Wikipedia:Reference desk/Archives/Miscellaneous/2017 August 24

Welcome to the Wikipedia Miscellaneous Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 24

Getting dark before sunset

I can tell that it gets darker even before sunset. As the sun gets low in the sky, the sky is not fully lit like in the middle of the day. After the sun sets, the sky gets darker more rapidly, obviously. After sunrise, the sky doesn't get fully lit until few hours later. The window in my bedroom where I sleep faces east. I can tell that right before the sun rises over the neighbor's roof over half hour after sunrise that begins shining in my room, my room was kind of dark. But after the sun stops shining in my room later in the morning when the sun gets too high to continue shining through the window, my room is brighter than what it was before the sun started shining. During the partial eclipse seen from near Chicago in early afternoon, the sky grew little darker, down to about the brightness of the sky as less than 30 minutes before sunset when the moon covered ⅞ of the sun, and it was cloudy from where I viewed from. PlanetStar 06:41, 24 August 2017 (UTC)

Yes, I cannot disagree with a single thing you say. I find myself in total accord with your point of view. Now, I wonder if I may prevail upon you very slightly, do you have a question? Richard Avery (talk) 07:17, 24 August 2017 (UTC)

Presumably the Q is why it isn't full brightness as soon as the Sun is fully visible. Three reasons:

1) Insolation angle. That means sunlight is hitting at a shallow angle, spread out over a wider area, so there is less per given area of the Earth's surface.

2) Twilight travels through more atmosphere, which absorbs, defracts, and reflects more of it. This is especially true if it passes through clouds. Since reds are affected more than blues, this accounts for the bluish color at twilight.

3) Ground clutter, like trees and buildings, block more of it then. Even if the Sun itself is not blocked, part of the brightly lit area of the sky around it may be. StuRat (talk) 14:00, 24 August 2017 (UTC)

Your 2) has the physics exactly backwards. Shorter (bluer) wavelengths are scattered more than longer (redder) ones – see Rayleigh scattering. {The poster formerly known as 87.81.230.195} 90.202.208.101 (talk) 21:17, 24 August 2017 (UTC)

Thanks for the correction. StuRat (talk) 04:31, 25 August 2017 (UTC)

Wikipedia has a rather extensive article called Twilight that should answer most of the OPs questions (if they have any). --Jayron32 11:13, 24 August 2017 (UTC)

My experience is the reverse. My kitchen faces east and is extremely gloomy. When I get up I need the electric light. Then shortly after dawn the sun shines through the windows and I can switch it off. Later in the day, when the sun is high but not directly visible I have to switch it on again. 81.151.100.122 (talk) 17:18, 24 August 2017 (UTC)

I agree with your reasons. When the sun is low in the sky, it isn't just trees and buildings blocking the sun as said in 3), but because of the low insolation angle as said in 1). When the sun is low in the sky but not low enough to appear yellow or orange, the light has to pass through more of the atmosphere in order to reach us, therefore slightly dimming the light. To test out my belief about the solar eclipse, in my area the moon covered 87% of the sun, then I calculate the angle of the sun away from the horizon using sine. 87% covered meant only 13% of the sun's disk is visible, so I find the sine that would equal 0.13, and I find it at 7. Therefore, when the moon covered 87% of the sun's disk during the eclipse from where I live, the sun would appear just as dim when it is near the highest point in the sky as if the sun is just 7 degrees above the horizon and totally unblocked by the moon. During the winter time, the sun would appear dimmer than during the summer due to the fact that sun is lower in the sky. But during the northern hemisphere winter, the Earth is located closer to the sun, compensating this. Do you guys agree or criticize my thoughts? PlanetStar 03:10, 25 August 2017 (UTC)

The distance of the Earth from the Sun doesn't vary by much (less than 3.5%), so that effect on light levels and temps is minimal. StuRat (talk) 04:31, 25 August 2017 (UTC)

That's still over 7% or 1/14, a significant amount. μηδείς (talk) 01:25, 26 August 2017 (UTC)

(She squared that amount to get the difference in light levels.) Significant yes, but compared to seasonal variations, such as the record temps in New York City, from 253K to 312K, that's a 23% difference. So, the distance is the lesser component. StuRat (talk) 12:17, 26 August 2017 (UTC)

StuRat, the record temperatures are 247K and 314K (241K if you count the pre-official recordkeeping observation of -26°F during the Little Ice Age). Sagittarian Milky Way (talk) 21:06, 28 August 2017 (UTC)

If we use your official numbers, that's about a 27% difference, making 7% an even lower portion. StuRat (talk) 21:18, 28 August 2017 (UTC)

Well, if the question is specific to the OP's location, and only on extreme days, that's one thing, and season will matter. But if total global insolation is what matters, then the 3.5% distance variation from the sun will be quite significant on average over the year. μηδείς (talk) 17:57, 26 August 2017 (UTC)

At solar noon (assuming for simplicity your sun is in the zenith) your hemisphere gets full sunlight, in other positions of the sun only part of it (and the missing part shines elsewhere). Hence the brightness of the day varies with the geometrical position of the sun (see grey area). --Pp.paul.4 (talk) 17:33, 26 August 2017 (UTC)