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Luriko-Ysabeth
2008-Mar-31, 07:41 PM
Er... I spent some time poking around and didn't see any evidence that this had been asked before, so I hope it is all right to ask.

I was rewatching some episodes of a show famed for never letting science get in the way of a good story (or even a mediocre story, depressingly enough; the mediocre ones tend to let my attention wander enough to start pondering things) in which some people lived on a (presumably terraformed) moon.

This got me thinking about the mechanics of living on a moon. I'm pretty sure that it is possible for a moon with an axial tilt to have seasons, as I think I remember reading something talking about Triton having a long, hot summer at the moment. However, I'm not sure whether the moon's orbit of its planet would affect said moon's seasons more than, say, perihelion and aphelion affect the Earth's seasons, or not.

Also, if the moon's days were somewhere in the neighborhood of twenty-four hours, which would probably make it easier on any Earthly colonists (potentially not necessary for humans, given that plenty of people live in Antarctica and Inuvik and Nome and whatnot, but it might help with anything you brought along to raise and eat), it still probably wouldn't be galloping around the planet that quickly. That is, if it did, would that cause problems for anything on the surface of the moon? And if it didn't, would this be an unstable state, and if so, how unstable?

Lunar eclipses probably would go on for a long time, too. Would it be long enough for it to get noticeably colder out, or is it another sort-of-evens-out thing? There are plenty of areas where the temperature goes down fast after the sun "goes down" on Earth.

The planet would probably be a big shiny THING in the sky. I wonder whether it would reflect enough light to see color? (I'm pretty sure you could see any color it might have, as I don't think an atmosphere would make that much difference, but.)

Thank you for your time.

AndreasJ
2008-Mar-31, 10:18 PM
Indeed, a moon can have seasons. The orbit around its planet shouldn't have much effect, partly because the change in distance to the star wouldn't change much and partly because the changes are comparatively short-period.

A large moon would probably be tidally locked, so the only way a 24h day would be likely is having it whizzing around its planet with that period. Doing so wouldn't have any particular effects for people or objects on the surface in any case.

I assume you meant to ask about solar eclipses? I imagine they could easily be long enough for temperature to drop quite noticeably.

The planet's brightness would depend a lot on the distance to the system's star. Bright or not, the planet should be a stupendous sight. As seen from the innermost of its large moons, Io, Jupiter has an angular diameter of 20, and as seen from tiny Metis one of 68!

Luriko-Ysabeth
2008-Apr-03, 05:43 PM
Thank you for your quick answer!

//A large moon would probably be tidally locked//

So from a lunar observer's perspective, the planet would go from crescent to full planet and back to nothing in one day (however long that took)?

//so the only way a 24h day would be likely is having it whizzing around its planet with that period. Doing so wouldn't have any particular effects for people or objects on the surface in any case.//

I suppose what I meant to ask for the second part of my question was "supposing something set the moon making revolutions that swiftly, would it be likely to maintain that speed indefinitely or slow down noticeably over time, and if so, how many centuries/millennia might that take?"

//I assume you meant to ask about solar eclipses?//

...the terminology I know is all earth-observer-centric, so, not knowing whether "solar eclipse" in this context would bring to mind "planet blocking the sun" or "casting a big round shadow on the planet," I decided to go with something that if mistaken should at least be comprehensible. ^_^

So if the planetary system were close enough to the star that anyone living on the moon didn't have to heat the place, the planet in question ought to be very bright indeed?

And anyone living there would be very lucky to have such a wonderful view, even if it did mean that they'd have to turn around to see the stars at night. ^_^

AndreasJ
2008-Apr-05, 09:09 PM
Thank you for your quick answer!

//A large moon would probably be tidally locked//

So from a lunar observer's perspective, the planet would go from crescent to full planet and back to nothing in one day (however long that took)?
Correct.

