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Because our moon is tidally locked with Earth, then the same half always faces the Earth regardless of where the Moon is in its orbit. As such, would a tidally locked satellite such as our moon (assuming it meets the criteria for an atmosphere) be able to have seasons?

I can see that if a satellite has a tilt like the Earth's and it orbits a planet orbiting a star then it would have seasons similar to how Earth does. But a moon that is tidally locked cannot do this as far as I can conceptualize. I'd love some clarification on this, thank you!

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The reason why Earth has seasons in the way it does is Earth's axial tilt relative to the Sun combined with its atmosphere.

Earth's moon has a much smaller (about 1.5°) axial tilt relative to the ecliptic than does the Earth (23°). Thus its seasonal variations are much smaller. Note that this is irrespective of the fact that Earth's moon is tidally locked to Earth! The contribution of Earth to the Moon's heating is negligible.

In order to actually notice seasonal variations, there has to be something to smooth out variations on a shorter time scale. Earth's moon essentially (but not completely!) lacks an atmosphere, so there is nothing to even out the differences even between sunlight and shadow, let alone over time. If Earth lacked an atmosphere, the temperature swings would be equally brutal here despite Earth's 23° axial tilt.

So what you need for a natural satellite to have some kind of seasons is a difference of insolation across its surface, which is usually caused by having a significant axial tilt relative to the sun, and a significant atmosphere to even out the extremes and thus make the seasons noticable against the noise of local temperature variations.

Compare Are there seasons on Luna? on Space Exploration.

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  • $\begingroup$ Ok, but can a satellite that is tidally locked with it's host planet have a large axial tilt relative to the sun? Wouldn't the tilt be constantly shifting due to the rotation about the planet since it is tidally locked? $\endgroup$ – Nerevar98 Feb 22 '17 at 21:02
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Yes but not as extreme as the Earth's. The Earth gets some if its seasonal difference from its orbit taking it closer or farther from the Sun. This would still affect the Moon. However, the Earth gets more seasonal effect from it's axial tilt (look at Australia vs US/Europe). That would not happen.

So, there would be some effect but not a lot. You would probably get more of an effect from the fact that the lunar "day" is about 28 Earth days long. That would produce a pretty hot spot that will travel slowly around the Moon.

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  • $\begingroup$ Earth's orbital eccentricity is not the driver of seasons. If it was, both hemispheres would be equally affected, but consider: the northern hemisphere experiences winter when Earth is the closest to the Sun! (Of course, by consequence, the southern hemisphere experiences summer when the Earth is closest to the Sun. But people have a tendency to ignore the southern hemisphere.) Also, Earth's orbit differs only about 3.4% between aphelion (farthest from the sun, 152.1 Gm) and perihelion (closest to the sun, 147.1 Gm); that's not enough to make any significant difference in the first place. $\endgroup$ – a CVn Feb 22 '17 at 20:14
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The rotation of the moon is strongly influenced by the Earth's gravity, in such a way as to make the axial tilt relative to the ecliptic constant at 1.5 degrees. (The direction the pole points in constantly changes to make this possible [wikipedia]) This is a very small tilt, compared with the Earth's (at 23 degrees) and so there is little variation in the amount of insolation that each hemisphere of the moon gets over a year or over a month. (This small angle allows for the craters near the moon's pole to remain in permanent shadow, and for water ice to be found there)

So our moon would not experience significant seasons even if it had an atmosphere. However, the "day" lasts for 28 Earth days, and so during the 14*24 hours that the sun is above the horizon, each side of the moon is alternately heated, then frozen. I don't know of any modelling of the atmosphere of a planet with a slow rotation, but it would be unlike the Earth's. The Earth has convection cells (Hadley cells) that move energy from the equator to the poles. The moon may have convection cells that move energy from the sunfacing side to the dark side.

A tidally locked moon can have an axial tilt relative to its sun. So it would be possible for seasons to exist, however remember that a tidally locked moon may have a low rotation rate, that is likely to be more significant than the seasons.

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  • $\begingroup$ It might just be possible for an atmosphere on the moon to experience superrotation, like the atmospheres of Titan and Venus. As far as I know, that's a phenomenon we don't fully understand, but convection currents probably have something to do with it. Assuming, of course, we somehow find a way to keep the atmosphere attached to the moon in the first place... $\endgroup$ – MozerShmozer Jul 31 '17 at 18:40
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The Moon would experience the seasons every ~27 days (depending on location and the lunar standstill every 9.8 years).

The logic for this argument is (assuming an atmosphere, i.e. widespread surface thermal transference and minimally discounting the intense semi-global storm systems):

During the ~10+ days of the cycle the Moon is closer to the Sun by ~240 thousand miles than the Earth, this almost completely eliminates axis tilt and inclination as a viable season indicator.

Due to tilt and inclination there is a portion that never receives Sunlight. This area can be equated to the polar regions on Earth, the the "atmosphere," if anything like Earth's, given the surface area (thermal mass) and reflectivity of the Moon's surface, a much more varied ground temperature versus air temperature than the equivalent polar regions on Earth.

The surface temperatures will most likely range from -160 degree F minimum (it will receive a lot of energy from Earth during the dark times + surface convection) to 200 degree F, with a very high probability of damaging wind storms Moon-wide constantly. Due to the small surface area and the thermal differences between the sunlit side and non-sunlit side.

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Well it would have a two phase day/night cycle anyway which is like having seasons in that it creates variations mean thermal inputs. As the Earth/Moon system turns the Moon, as seen from Earth, is lit by the Sun by varying degrees, this is what creates the lunar cycle we know. Therefore the Moon has a Solar day that's about 672 hours long but it also goes through an Earth day cycle wherein more or less of the Earth is lit and therefore radiating thermal energy towards the Moon. Between them these two cycles are going to create a season-like variation in temperatures. You've also hit on the fact that the Moon will have axial procession in it's Solar day cycle which will create a longer seasonal cycle and of course there's the orbital eccentricity of Earth to take into account. With a larger primary world the thermal and light inputs from the world the satellite is locked to becomes more important as well as they do with greater distance from the stellar primary of the system.

Really it's not so much about whether there can be seasons as what kind and how much they influence the environment. The Moon has several sets of seasons all of which have little impact because of the lack of any atmosphere to acquire and store thermal energy.

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