Wikipedia:Reference desk/Archives/Science/2018 June 15

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June 15

Phobos orbital height - I must be missing something

Phobos orbits at a height above the Martian datum of 6km. Olympus Mons is 26km in height. Maybe Phobos has a stable orbit and won't collide with it, but I feel like I'm missing something or misinterpreting the data? Surely Phobos can't really be that low? Dr-ziego (talk) 12:10, 15 June 2018 (UTC)

Where did you get the idea that Phobos orbits 6 km above Mars? It's about 6,000 km, per our article. Matt Deres (talk) 12:19, 15 June 2018 (UTC)

It'll collide eventually causing a titanic explosion. Sagittarian Milky Way (talk) 13:48, 15 June 2018 (UTC)

Phobos has an orbital inclination of 1.093° degrees, relative to the Martian equator. Olympus Mons is located 18.65° North of the Martian equator. Therefore, Phobos never overflies Olympus Mons. LongHairedFop (talk) 13:39, 16 June 2018 (UTC)

Oh right, that too. Sagittarian Milky Way (talk) 13:54, 16 June 2018 (UTC)

On another hand, Pavonis Mons is on the equator. —Tamfang (talk) 07:29, 17 June 2018 (UTC)

Pavonis is 1.48°North, so Phobos doesn't overfly it. However, IIRC, the inclination of moon's orbits can vary over times, so eventually Phobos will overfly it. LongHairedFop (talk) 22:05, 17 June 2018 (UTC)

I don't know what's at 1.48°; the highest point? the middle of the caldera? The south rim crosses the equator. —Tamfang (talk) 03:08, 22 June 2018 (UTC)

It could make a Hole around Mars as postulated by Jerome Bixby. Sjö (talk) 07:14, 23 June 2018 (UTC)

What airliner requires the most runway at over 40 Celsius and 0 to 1000 feet elevation?

I've heard it might not actually be the Airbus A380. Sagittarian Milky Way (talk) 13:44, 15 June 2018 (UTC)

For passenger aircraft, it is the Airbus A380. If you look into cargo planes, the AN-225 Mriya requires more (about 9,000 ft for the A300 and 10,000 for the Mriya). 209.149.113.5 (talk) 14:00, 15 June 2018 (UTC)

Isn't there a big difference in minimum recommended runway between frigid, low ladenness and good weather and hot, highly laden and unfavorable wind? Or landing in tropical rain while highly laden for landing lengths? Sagittarian Milky Way (talk) 14:22, 15 June 2018 (UTC)

Yes. A pilot should use the temperature, humidity, wind speed/direction, and air pressure to calculate the "density altitude", which is the altitude the aircraft appears to be at if the conditions were optimal. Then, with the density altitude, the minimum required runway length is calculated. There are cases where normal airports shut down runways because they are too short for the calculated density altitude. Minimum length can get very long. In Tibet, there is a runway that is at least 3 miles long - and sometimes it can get too short. 209.149.113.5 (talk) 14:49, 15 June 2018 (UTC)

This isn't under the above conditions and also doesn't seem to clearly specify it's referring to the rare 747-8I (passenger) variant but for "minimum requirements that apply to an aircraft at Maximum Certified Takeoff Weight (MTOW), taking off at sea level under ISA condition" gives Airbus A380-800 2900 metres and Boeing 747-8 3050 metres [1] Nil Einne (talk) 16:44, 16 June 2018 (UTC)

This quick guide also gives similar figures for the MLW [2]. Actually the 747-400 is the also higher. Nil Einne (talk) 16:59, 16 June 2018 (UTC)

I think you should be able to work out from these documents if the above is correct for the specified conditions [3] [4] Nil Einne (talk) 17:11, 16 June 2018 (UTC)

So it appears that at maximum takeoff weight, 0 pressure altitude and 30C sea level equivalent (it doesn't give more) the 747-8I needs a longer takeoff than the A-380-800 but it's roughly the same at 1,000 feet and the other way around above that. Sagittarian Milky Way (talk) 22:30, 16 June 2018 (UTC)