//so the only way a 24h day would be likely is having it whizzing around its planet with that period. Doing so wouldn't have any particular effects for people or objects on the surface in any case.//

I suppose what I meant to ask for the second part of my question was "supposing something set the moon making revolutions that swiftly, would it be likely to maintain that speed indefinitely or slow down noticeably over time, and if so, how many centuries/millennia might that take?"
Assuming the primary's rotational period to be less than the moon's orbital period, the moon would very slowly spiral outward while slowing down in proportion to keep its day equal to its orbital period. But on a time scale of millennia the change would only be detectable to precision instruments.

//I assume you meant to ask about solar eclipses?//

...the terminology I know is all earth-observer-centric, so, not knowing whether "solar eclipse" in this context would bring to mind "planet blocking the sun" or "casting a big round shadow on the planet," I decided to go with something that if mistaken should at least be comprehensible. ^_^
For a moon-bound observer, a solar eclipse would be the planet blocking the Sun. The moon casting a shadow of the planet might be a neat sight, but I don't see how it could be called an "eclipse".

So if the planetary system were close enough to the star that anyone living on the moon didn't have to heat the place, the planet in question ought to be very bright indeed?
Indeed.

And anyone living there would be very lucky to have such a wonderful view, even if it did mean that they'd have to turn around to see the stars at night. ^_^
If you lived at the subprimary point, where the primary hangs at the zenith, seeing the stars at all might be hard - when full it would shine thousands of times brighter than the full moon as seen from Earth. Too get a good view of the stars, you might do well to take a trip to the other side of the world where the primary is eternally hidden by the moon's own bulk!

Drunk Vegan
2008-Apr-05, 09:37 PM
Too get a good view of the stars, you might do well to take a trip to the other side of the world where the primary is eternally hidden by the moon's own bulk!

Actually there wouldn't be a perpetual dark on either side of the moon, because while it's tidally locked it is still rotating. The same is true of our moon, there's no "dark side" just a side that we can't see from Earth during the period when it is lit.

ctcoker
2008-Apr-06, 01:28 AM
Correction Drunk Vegan, we cannot see the far side of the Moon at all. You are right that there is no "dark side" of the Moon, but the Moon always shows the same face to us, whether it is full or new, because it is tidally locked.

Neverfly
2008-Apr-06, 07:50 AM
Correction Drunk Vegan, we cannot see the far side of the Moon at all. You are right that there is no "dark side" of the Moon, but the Moon always shows the same face to us, whether it is full or new, because it is tidally locked.

Clarification you mean.;)
I understood what Drunk Vegan said. But it could be mistakenly misinterpreted to say we could see the far side of the moon during the time it isn't lit.

What about axial tilt on moons?
The Earth's seasons are more a product of the tilt than of the sun aren't they?

It's winter season during the time we are closest to the sun (Northern Hemisphere.)

AndreasJ
2008-Apr-06, 06:28 PM
Actually there wouldn't be a perpetual dark on either side of the moon
Nobody said there would. I said you couldn't see the primary, that is the planet the moon orbits, from the anti-primary side, not that you couldn't ever see the sun. In terms of the Earth's Moon, I was talking about the Farside, not the mythical "Dark Side of the Moon".



What about axial tilt on moons?
The Earth's seasons are more a product of the tilt than of the sun aren't they?
The Earth's seasons are indeed overwhelmingly the effect of the axial tilt rather than the varying Sun-Earth distance over the year. The same would be true of any moon with a substantial tilt and whose primary had a near-circular orbit.

eburacum45
2008-Apr-06, 06:46 PM
The moons of a large planet tend to orbit more or less in the plane of that planet's equator. This means that the tilt of the moon's orbit will be largely dependent on the tilt of the planet itself. So a moon orbiting a planet with a large axial tilt would quite probably have a large tilt itself compared to the star, and so would experience more marked seasons than a moon orbiting a world with little or no tilt.

Most moons orbiting large planets have orbital periods lasting several days, so would have days lasting much longer than our own. This would also affect the weather on a moon with an atmosphere quite a lot; the nights might be cooler and the days hotter than our own, for instance, even if the planet received the same average warmth from the local star as we receive from the Sun.

