTC)

new link

http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/rise-set-twi-defs for def.s of twilight in 1st footnote. —Preceding unsigned comment added by 65.100.114.241 (talk) 22:43, 7 August 2010 (UTC)

In Hinduism?

This edit (diff) on June 25, 2010 introduced the subsection regarding twilight's significance in Hinduism. The section was "cleaned up" (i.e. some "no citation" templates were added) a few days later, but has not been modified or changed in any way since then. Since there are no citations or references for any of those claims and over a month has passed since those edits, I propose the whole section be deleted. The original author has had plenty of time to provide references, and he/she has not yet, so no point holding our breath.  Amit  ►  21:05, 11 August 2010 (UTC)

Mind you, I did spend some time on my own looking for sources that confirm any of these facts. I am Hindu, btw, so I figured I'd know more about what to look for than others. Long story short, I couldn't find any reliable sources for any of these claims. Further reason to remove the section. I'm going to be BOLD soon and just make the edit, but I wanted to vet it out here first.  Amit  ►  21:16, 11 August 2010 (UTC)

Sounds reasonable to me. WP:BURDEN is one of Wikipedia's key quality control mechanisms. Adrian J. Hunter^{(talk•contribs)} 10:41, 12 August 2010 (UTC)

 Done - went ahead and took it out. Thanks for pointing me to the relevant guideline... I'm still getting used to some of the shortcuts here.  Amit  ►  13:31, 12 August 2010 (UTC)

Twilight Cause

There are two twilights to consider,there is is the daily twilight due to daily rotation and then there is polar twilight which has its origins in the specific orbital behavior of our planet moves around the central Sun. The primary cause for the daily twilight and its duration is strictly a matter of daily rotation and latitudinal speeds of a round Earth with a maximum rotational speed of 1037.5 miles per hour at the equator,reducing to 520 miles per hour at 60 degrees latitudes North and South and reducing further to zero at the geographical poles.As the rapid equatorial speed translates into a rapid transition from daylight to darkness,the longer twilight period at 60 degree latitudes can then be accounted for by the slower latitudinal speed of 520 miles per hour.Oriel36 (talk) 15:21, 6 August 2010 (UTC)

How would the twilight period vary with latitude if the earth were conical instead of spherical? What would be the effect of surface speed if all locations on the same "meridian" saw exactly the same solar elevation at local noon? -Ac44ck (talk) 13:10, 7 August 2010 (UTC)

You poor thing,the Earth is round and rotating with specific dimensions correlating speed with distance traveled,specifically turning 15 degrees/1037.5 miles per hour at the equator and a full equatorial circumference of 24901 miles in 24 hours.There are fools who cannot express this basic planetary fact with confidence as they do not believe the Earth rotates 15 degrees/1037.5 miles per hour at the equator and unfortunately are in the majority and can be identified by invoking hypothetical conical Earths.

Twilight length variations at different latitudes have an extremely simple cause - the faster a location spins the shorter the twilight,the slower it spins the longer the twilight.Pity we live in an era when the link between the dimensions and rotation does not survive as most refuse to accept that the 24901 mile equator turns through 360 degrees in 24 hours and that,unfortunately,is a dismal fact.Oriel36 (talk) 19:07, 17 August 2010 (UTC)

Repetition does not ameliorate your fallacies. —Tamfang (talk) 07:11, 18 August 2010 (UTC)

Try again.

Consider a cone rotating at the same rate as the earth and tapered such that that the surface speed is 1037.5 miles per hour at zero latitude and 520 miles per hour at 60 degrees latitude. The surface speed at 90 degrees latitude would be zero, just as it is for the spherical Earth.

This cone is also "round and rotating with specific dimensions correlating speed with distance traveled,specifically turning 15 degrees/1037.5 miles per hour at [zero latitude] and a [maximum] circumference of 24901 miles in 24 hours."

Using your choice of value for the length of twilight at zero degrees latitude on this cone, what would be the length of twilight at 45 degrees latitude? - Ac44ck (talk) 08:14, 22 August 2010 (UTC)

It's a bit strange to speak of "degrees latitude" on a cone. —Tamfang (talk) 23:17, 10 September 2010 (UTC)

Perhaps, but is it not easily envisioned? Geodetic latitude doesn't translate well; all normals on the cone are at the same angle to the axis of rotation. But geocentric latitude is quite comparable. The fixed point for all radii to the surface in geocentric latitude would be the center of the largest circle, just like on a sphere. The largest circle would be at zero latitude, just like on a sphere. The point with zero surface speed would be at 90° latitude, just like on a sphere. Surface speed would vary with latitude; linearly as opposed to sinusoidally, but the rate of variation is irrelevant for the purpose at hand. A conical planet is suitable for examining Oriel36's claim that surface speed governs the length of twilight. Would not coordinates of latitude and longitude be useful on such a planet? - Ac44ck (talk) 04:57, 11 September 2010 (UTC)

Certainly, but I wouldn't call the latitude measure an angle, nor define it by the arcccos of relative circumference as you implicitly did above. That's all. —Tamfang (talk) 02:43, 13 September 2010 (UTC)

My label is strange, the measure isn't an angle, and you would do it some other way.
So your proposal is ...? - Ac44ck (talk) 03:22, 14 September 2010 (UTC)

I'd use linear measure, of course. Whether to use 90 units or some other number is not important. Here, I'd have put the question to Oriel with "halfway between rim and apex" rather than defining a coordinate system which the reader would then have to interpret. Since you ask. —Tamfang (talk) 21:31, 14 September 2010 (UTC)

Angle of sun vs horizon

As currently written, parts of the table in "Definitions" doesn't seem to make any sense. It claims that when the "center of [the] sun's disk is at [the] horizon" (third line), that the "sun's centre relative to [the] mathematical horizon" is theta-sub-s of -0deg35min.

