The moon would be similar to Earth in size and composition and the gas giant comparable to neptune or uranus.

  • Will the periods of eclipse of the the giant have an effect on climate?
  • What about the tidal force?
  • Will being closer to the sun or farther from it on the other side of the giant have any repercussions?
  • $\begingroup$ Ye gads! that's an interesting one, it's probably going to get a lot of tidal forces pushing & pulling it's core, hard to see how it wouldn't, might well be inside the gas giants magnetosphere if it has one as well [clicks follow & bookmark, pulls up a stump & waits expectantly for answers]. $\endgroup$
    – Pelinore
    Commented Mar 16, 2021 at 15:03
  • $\begingroup$ I've rejiggled your answer slightly to improve the english and formatting; please check that I haven't changed the meaning of anything you wanted. That said, you have four questions here... it might be a good idea to make it clear that the main question is in the title, and the bullet points are just hints or bonus questions. $\endgroup$ Commented Mar 16, 2021 at 16:10
  • $\begingroup$ I'm sorry, english is not my mother tongue. it sound right tank you! $\endgroup$
    – Veknor
    Commented Mar 16, 2021 at 16:13
  • $\begingroup$ This question has a little bearing on yours. $\endgroup$
    – Paul
    Commented Mar 16, 2021 at 22:12

2 Answers 2


Will the periods of eclipse of the the giant have an effect on climate?

Very little compared to everything else.

Your moon will almost certainly be tidally locked, meaning that one hemisphere will always face its parent gas giant, and one half will always face away. It probably won't have a moon as big as Earth's (due to gravitational interference from the gas giant) and that means that the huge angular momentum the Moon brings to our system will be absent and so tidal locking will inevitably occur.

Similarly, the geodynamo that drives Earth's magnetic field may well have cooled to the point where your moon's magnetic field is reduced or absent compared to Earth, because without Earth's spin or tidal forces induced by the Moon, cooling will occur much more quickly than it has on Earth itself. Part of the reason that Mars lost its atmosphere was the lack of a magnetic field, but we know that Titan has kept its atmosphere at least in part due to protection from Saturn's magnetosphere.

Lets say your gas giant is like Uranus. Uranus' magnetopause is at a distance of ~460000 km from its centre. If we put your world at ~400000km, it could have a 7.6 Earth-day long orbital period, which could be called either its day or its month, up to you. That means that its periods of daylight will be long and hot, and periods of night-time will be long and cold. A thick atmosphere with superrotation (as found on Venus and Titan) will help keep heat in and distribute it evenly across the world's surface. It could be quite windy!

Preventing a runaway greenhouse effect might be difficult in these circumstances. I'm not sure how it could be done. You might end up with a hot, wet, pressure-cooker like water world.

The gas giant will have an angular diameter of ~14°41'... that's pretty big, nearly 30 times wider than the full moon seen from Earth. It will have an apparent magnitude of -19, more than 300 times brighter than the full moon. The gas-giant facing hemisphere of your world will have bright nights every night (but for cloud cover), and wildlife will adapt accordingly.

What about the tidal force?

This is usually a hideously difficult question to answer well, but as I'm assuming that your world will already be tidally locked it does simplify some things.

Real-life moons like Io interact with other moons in their local system (like Ganymede and Europa) which tweaks their orbit to be non-circular, resulting in tidal heating. Your moon is absolutely vast by comparison, making it a) hard to tweak and b) even less likely to be near any other moons. Without these peturbations, tidal effects will circularise you moon's orbit in a relatively short period of time, astronomically speaking.

It can therefore be reasonably said that there are no tidal effects, and indeed your moon would in fact experience fewer tidal effects than the Earth does from its own Moon (because the Earth rotates, and your world does not). This means the seas will have solar tides only, for example.

Will being closer to the sun or farther from it on the other side of the giant have any repercussions?

Not nearly as much as you might think! If your gas giant was the size of Uranus, and it was as far from a star like the Sun as Earth is now, your moon would probably have a maximum stable orbit radius of no more than 1.8 million kilometres. If its orbital plane lay in the same plane as the gas giant's orbit about the star, you'll get a difference of solar irradiance of ~5% between closest and furthest points. Earth itself has its perihelion some 5 million km closer to the sun than its aphelion, and it makes less difference than the axial tilt to our experience of winter and summer.

Footnote: here's my thinking on tidal locking.

The time for a body to become tidally locked can be approximated by $$T_{lock} \approx {\omega a^6 I Q \over 3Gm_p^2 k_2 r^5}$$ where $\omega$ is Earth's spin rate (~2π rad/day), $a$ the orbital semimajor axis, $I$ is the moment of inertia (which is 0.331 x the mass of the Earth x the radius of the Earth squared), $Q$ is the dissipation function, $G$ is the gravitational constant, $m_p$ is the mass of Uranus, $k_2$ is the Love number of the Earth and $r$ is Earth's radius. For Earth, Q appears to be about 100 and $k_2$ appears to be about 0.308.

Your moon will not be orbiting beyond more than about half of the gas giant's Hill Radius, which for Uranus at 1AU from the Sun gives a limit of ~1.8 million km and for Neptune ~1.9 million km.

This gives a tidal locking timescale of ~130 million years for either case... sufficiently quickly that your moon could be doomed to a Mars like fate as its geodynamo cools, the magnetic field weakens and the solar wind blows the atmosphere off. Maybe the atmosphere will remain (as it has on Venus, though you might not want to live there) but I don't think that the tidal locking can be avoided in your scenario.

A closer moon will lock much faster, but will be protected from the solar wind. It'll also get much brighter nights on the planet-facing side, with a generally awesome view. What's not to like?

  • $\begingroup$ How about the difference in 'day' & 'night' on this moon if it's tidally locked, between the gas giant facing side & the other? .. going to be very different environments there, leading to (not part of the question I know) very different fauna & flora... liable to lead to (moving back to the question) some interesting convection currents & weather patterns, probably quite extreme ones. $\endgroup$
    – Pelinore
    Commented Mar 16, 2021 at 22:49
  • $\begingroup$ Would it be possible to not be tidally lock even if its has a really slow rotation so the Gas planet would be visible on every side of the moon once in a while? $\endgroup$
    – Veknor
    Commented Mar 17, 2021 at 0:14
  • $\begingroup$ @Pelinore follow the link on super-rotation ;-) I would expect there to be a difference in wildlife between the planet-facing side and the non-planet-facing side, given how bright the nights will be on the facing side. $\endgroup$ Commented Mar 17, 2021 at 9:29
  • $\begingroup$ @Veknor it seems very unlikely... the tidal locking timescale is well under a billion years, so by the time anything interesting evolves any rotation will be long gone. There are other questions on this site about habitable moons with their own moons... this might prevent tidal locking (and allow a magnetosphere) but requires a brown dwarf parent instead of a mere gas giant (see here for example). $\endgroup$ Commented Mar 17, 2021 at 10:00
  • $\begingroup$ thanks for the answer $\endgroup$
    – Veknor
    Commented Mar 17, 2021 at 17:18

I suggest that yu use the Search on Worldbuilding feature at the top of the worldbulding bar and search for something like "Habitable Moons".

I also suggest that you search for articles about the possibility of Habitable exomoons by Rene Heller & others.




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