Solar noons and midnights

Earth in its Orbit

The occurrence of the "Number of solar noons (middays)" is always equal to "Number of solar midnights"

The diagram (not to the scale) on the right-hand side depicts the path traced by the earth for day and night in its orbit (either circular or elliptical) around the sun. Any point on the outer circle represents solar midnight while on inner circle solar noon. The length of an outer circle is greater than the length of the inner circle and hence Arc I > Arc II. This means the appearance of midnight points (anti noon) are more than middays points (solar noon) when the earth revolves around the sun in its orbit – Any special reasons

As # of midnights = # of solar noons when the length of arc I = length of arc II but since arc I > arc II, therefore

Are solar noons and solar midnights equal in numbers in the arc I and arc II or after the completion of 4 years?--Eclectic Eccentric Kamikaze (talk) 13:57, 15 June 2018 (UTC)

... or another way of looking at the situation is that the midnight point is always moving faster than the noon point. Dbfirs 14:46, 15 June 2018 (UTC)

The first thing to bear in mind is tidal force. If the Earth did not rotate, or more precisely, were tidally locked, its outer edge would have to revolve faster than the inner edge, like a phonograph record. This doesn't precisely match the expected orbital period, so there are tides raised by the Sun - the other part of the planet gets a gentle nudge like it should fly off to space, while the inner has a gentle nudge to fall toward the Sun, though of course if separated somehow neither would get far before returning where they were in some sort of elliptical orbit, sans planetary integrity. So the point is, the "midnight" part of Earth is always going a bit faster to make up the distance. Wnt (talk) 18:48, 15 June 2018 (UTC)

In other words: there are just as many middays on the inner circle as midnights on the outer one, but space (not time!) between midnights is a bit larger. Right? 194.174.76.21 (talk) 10:15, 18 June 2018 (UTC) Marco Pagliero Berlin

With rotating systems, it is often more useful to consider things in terms of angles, rather than distances. The earth is moving 2pi radians per year. There are 365.25(ish) middays and midnights for a point on the surface in each year. Therefore, each successive midday is separated by 2pi/365.25, and the same for each successive midnight. The midnights are further out, so the linear distance between them is greater, but that's not relevant here. MChesterMC (talk) 10:31, 18 June 2018 (UTC)

Does it mean time dilate during nighttime when the earth orbits the sun? - The Earth spins at the same speed no matter if it's day or night.

Any point on the outer circle, which represents midnight if connected to the center of the sun via a straight line passes through the noon (inner circle). When there is midnight, there is a noon, therefore, duration/occurrence of midnights must be equivalent to the duration/occurrence of noons and hence their lengths.50.66.1.32 (talk) 18:20, 18 June 2018 (UTC)eek

Attribution note - the diagrams at Talk:Season#Earth's Rotation and its Orbital Motion were uploaded by Eclectic Eccentric Kamikaze, and not as stated by Dbfirs. Just to clear up any possible misconceptions from the present discussion, clocks set to mean solar time and sundials when adjusted to mean solar time measure time at the same rate whatever the time of day and wherever on the surface of the Earth they may be located. 86.132.186.246 (talk) 13:26, 19 June 2018 (UTC)

The statement I made was about the text, not the diagrams, but I agree with your comment about normal clocks. A sundial could be adjusted to mean solar time by using a clock. Dbfirs 19:19, 19 June 2018 (UTC)

I believe most people who are not dab-hand in astronomy are messed up with sidereal and solar times. Astronomy isn’t my field - My biggest apology for the diagram in Talk:Season#Earth's Rotation and its Orbital Motion.