That obviously doesn't make much sense. The two possibilities I see are that we're distinguishing the center of the sun's *disk* from the center of the (spherical) sun -- but those should be essentially identical, or that the row and column descriptions are talking about two different horizons. If they're talking about two different horizons, they should call that out explicitly; as it stands, the parenthetical below the table which defines what 'horizon' means in the table implicitly defines both uses of horizon to mean the same thing.

And if it's not one of those, I just don't get it.

Plus using both 'center' and 'centre' in one table is goofy, which makes me wonder if this table has been edited inconsistently. 24.16.57.110 (talk) 13:59, 19 October 2010 (UTC)

Without looking, I guess the difference is atmospheric refraction, which brings the sun's image (its "disk") above the horizon when the physical sun is below the "mathematical" plane. —Tamfang (talk) 18:08, 24 October 2010 (UTC)

That seems to be the intent, but the wording is inconsistent. The second row of the table gives a negative "solar elevation angle" value for "Sun's lower limb at horizon." But "solar elevation angle" is defined as "the position of the geometric center of the sun relative to the horizon." If the geometric center of the solar disk is below the "horizon" then the lower limb cannot be "at horizon." -Ac44ck (talk) 04:30, 25 October 2010 (UTC)

Caption of daylight-hours graph is in error

The caption in this graph in in this version is badly off.

"The number of daylight hours depends on the latitude and time of year. There are brief times in March and September where continuous daylight exists at locations near both poles."

Please give me a list of locations that are near both poles. ;) ... OK, we'll start with the small grammar nit. It is impossible for a location to be "near" (in the sense used here) "both" poles. It can be near one or the other, but not both. Hence, "... at locations near either pole."

I'd fix that myself, but there is a much bigger issue in the entire sentence. "There are brief times in March and September where continuous daylight exists..." Actually, as the graph shows, the entire period between the March and September equinoxes is in continuous (not brief) daylight at each pole. At 80N, the graph indicates continuous daylight from about April 1 to about September 7, hardly "brief". At 70N, about May 1 to Aug 10, still not "brief" and nowhere near "March and September". At somewhere around 60N, there is "continuous daylight" only on the summer solstice, about June 22.

And vice versa (complete darkness) in the Southern latitudes, with the periods of continuous daylight centering around Dec 22 at 60S,.and expanding away from December as one approaches 90S.

Thus "brief times in March and September" is totally inaccurate. It's true that the briefest period of continuous daylight occurs the farthest from the poles (among all locations that experience this), but the times center around the solstices, not the equinoxes. I claim no knowledge of astronomy, etc., so hope that an expert will word this properly. Unimaginative Username (talk) 02:35, 14 June 2011 (UTC)

I've fixed the caption. On a related note, in the "Length" section: "In the Arctic and Antarctic regions, twilight (if there is any) can last for several hours." I think "several weeks" might be more accurate. Isn't it true that the sun may be just below the horizon, but the sky still illuminated, for continuous periods of (Ant)arctic spring and autumn? The later sentence, "At the poles, civil twilight can be as long as two weeks, while at the equator, conditions can go from day to night in as little as twenty minutes," seems to agree with me. Doesn't that indicate internal inconsistency (direct contradiction) in the article? "Pij" (talk) 22:10, 30 June 2011 (UTC)

The rapid transition from daylight to darkness at the equator is due to daily rotation as a location at the equatorial latitude moves at 1037.5 miles per hour through the circle of illumination hence the daily absence of twilight in any meaningful way,the polar twilight is from an entirely different cause and only occurs at the orbital points of the equinoxes as those null rotational points (North/South poles) swing through the circle of illumination as an effect of the orbital behavior of the Earth.It is therefore not possible to represent twilight at the equator and twilight at the poles within the same graph,the causes are different and destroys the ability to describe the causes for the Earth's day/night cycles,one belonging to the daily rotation of the Earth and the other arising from the planet's orbital behavior,in short,contemporary explanations try to do too much with too little information.Gkell1 (talk) 03:27, 2 July 2011 (UTC)

Welcome back, Oriel36. —Tamfang (talk) 03:56, 2 July 2011 (UTC)

Thank you,as long as some pony tailed geek or over grown pupil doesn't decide to throw a tantrum and pull the plug,there are many things to discuss,adjust and adapt to Gkell1 (talk) 17:31, 4 July 2011 (UTC)

Nautical twilight

"In general, nautical twilight ends when navigation via the horizon at sea is no longer possible." I think this sentence is referring to evening nautical twilight. If so (and it needs to be stated), it could perhaps be re-worded so: "In general, evening nautical twilight ends when navigation via the horizon at sea is no longer possible, and morning nautical twilight begins when there is sufficient light to navigate via the horizon." I don't really like it, though. Too clumsy. "Pij" (talk) 22:01, 30 June 2011 (UTC)

Duration of Twilight

I would like to contribute on the main page with an explanation of the duration of twilight. Don't have the time now write properly but wanted to point out that while it is written on the main page that twilight depends on latitude of observer and is shorter at the equator, it is not mentioned the fact that twilight depends also on the day, i.e. it is shorter at the equinoxes and longer at the solstices. An explanation not requiring spheric geometry notions and that can (hopefully) be easy to visualise/depict can be the following:

The duration of twilight (for a given location i.e. latitude and longitude of observer) is a function of both (1) the speed the sun moves across the sky during the day in its apparent motion due to earth rotation along its axis, and (2) the apparent trajectory of the sun (more or less inclined with respect to the horizon) due to the same apparent motion when the sun sets or rises above the horizon.

