Wikipedia:Reference desk/Archives/Science/2017 May 5

= May 5 =

Earth tidal heating
If Earth is in interstellar space, and the Moon is orbiting it in a circular distance d, then what d will allow tidal heating to maintain a temperature of 15 degrees Celsius or 59 degrees Fahrenheit?

What if Earth is orbiting Jupiter in a circular orbit?

Are seasons possible?32ieww (talk) 02:24, 5 May 2017 (UTC)


 * There is a way of working out the answer, but I'm not going to do the heavy lifting for you. Work out the radiated heat energy from the earth using the Boltzman equation, at 288 K, then you'll know how much power we need to suck out of the moon to get that, then do the orbital mechanics on the moon. At the moment we get about 3.7TW from the moon. I'd guess the power extracted is proportional to the tidal velocities cubed. The rest is easy. No, no seasons Greglocock (talk) 05:25, 5 May 2017 (UTC)
 * Well, seasons are possible if you allow the moon to have an ellipsoidal orbit, with the tide level hence tidal heating being higher when the moon is closer. I probably need to add that "the Sun is closer to the Earth in summer" is an incorrect explanation for seasons, which can easily be proven false if you remember that "summer" and "winter" are not simultaneous in the northern and southern hemispheres. Tigraan Click here to contact me 08:37, 5 May 2017 (UTC)
 * Hmmm, actually it is not obvious how tidal heating depends with tidal height. Surely it is nondecreasing, but maybe it increases sufficiently slowly that the main effect is the period of the tides. The formula for the satellite tidal heating found at tidal heating does not involve height... Tigraan Click here to contact me 08:50, 5 May 2017 (UTC)
 * As in surface temperature.32ieww (talk) 02:52, 9 May 2017 (UTC)


 * You might want to ask at the Math Desk, if you need help with the heavy lifting. μηδείς (talk) 00:20, 6 May 2017 (UTC)
 * What weights are involved?32ieww (talk) 02:52, 9 May 2017 (UTC)
 * Comments:


 * 1) Maintain that temp where ? In the atmosphere ?  That's a tall order considering the atmosphere is almost entirely heated by the Sun, at present.  Tidal forces sufficient to warm the atmosphere to that degree might well tear the Earth and/or Moon apart.


 * 2) Radioactive decay also is a major source of heat in the interior of the planet.


 * 3) As for seasons, you could have a highly eccentric orbit for the Moon, which would cause higher tidal heating when it was near. Of course, at the small distances needed to warm the Earth that much, the Moon's orbit wouldn't last a year, or even a month.  StuRat (talk) 01:10, 6 May 2017 (UTC)

Why we cannot see crescent of Venus
for more explanation, we know that nearest object after moon to earth , is Venus and as it is bigger than mars and near to sun , so we might be able to see its crescent. --Akbarmohammadzade (talk) 15:39, 5 May 2017 (UTC)
 * We can see the phases of Venus, though it's usually too small to see with the naked eye, unlike the moon which is much larger. Wikipedia has an article titled phases of Venus, which describes it more fully.  -- Jayron 32 15:41, 5 May 2017 (UTC)


 * Back when I had a telescope, I saw the crescent of Venus many times. It occurs when Venus is closer to us. Although a crescent, its highly reflective atmosphere makes it very bright. As Venus approaches the far side of the sun it becomes more "full" but also appears smaller, of course. ←Baseball Bugs What's up, Doc? carrots→ 16:25, 5 May 2017 (UTC)
 * You should see Venus from Mercury. There it can be full without being far away and so reaches magnitude -7.7 which is as bright as a crescent Moon on Earth. Venus gets to a see a Full Earth, which is bigger and more impressive. Sagittarian Milky Way (talk) 07:36, 6 May 2017 (UTC)


 * That's OR, and you know it! μηδείς (talk) 00:11, 6 May 2017 (UTC)
 * So is the OP's claim that crescent Venus can't be seen. And by an amazing coincidence, my recollection of seeing the crescent through a telescope matches the photo you posted! ←Baseball Bugs What's up, Doc? carrots→ 01:01, 6 May 2017 (UTC)


 * My interpretation of the OP's question is not that they think Venus doesn't exhibit a crescent telescopically, but that we ought to be able to see it with the naked eye, but seemingly can't.
 * In fact, it is just possible under some circumstances to see the crescent with the naked eye, as hinted at by Jayron above. From the Phases of Venus article Jayron linked, in the Naked eye observations section:
 * "The extreme crescent phase of Venus can be seen without a telescope by those with exceptionally acute eyesight, at the limit of human perception. The angular resolution of the naked eye is about 1 minute of arc. The apparent disk of Venus' extreme crescent measures between 60.2 and 66 seconds of arc,[4] depending on the distance from Earth. Nevertheless it is possible for observers with extremely acute eyesight to see a crescent Venus under ideal atmospheric circumstances."
 * {The poster formerly known as 87.81.230.195} 2.122.60.183 (talk) 09:44, 6 May 2017 (UTC)
 * Could be. Maybe the OP will come back here and clarify. ←Baseball Bugs What's up, Doc? carrots→ 11:10, 6 May 2017 (UTC)