Please read EEK 139 if it doesn’t bother you. Scroll up couple of posts to "Reply #23 on: June 20, 2018, 02:38:00 PM". I believe it may help. — Preceding unsigned comment added by 50.66.1.32 (talk) 19:34, 21 June 2018 (UTC)

Bees and butterflies question

There has been an abortion debate in Argentina in those days, as the Congress is discussing a bill to legalize it. But there is a part of the text of the bill that I did not understand: translated to English, instead of talking about women, it talks about "women and people that may become pregnant". It feels weird having to ask this, but does such wording make sense, can someone other than a woman become pregnant? I know that there are transsexual people and sex reassignment surgery, but is that enough to make a person that was once a man to become pregnant and deliver a baby? Did that ever happened? Or is it just political correctness gone mad? Cambalachero (talk) 19:17, 15 June 2018 (UTC)

"No results found for 'women and people that may become pregnant'".[5] Maybe it is not available in English. Can you post the text in the appropriate language? Bus stop (talk) 19:25, 15 June 2018 (UTC)

If "women" is taken to mean "adult females" then of course other people who may become pregnant are females below the age of adulthood. DuncanHill (talk) 19:28, 15 June 2018 (UTC)

And see also male pregnancy. DuncanHill (talk) 19:29, 15 June 2018 (UTC)

Also, women can consider themselves men and still become pregnant. Bus stop (talk) 19:40, 15 June 2018 (UTC)

That's under transgender pregnancy. Also, some people regard themselves as a third sex, etc. My feeling is that "women and people who may become pregnant" is inelegant; one (presumably the latter) should suffice for most intentions. True male pregnancy in humans is probably not far off anyway, so "people" would seem like a reasonable replacement in a forward-looking statement. Wnt (talk) 21:45, 15 June 2018 (UTC)

It's downright criminal that every news outlet to discuss this bill doesn't tell us what it's frickin' name is!. Makes it very hard to search for. Still looking - I was hoping to just read the Spanish version directly. Someguy1221 (talk) 22:16, 15 June 2018 (UTC)

OK, I think this is the bill that was just voted on, but I don't see what the OP is referring to anywhere. Someguy1221 (talk) 22:54, 15 June 2018 (UTC)

The term in Spanish is "mujeres y personas gestantes". I have not found a single-word translation of "gestante" to English, so I made the translation myself. The word would be an adjetive for a living being with the hability of becoming pregnant. In any case, I was not asking about the bill (that's a local bill in a Spanish-speaking country, after all; I do not expect many people abroad to be following those news events) but about the pregnancy aspect. If I understood the article, barring discussions of what would be theoretically possible there are no actual cases of children born from male-to-female transgender people. There are some named cases of children born from female-to-male transgender people (that is, someone who was born as a woman, and retains all the required organs), but how often does that happen? Do they get pregnant as easily and frequently as straight women, or are those special fortunate cases? Cambalachero (talk) 00:36, 16 June 2018 (UTC)

[6]. It has apparently been the standard language used by the Ministry of Health since 2015 in cases regarding abortion, and explicitly with transmen and similar situations in mind. As for how often this happens, we have an article on Transgender pregnancy, but no statistics, and I also could not find any statistics on this in the scientific/medical literature. I would assume that it is quite rare. However, as to how easy it is, that depends. The majority of transmen do not undergo sex reassignment, leaving them physically as capable of conceiving and bearing a child as they would be otherwise. There is insufficient data to say what the long-term effects of hormone transition therapy are on the fertility of transmen. It's believed that such therapy impairs fertility, especially while it is being taken (and may also impact fetal development), but it's not clear to what extent. There is a review on the subject here, but mostly it just clarifies that we don't know a lot. Someguy1221 (talk) 00:53, 16 June 2018 (UTC)

"Pregnant people" is also used more and more in English these days, rather than the phrase "pregnant women" that was nearly exclusively used historically. Some examples: [7] [8] [9] [10]

Something of an aside, but "bees and butterflies"? Are they the Argentine equivalent of the birds and the bees? DuncanHill (talk) 00:59, 16 June 2018 (UTC)

The equivalent of Spanish gestante is English gestant from gestation, which is a rather unsual word compared to pregnant. --62.99.192.174 (talk) 23:16, 16 June 2018 (UTC)

I am reminded of the term Embarazada. Bus stop (talk) 23:25, 16 June 2018 (UTC)

Public Health England no longer offers cancer screening to "women". It offers it to "anyone with a cervix". 86.132.186.246 (talk) 10:47, 21 June 2018 (UTC)