The speed the sun moves across the sky (1) is due to the apparent motion of the celestial sphere (including all stars and planets) due to earth's rotation and can be measured in rad/sec or degrees/sec or degrees/ min. This speed is not the same for all celestial objects. It is highest for objects on the (celestial) equator (ie. declination zero) while it decreases for objects far from the celestial equator (with higher declination) and is obviously zero for objects at the north and south celestial pole (e.g. declination plus of minus 90 deg e.g. Polar star which in fact is almost fixed and does not move during the earth's day). This speed as said is basically the speed or objects on the celestial sphere as it rotates. Clearly objects near the north pole and south pole will move slower whereas objects close to the equator moves faster. Note this speed is different from the angular velocity of the celestial sphere around its axis (south/north celestial pole) and which is roughly 360 deg every 24 hrs and obviously the same for all objects (angular speed around the axis) While during the day all celestial bodies move across the sky as per the earth rotation, it is also obviously clear that during the year the sun and the planets as well as the moon trace apparent paths on the celestial sphere as part of their orbital motions with respect to other distant objects which are "fixed" in this sphere At the equinoxes, the sun path crosses the equator thus the sun is on the equator and therefore the apparent daily motion due to earth rotation occurs with the highest speed compared to an object at higher declination. At the solstices, the sun is at either 23 degrees 27 declination north or south thus moves slower. The highest speed at equinox means also that the sun will reach 6 degrees (civil twilight) , 12 degrees (nautical) , 18 degrees (astronomical twilight) below the horizon faster than at the solstices. This is factor (1) contributing to shorter twilights at equinox for any location.

Now factor 2. Factor (2) is about the trajectory. This is a bit more difficult to visualise but for the sake of simplicity, think you are on the equator or close (e.g. Singapore) when is the equinox day (around 20-21 March or September). On this/se day(s) and at this location the sun sets and rises perpendicular to the horizon, i.e. with an angle of 90 deg (note also on this day it is also the shortest of all twilights possible on earth). Now same location at the solstice. On this day the sun is either 23 deg decl north or south. The apparent trajectory of an object at such declination when it sets (or rises) intersect the horizon at an angle less than 90 degrees. For objects at higher declinations, this angle further decreases to a point where the object no longer sets as when it reaches the horizon, it is tangent. Think about the trajectories / motion of constellations and those with stars that never set below the horizon. They trace circular paths around the north celestial pole. So for this same reason, any object on the celestial equator will trace a trajectory whose angle with the horizon is the maximum possible amomgst all celestial objects for the same location. In other words, the closest to the equator is the object (declination 0), the "steeper" (more vertical) is the intersection angle with the horizon. For the sun, as at the equinox its declination is zero, this angle is the highest and the trajectory more vertical so the Sun "goes down" with a steeper trajectory and an angle closer to 90 deg. That's factor (2).

Both factor 1) and 2) ensures that at the equinox, the sun traces a steeper apparent trajectory and at the same time a higher apparent motion speed across the sky resulting in a vertical speed component which is the greater possible. Note that the trajectory of course is not a straight line. It is a circle on the celestial sphere. Factor (1) and (2) will cause the sun to go "down" i.e. vertically below the horizon quicker and thus twilight will be shorter. It is interesting to note that both factors (1) and (2) impacts "symmetrically" the duration of twilight at both winter and summer solstice. Even if at summer solstice (in the northern hemisphere) the sun is higher above the horizon and reaches higher elevations, the way the sun sets (rises) is the same as at the winter solstice when it is lower (maximum elevation of the sun is lower). Even if the trajectory is a lower elevation one, the sunset angle is the same as in the summer solstice. And the same applies for the apparent speed which is the same at both summer and winter solstice. Hope the explanation has been enough clear. And not have done any error. — Preceding unsigned comment added by Mdimauro (talk • contribs) 23:06, 30 September 2011 (UTC)

I am afraid you are all over the place so just restrict it to the changing relationship between daily and orbital characteristics and the daily and seasonal variations will appear separately leaving the reader to comprehend that there are two types of twilight events involved.To determine why twilight lengths are shortest at the equator,it needs only to determine the maximum equatorial speed and work in slower rotational values towards the polar coordinates,to comprehend the orbital daylight/darkness cycle with polar twilight at the equinoxes,it is best to appreciate what the Earth does where rotation is residual at the North/South poles.Alas,there is no current acceptance of the polar daylight/darkness cycle as arising strictly from the orbital behavior of the Earth in that the polar coordinates turn roughly 10366 miles with respect to the central Sun and coincident with the orbital period of the planet.The polar twilight lasts a number of weeks as the poles turn in a circle through the terminator and either into orbital darkness or into the light of the central Sun depending on what orbital point the Earth exists at in its annual cycle19:01, 5 October 2011 (UTC) — Preceding unsigned comment added by Gkell1 (talk • contribs)

Definitions table

Hello, new here, so please bare with me if I'm not adding my comment in the wrong place, I need to learn! I thought I'd contribute about the definitions table, and especially its reference as I want to reference original research in my own work. On the reference itself, I've read the document linked to the definitions table [3] "Low precision formulae for planetary positions" and I cannot find any reference to the values of the definitions table. However, the document itself references the http://www.usno.navy.mil/USNO/astronomical-applications/publications/exp-supp which might hold these values. Am I missing something in the first referenced document [3] on the page? ekynoxe (talk) 20:37, 26 October 2011 (UTC)

Ideal versus True Horizon?