3488
2008-Apr-06, 07:03 PM
You may be interested in the below?

Saturn's Titan: Saturn's own seasons are four months later.
Northern Sping / Southern Autumn Equinox: August 2009.
Northern Summer / Southern Winter Solstice: May 2017.
Northern Autumn / Southern Spring Equinox: February 2025.
Northern Winter / Southern Summer Solstice: December 2032.


How about the moons of Uranus?

2007 Northern Spring / Southern Autumn Equinox.
2028 Northern Summer / Southern Winter Solstice.
2049 Northern Autumn / Southern Spring Equinox.
2070 Northern Winter / Southern Summer Solstice.

You may also be interested in the below? Pole stars from Planets & their major moons.

Mercury: Omicron Draconis (north) Alpha Pictoris (south).

Venus: 42 Draconis (north) Eta 1 Doradus (south).

Earth : Polaris / Alpha Ursae Majoris (north) Sigma Octantis (south).
The Moon: 37 Draconis (north) Epsilon Doradus (south).

Mars: Deneb / Alpha Cygni & Alderamin / Alpha Cephei (north) Delta Velorum & Kappa Velorum (south).
Phobos & Deimos both share these.

Jupiter: 36 Draconis (north) Delta Doradus (south).
Amalthea, Thebe, Io, Europa, Ganymede & Callisto share these.

Saturn & Titan: Polaris / Alpha Ursae Majoris (north, 6 degrees away) Delta Octantis (south).
Mimas, Enceladus, Telesto, Tethys, Dione & Rhea share these.
Iapetus: 73 Draconis (north) Eta Volantis (south).
Phoebe: Omicron Cephei (north) Lambda Muscae (south).
Hyperion does not have a stable axis, so is therefore tumbling, thus does not have a 'polar star' as such.

Uranus: Sabik / Eta Ophiuchi (north) 15 Orionis (south).
The main moons of Uranus (Puck, Miranda, Ariel, Umbriel, Titania & Oberon) share these.

Neptune: Delta Cygni (north) Noas / Zeta Puppis (south).
Triton: Gamma Sagittae (north) 11 Puppis (south).

Pluto & Charon: 18 Delphini (north) 15 Hydrae (south).
The moons Nix & Hydra will probably share these as they orbit Pluto in the same plane as Charon.

Interesting to see how the polar points of Earth & Saturn are very similar & those of Mars & Neptune.

Andrew Brown.

neilzero
2009-Dec-21, 08:15 PM
Hi Eburacum: Perhaps I miss understood. The Moon's orbital tilt (5 degrees?) is with respect to the plane of Earth's orbit. The almost 23 degrees tilt of Earth is almost irrelevant to the Moon's orbit or seasons. Our moon does not stay over the Equator. Near the North Pole of Earth, the moon is frequently above the horizon on December 22, even though the Sun is far below the horizon for several months of continuous night without any twilight, so I have been told. Neil

chornedsnorkack
2009-Dec-21, 08:38 PM
Hi Eburacum: Perhaps I miss understood. The Moon's orbital tilt (5 degrees?) is with respect to the plane of Earth's orbit. The almost 23 degrees tilt of Earth is almost irrelevant to the Moon's orbit or seasons. Our moon does not stay over the Equator. Near the North Pole of Earth, the moon is frequently above the horizon on December 22, even though the Sun is far below the horizon for several months of continuous night without any twilight, so I have been told. Neil

This is true.

It happens because Moon is relatively so far from Earth that the perturbations of Sun are much stronger than the perturbations due to the equatorial flattening of Earth.

The inner satellites of the gas giants are relatively very much closer to the planet, and the planets themselves are more flattened, so the perturbations from the flattening of planet are stronger and the satellites stay near the equator.

neilzero
2009-Dec-21, 09:44 PM
Our 4 gas giant planets spin rapidly. Is it not probable that some gas giant planets take a month to turn once, and thus would have very little equatorial bulge? Neil