In the twilight article, under the definitions table, the phrase "ideal horizon" is used: ("For these definitions, an ideal horizon 90° from the zenith is used"). However, on the horizon page the term used that seems to refer to the same concept is "true horizon", which seems to be a technical term since it's put in italics. Does "ideal horizon" differ in meaning in any important way from "true horizon"? If not, should we use "true horizon" on the twilight page for consistency? Showeropera (talk) 05:11, 22 December 2011 (UTC)

Say something once, why say it again?

Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below the horizon. . . . Twilight is technically defined as the period between sunset and sunrise during which there is natural light provided by the upper atmosphere, which receives direct sunlight and scatters part of it towards the earth's surface.

Does this last sentence add anything? —Tamfang (talk) 23:23, 30 July 2011 (UTC)

You're right; it says exactly the same thing as the first sentence. I took it out. TWCarlson (talk) 15:41, 31 July 2012 (UTC)

Polar Twilight

There is a distinction between daily twilight and polar twilight where the causes are separate and no indication is given in the main article that they are.The polar twilight is an event which occurs around the equinox as those null rotational points (North/South poles) swing through the circle of illumination about a traveling orbital axis which stretches through the center of the Earth from Arctic to Antarctic circles.The polar daylight/darkness cycle where roughly six months of daylight follows 6 months of darkness produces that twilight at specific orbital points hence the concentration of the cause as dynamically distinct.It is therefore crucial to look at the orbital behavior of the planet separate to the daily cycle and the other twilight associated with daily rotation.As it does tie in with the seasonal explanation and the variations in the natural noon cycle due to daily rotation and orbital motions as separate motions with combined effects that vary ,the issue is high importance as opposed to mid-class importance.Gkell1 (talk) 20:39, 13 September 2011 (UTC)

The cause of twilight is the same at any latitude: when the sun is below but near the horizon, the upper atmosphere is illuminated. What differs is the cycle in which this occurs. —Tamfang (talk) 07:16, 12 October 2011 (UTC)

The polar coordinates turn in a roughly 10,366 miles cycle/circle to the central Sun and coincident with the orbital period of the planet so those polar coordinates will turn through the circle of illumination at the equonoxes hence the polar twilight as distinct from the daily twilight experienced at lower latitudes.It replaces 'tilt' towards and away from the Sun and introduces the idea that aside from daily rotation our home planet has a single daylight/darkness cycle arising solely from the orbital motion of the Earth and the polar coordinates act like a beacon for that motion and especially noteworthy at the equinoxes as orbital daylight turns to roughly 6 months of orbital darkness or visa versa.I would adjust the header to - 'Orbital Twilight'- denoting its distinct cause and again,this is a major modification which introduces an additional day/night cycle to the Earth .There is no reason to ignore the cycle nor leave the explanation unattended as it has been for a number of years.Gkell1 (talk) 15:57, 12 October 2011 (UTC)

This is somewhat compelling, but appears to be original research. Can you provide any sources? TWCarlson (talk) 15:58, 31 July 2012 (UTC)

No, because he has been banned for trolling. It's hard to tell whether Gerald Kelleher (formerly Oriel36 (talk · contribs)) misunderstands the interaction of axial and orbital motions as grossly as he seems to, or understands it well enough but has his own incompatible language for it, or what. —Tamfang (talk) 20:39, 31 July 2012 (UTC)

Interesting. I should have checked his account and I would have seen that. I have no doubt the explanation makes sense in his own mind, but I am thoroughly confused. Thanks for the reply. TWCarlson (talk) 11:30, 2 August 2012 (UTC)

new civil location

Baltasound

is an example of a uk location that experiences civil twilight all year, no uk locations are listed at the moment so someone should add it — Preceding unsigned comment added by 94.193.11.45 (talk) 00:37, 4 December 2011 (UTC)

Proposed image and diagram changes

I'm proposing a change to the layout of the images and diagrams on this page. See my draft of the Intro and Definition sections on my sandbox page:

User:TWCarlson/sandbox

I want to replace the simple overview diagram with a more thorough and colorful one. Then I want to include a detail view of the three different types of twilight.

The only problem is, this creates too many images, so I thought the current lead image should be moved to the gallery. That would make the diagram of twilight be the first thing seen in the article, rather than an image of twilight. Is this acceptable? If not, are the diagrams good and is there any way to still use them in some way?

Thanks. TWCarlson (talk) 20:37, 19 September 2012 (UTC)

I like the new diagrams very much and I agree the page can be improved. However the photo you've chosen to represent twilight is actually a dusk photo, as there would be no need for the lights of a City to be on before Dusk, or just before Sunrise. Twilight begins just after the Sun sets, and also ends just before Sunrise. Twilight is actually quite a bright time of day. I also think the Piccadilly Circus photo is closer to dusk as well; but the gallery photos need not hit the mark precisely, they are there to marvel at for the most part. In summery, yes...you are headed in the right direction with the new and informative charts; and I'm sure the perfect twilight photos will find their way into the article eventually.

Thanks Pocketthis (talk) 22:32, 19 September 2012 (UTC)

Thanks for your comments.

I'm not an expert on this, but I think the photo may represent nautical twilight rather than civil. Certainly the most common everyday notion of twilight would be civil twilight, and this photo probably is after that. But since there is still some light in the sky, it probably meets the nautical definition.

I'm going to tweak the diagrams a bit more and then I'll probably just boldly make the changes to the article and then see how any additional changes evolve. TWCarlson (talk) 12:16, 21 September 2012 (UTC)

Riga

Riga Bridge looks very much like London Eye Twilight ! — Preceding unsigned comment added by 195.240.153.18 (talk) 13:54, 12 November 2012 (UTC)

You're right, the image has the wrong caption. It looks like the article is using a confusing system where it cycles between various images based on the number of edits the article has received, and someone has (at some point) broken this by adding a new image into the sequence without adding a caption. I've cut the cycling image system and just used the first one instead. --McGeddon (talk) 14:04, 12 November 2012 (UTC)

Daylight length SVG is inaccurate

I live at 42.8 degrees North and know that the minimum length of day here goes down to 8 hours and 22 minutes, which seems to disagree substantially from the daylight length graphic. Perhaps it can be corrected? — Preceding unsigned comment added by 173.14.140.253 (talk) 03:46, 1 December 2012 (UTC)

Did you click on the graphic and read its description? The length plotted is "the period from the beginning of civil twilight in the morning to the end of civil twilight in the evening", not the time from sunrise to sunset. It looks about right for your latitude at the winter solstice. Hertz1888 (talk) 04:27, 1 December 2012 (UTC)

Ah... yes, I saw that be didn't appreciate that the difference was that large. It would be nice if this was re-created to use the daylight hours more people are used to seeing though. This is a beautiful graphic, really an exceptional example of good data visualization - I was going to point it out in a lecture, but everyone here in New England knows 'it gets dahhhk at 4:30' and will automatically (and ruefully) point out that we seem to get a lot less daylight in the winter than this pic shows - using 'civil' twilight somewhat detracts from the graphic for this reason, even if it is, as I see now, technically accurate. — Preceding unsigned comment added by 173.14.140.253 (talk) 04:49, 1 December 2012 (UTC)

This is what you get up to when I'm not looking?

The lead now says:

Twilight is the illumination that is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere, so the surface of the Earth is neither completely lit nor completely dark. The Sun itself is not directly visible because it is below the horizon. Owing to its distinctive quality, primarily the absence of shadows, twilight has long been popular with photographers, who refer to it as Sweet Light, and painters, who refer to it as the "blue hour", after the French expression l'heure bleue.

The bit about painters is the only clear hint that it's a time of day! Whose bright idea was it to take out "the time between dawn and sunrise or between sunset and dusk, during which"? —Tamfang (talk) 08:56, 29 January 2013 (UTC)

Not my idea, bright or otherwise. Agree that it is a time of day. I have restored the normal or usual definition, which is backed by the citation. Still missing is the previous language regarding the exceptional conditions of all-night or round the clock twilight. Hertz1888 (talk) 09:41, 29 January 2013 (UTC)

• The edit was made by a good editor: D0wenWilliams, on January 21st. I didn't notice the removal of the times of day, or I would have gone to his talk page and conferred with him, as I often do. His explanation for the edit was that at certain Latitudes such as the poles, twilight can last the entire day, so it's not necessarily a time of day as most of us experience it. If you would like to discuss it with him, you can reach his talk page here: http://en.wikipedia.org/wiki/User_talk:DOwenWilliams. However, as the article reads now, you have corrected the 1st paragraph, and the longer twilight times are mentioned in "Times and occurrences"; so all appears to be just fine in my humble opinion; however, I am not a science editor. Thanks Pocketthis (talk) 15:43, 29 January 2013 (UTC)

I just looked up "Twilight" in the Oxford and Webster's dictionaries. They both say that it is a type of light. Webster's says that it can also be a period of time when the light occurs. I've edited the article to show this.

Twilight is not always a period between sunrise/set and dawn/dusk, even in temperate latitudes. I have seen twilight in the northern sky at midnight near the summer solstice in England, just north of London, at about 52 degrees North latitude. Using the "astronomical" definition of twilight (Sun within 18 degrees of the horizon), night-long twilight occurs anywhere with latitude more than 48.5 degrees, North or South.

DOwenWilliams (talk) 16:08, 29 January 2013 (UTC)

I think the article is nicely balanced now, doing justice to both usages of the word. I can see that the editing is in good hands. I believe that any overreaction that may have occurred was intended in a constructive spirit. Hertz1888 (talk) 03:39, 30 January 2013 (UTC)

I think we can give ourselves a communal pat on the back for the above discussion. Everyone contributed constructively, without arrogant dogmatism or disrespect for others' views. As a result, I think we all learned something. I know I did. I wish discussions on Wikipedia were always as constructive as this. Thanks. DOwenWilliams (talk) 16:23, 30 January 2013 (UTC)

• I wish every science editor on Wikipedia had the same communication skills, along with the strong desire to get things "right" as you do.

You have always had my respect. That's why going to bat for you was a "no brainer". I knew you would rush to the article, and do all the needed research to satisfy all involved, and the end result would be an improved and informing article. Thanks David. Pocketthis (talk) 17:13, 30 January 2013 (UTC)

I have worked as a high-school science and math teacher, and also as a computer magazine editor. Old habits die hard!

Getting back to the meanings of "twilight": I wonder if there are differences between English dialects. I was raised in Britain, and clearly came to understand that "twilight" is a type of natural light. When I saw the intro to the WP article saying it was a period of time, my immmediate reaction was that that was wrong. I checked the Oxford (i.e. British) dictionary, which confirmed my ideas, so I edited the article accordingly. But then people (maybe Americans) disagreed with me, and I found that the Webster's (American) dictionary does include the "time" meaning, along with the "light" one. So both meanings are acceptable, but maybe not universally understood.

English is a wonderful language...!

DOwenWilliams (talk) 19:55, 30 January 2013 (UTC)

New moon?

How does a full moon affect the various twilight definitions? I would think that a night sky illuminated by a full moon with the sun 25° below the horizon is brighter than a a night sky with no moon and the sun 15° below the horizon? Derekt75 (talk) 23:18, 30 January 2013 (UTC)

• Good question, and I shouldn't be answering it...lol. However, as a photographer, my opinion would be that the brightest moon light is still darker than the darkest time of twilight....even dusk. I have seen full moons here in the High Desert that seem extremely bright, and even cast a crisp shadow of myself, should I venture out to enjoy the evening; so I can see your question making sense, but I'll stick to my theory until D0wen or another science editor tells me I'm playing scientist again, and should keep to my areas of expertise. Great question, thanks for asking it. It gives me something new to ponder today. Pocketthis (talk) 23:39, 30 January 2013 (UTC)

It's easy to walk around without bumping into things by the light of the full moon. This is not possible if the only light around is "astronomical" twilight. So moonlight must be brighter.

Long ago, I took some photographs by full moonlight. I think I calculated that it was a million time dimmer than full sunlight, and set the camera accordingly. The photographs came out just fine. They looked exactly like pictures taken in sunlight. I decided that if I wanted pictures that looked like that, I might as well use the Sun.

Twilight is defined as sunlight that has been scattered in the atmosphere. Moonlight doesn't fit the definition. Certainly, it contributes to the ambient light, but it isn't twilight. The same is true, of course, of the aurorae, light produced by meteor swarms, etc.

DOwenWilliams (talk) 03:37, 31 January 2013 (UTC)

The advantage of being able to set your camera up for moonlight as opposed to Twilight, is moonlight is constant. Evening twilight gets dimmer by the second, and it's almost impossible to meter. Of course there is a point at the very end of twilight just before nightfall, when it is too dark to negotiate a proper path to walk, however, I believe most of twilight time is brighter than moonlight, so the moon being out wouldn't change the lighting environment. I'll be looking for the next full moon on a clear evening to test my opinion. I have taken photos of the full moon during morning twilight, and the moonlight was not a factor at all. http://commons.wikimedia.org/wiki/File:Mountain_Moonset.jpg ......Interesting subject. Pocketthis (talk) 15:39, 2 February 2013 (UTC)

Moonlight is fairly constant over a period of a few minutes or hours, but it varies greatly according to the phase of the moon. This is partly because the moon appears larger when it is full than at other phases, and partly because of the properties of its surface, which make it somewhat "reflective", like the reflective material used on road signs. Sunlight that falls on it is reflected more strongly back toward the sun than in other directions. At full moon, the earth, as seen from the moon, is close to the sun in the sky, so much of the sunlight that falls on the moon is reflected toward the earth, making the moon look very bright to us. At other phases of the moon, only small fractions of the sunlight that strikes the moon are reflected in our direction. DOwenWilliams (talk) 16:27, 2 February 2013 (UTC)

My reply was addressed to "Full Moon" conditions; however, I always appreciate your detailed educational posts. Thanks Pocketthis (talk) 16:57, 2 February 2013 (UTC)

Thanks. I've been remembering those photos I took by moonlight long (about 50 years!) ago. I had a light meter, but it wasn't sensitive enough to respond to moonlight, so I had to do some calculations. The angular radius of the moon, seen from the earth, is about 1⁄4 degree. Call it 1⁄250 radian. So the earth-moon distance is about 250 moon radii. Suppose that the surface of the moon uniformly diffuses 100% of the sunlight that falls on it. By the time it gets to the earth, the light will have diminished in intensity by a factor of 250², or about 60,000. So moonlight would be 60,000 times dimmer than sunlight if the moon diffused all the light it receives from the sun. But it actually diffuses only a few percent of the light, so I threw in another factor of 20 or so, leading to the conclusion that full moonlight is about a million times dimmer than sunlight.

I knew that in full sunlight I would set the camera to about f/16 and 1⁄100 second exposure. (Slow film!) The widest aperture it had was f/2.8. At that aperture, I figured that in full moonlight I should use an exposure time of about five minutes. So that's what I used, and got pictures that looked like they'd been taken in full sunlight.

I imagine it would be a lot easier now!

DOwenWilliams (talk) 20:57, 2 February 2013 (UTC)

Your guesstimate factor of 20 was close.

Apparent_magnitude#Example_-_Sun_and_Moon

Sun appears about 400,000 times brighter than the full moon.

- Ac44ck (talk) 04:40, 3 February 2013 (UTC)

Interesting. My "guesstimate" took no account of the retro-reflective property of the lunar surface, which I don't think was understood back then. The Apollo astronauts found large numbers of little glassy spheres on the surface, which are thought to be solidified droplets of molten rock that were produced during impacts. The droplets refract and reflect light preferentially back in the direction from which it came. Little glass spheres are also used in reflective paint. I took those pictures about ten years before Apollo. If I had known about the spheres, I might have put in another factor of two or so, making my estimate of moonlight 500,000 times dimmer than sunlight. That would have been even better! Fortunately, a factor of two doesn't make much difference in photography. DOwenWilliams (talk) 16:17, 3 February 2013 (UTC)

I guess this is my question: if the main contribution to ambient light is not from sunlight scattered in earth's atmosphere, is it considered "twilight" when the sun is the specified number of degrees from the horizon, or does it simply become "night"? If astronomical twilight is never visible in Manhattan, does it happen? 68.189.124.139 (talk) 23:48, 1 February 2013 (UTC)

Yes. I think so. If some of the ambient lighting is from scattered sunlight, then that is twilight. If there's other light too, it makes no difference to the fact that twilight is occurring. DOwenWilliams (talk) 01:32, 2 February 2013 (UTC)

Proposed merger (dawn and dusk)

Since almost all the info required to understand dawn and dusk is intrinsically part of the twilight article, I suggest merging these articles into twilight.(66.146.195.82 (talk) 12:57, 21 July 2013 (UTC))

• Absolutely not......a big NO. Dusk is only after "evening" Twiight, and has nothing to do with morning Twilight. Bad Idea! Pocketthis (talk) 15:02, 21 July 2013 (UTC)
• Strongly oppose merger, as per Pocketthis. Separate handling, per the current set of articles, is essential for a clear presentation of the respective principles involved. Hertz1888 (talk) 16:00, 21 July 2013 (UTC)

Beginning and end of the various twilights

Quote: "Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon." It seems like it begins in "the morning", lasts the whole day, and ends in the evening at a specified sun angle. Same with astronomical twilight.

It is not explicitly stated in the main sections for nautical twilight and astronomical twilight when they end in the morning and when the begin in the evening. It is possible for a reader like me to wonder whether nautical twilight in the evening starts at sunset or at civil dusk, etc.

I suggest that these problems can be solved by writing out fully in the sections for nautical and astronomical twilights when they begin and when they end.

Gbwi (talk) 12:22, 5 October 2013 (UTC)

Possible photo

Possible photo for twilight.

For possible inclusion in article.--Tomwsulcer (talk) 13:01, 22 December 2013 (UTC)

• Hello Tom. I also am a boomer (read your user page), and made a living as a photographer for the past 35 years. You have some nice photos on your user page, I enjoyed looking at them. In the case of this photo, in my opinion you have a "Dusk" photo. Hard to get because of the lighting. Also, the composition of the photo needs some editing. If you still have the original, balance the photo by coming lower, and showing the base of the garage. There will still be plenty of sky left to see. Also, spin the photo so the objexts sit at 90 degrees to better balance it, by simply making sure the base of the garage is perfectly horizontal. Then, put it in the Dusk discussion page, and we'll have another peek at it. Thanks-Pocketthis (talk) 17:53, 22 December 2013 (UTC)

• • Hey thanks Pocketthis. I deleted the original (actually in my computer trash bin but it would take me an hour to search through). Or maybe I'll try to redo the shot when lighting is similar outside. Cool you're a photographer; I'm only an amateur photographer but I carry my camera wherever I go and sometimes get some good shots. Good idea about Dusk page; I'll post on its talk page.--Tomwsulcer (talk) 18:50, 22 December 2013 (UTC) Btw just saw your user page. WOW -- great photos. You're a pro!--Tomwsulcer (talk) 18:52, 22 December 2013 (UTC) Also I'd like to post one of your photos on my user page here that is assuming you have no objections.--Tomwsulcer (talk) 19:07, 22 December 2013 (UTC)

• • • Post away my friend, I'm flattered you'd like to display a photo of mine. You're not so bad yourself. My photos these days are far from technologically accurate. I sold all my pro equipment when I retired and just take 'point and shoots' with my digital Leica. I still strive for composition, which is just as important as the technical aspects of a photo. In fact, I think composition may be more important. Happy Holidays.-Pocketthis (talk) 23:54, 22 December 2013 (UTC)

Civil, Nautical, & Astronomical - What are the Reasons?

What is the purpose for the three different "versions" to twilight? The article mentions that nautical twilight is used in military planning; but is that it's origin? If so, what about the other two? Any addition of this info would greatly help both me, and the article in general. — Preceding unsigned comment added by 97.124.76.228 (talk) 19:23, 4 May 2013 (UTC)

Civil twilight is bright enough that normal daytime activities can be performed without additional lighting. For example, a car can be driven safely without using its headlights. Many jurisdictions have laws that compel drivers to turn their headlights on at civil dusk. Defining the time when this occurs is therefore necessary for the implementation of these laws.

Nautical twilight is essentially bright enough to allow a ship to be navigated safely without relying on navigational aids such as lighthouses. In warfare, lighthouses cannot be relied on, so planners must ensure that activities will occur when nautical twilight is present.

Viewing very faint astronomical objects such as distant galaxies can be done only if the sky is almost perfectly dark. Knowing the times of astronomical dusk and dawn allows astronomers to plan their activities appropriately.

DOwenWilliams (talk) 20:00, 4 May 2013 (UTC)

The reference given is a US one. Are these 3definitions US only, or are they recognised internationally? I'vebeen a seaman for 40 years and never heard of them. In the UK lighting-up time is legally defined in terms of minutes after sunset, not solar elevation. The two methods will diverge widely during the year, especially in Shetland -User:Brunnian — Preceding unsigned comment added by 195.137.63.170 (talk) 08:01, 18 February 2014 (UTC)

Interesting question. As far as I can see from http://astro.ukho.gov.uk/nao/miscellanea/birs2.html, the British government uses the same definitions of the three kinds of twilight as we do here. There is also a period called "Hours of Darkness" which extends from 30 minutes after sunset to 30 minutes before sunrise. Laws concerning use of car headlights, etc., are written in terms of hours of darkness. DOwenWilliams (talk) 21:53, 18 February 2014 (UTC)

Shrunk the Gallery

I thought the gallery was getting out of control, so I deleted and replaced, and moved around some shots, and cut the Gallery to one line. Any of you whose photos didn't make the cut, I promise you that I removed at least as many of my own shots as others when editing. If you want to scream at me, this is the place...:-)→Pocketthis (talk) 17:52, 30 May 2015 (UTC)

Article contradicts itself on twilight definitions

Diagram shows twilight as...

Civil Twilight: Sun 0-6º below the horizon

Nautical Twilight: Sun 6-12º below the horizon

Astronomical Twilight: Sun 12-18º below the horizon

The text in the article shows twilight as...

Civil Twilight: Sun 0-6º below the horizon

Nautical Twilight: Sun 0-12º below the horizon

Astronomical Twilight: Sun 0-18º below the horizon

The diagram is right, not the text. See this article by Cambridge University.

http://www.ast.cam.ac.uk/public/ask/2445 Appple (talk) 01:30, 20 August 2015 (UTC)

See Bowditch's The American Practical Navigator, pp. 227-228 & table 1516, which agrees with the definition as given in the text. AstroLynx (talk) 14:16, 11 August 2015 (UTC)

Further note that the Cambridge web article cited above does not cite a reliable source but only WP (circular reasoning). AstroLynx (talk) 14:20, 11 August 2015 (UTC)

The American Practical Navigator is a good source you have there. It's possible that it's right. It shows twilight as...

Civil Twilight: Center of sun 0º50' - 6º below the horizon

Nautical Twilight: Center of sun 0º50' - 12º below the horizon

Astronomical Twilight: Center of sun 0º50' - 18º below the horizon

However, the Glossary of Marine Navigation which is put out by the same agency says differently.

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#22

“ civil twilight. . 1. The period of incomplete darkness when the upper limb of the sun is below the visible horizon, and the center of the sun is not more than 6° below the celestial horizon.”

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#82

“ nautical twilight. . 1. The time of incomplete darkness which begins (morning) or ends (evening) when the center of the sun is 12° below the celestial horizon. The times of nautical twilight are tabulated in the nautical twilight are tabulated in the Nautical Almanac; at the times given the horizon is generally not visible and it is too dark for marine sextant observations. See also FIRST LIGHT.”

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#9

“ astronomical twilight. . 1. The period of incomplete darkness when the center of the sun is more than 12° but not more than 18° below the celestial horizon. SEE ALSO CIVIL TWILIGHT, NAUTICAL TWILIGHT.”Appple (talk) 01:30, 20 August 2015 (UTC)

Evidently, there is much confusion about the definitions (even among those who should know). Perhaps it is best to mention this in the article, the reader can then decide which definition s/he finds most practical. AstroLynx (talk) 11:21, 16 August 2015 (UTC)

That sounds like a good plan.Appple (talk) 01:30, 20 August 2015 (UTC)

@Appple please sign your postings by typing four tildes at the end so that I know to whom I am responding. AstroLynx (talk) 08:06, 17 August 2015 (UTC)

Article updated with both definitions. Appple (talk) 03:09, 20 August 2015 (UTC)

As I mentioned earlier, the Cambridge reference ("Ask an Astronomer") is not reliable as it refers to WP as source - so what you have here is circular reasoning. Either substitute it with an another verifiable source or leave it out all together. AstroLynx (talk) 07:48, 20 August 2015 (UTC)

In addition, the "Glossary of Marine Navigation" which you cite for the other definition of twilight is the appendix printed at the end of the "American Practical Navigator". Apparently, the authors of the glossary did not check their facts with the information in the earlier chapters of the manual. AstroLynx (talk) 07:59, 20 August 2015 (UTC)

@AstroLynx: Why haven't you removed or at least tagged the unreliable source(s)? Readers and editors should be made aware of the sourcing problems. --Florian Blaschke (talk) 15:25, 22 September 2015 (UTC)

I already mentioned in this section that some of the sources used by Appple were unreliable. In the mean time I have been looking at some other twilight-related sources but have not yet decided how best to correct the current text. Please feel free to do so yourself if you cannot wait. AstroLynx (talk) 15:35, 22 September 2015 (UTC)