76 votes
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Is Earth's moon the only one where a total eclipse of the sun covers the entire sun?

As planets get farther from the Sun, the Sun takes up a smaller part of the sky. The Sun is about 31 arc-minutes when viewed from Earth, but just 6-7 from Jupiter and 3-4 from Saturn. Less than 2 ...
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48 votes
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Why don't the inner moons of Jupiter have tidally-induced volcanism?

It’s because they are much smaller than Io. Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other. When an ...
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  • 4,259
44 votes

Why is there a mountain inside the Herschel crater on Mimas?

In the extreme energy of a large impact, the rock behaves like a liquid (It isn't actually completely melted, though some is. The extreme forces cause the rock to flow). As the impactor hits the moon, ...
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38 votes

Do celestial objects need to be big to have liquid water on their surfaces?

Liquid water can't exist in a vacuum. If there is no pressure, then the boiling point will drop to the freezing point and so there will either be ice or water vapour. And if the world is "small&...
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25 votes
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Why do planets and satellites in the Solar system look so wildly different if they came from more or less the same matter?

This questions can be split in two; for planets and satellites. The diversity of planets reflects in part the diversity in terms of chemical composition of the protoplanetary disk. We know that UV ...
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  • 3,556
24 votes

Can a natural satellite exist in a geostationary orbit?

Yes. Charon is in Pluto synchronous orbit. Pluto and Charon are mutually tide locked.
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  • 1,332
23 votes
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Which Saturn satellite passes closest to Saturn's rings and at what distance?

Pan, Daphnis, and various other moonlets, I would argue, are inside the rings. If you explicitly discount the Encke gap (which Pan orbits in) and the Keeler gap (which Daphnis orbits in) as being ...
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  • 3,928
20 votes
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Can a natural satellite exist in a geostationary orbit?

Of course, a natural satellite (moon) could have an orbital period equal to the spin period of its host (provided such an orbit would be accessible). However, the tidal friction that may generate such ...
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  • 5,276
20 votes

Do moons have moons?

There is a prior answer here claiming that "a moon with a moon would be an unstable system". That is incorrect. Intuitively: Of course satellites can have satellites with long-term stable orbits. ...
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20 votes
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Did we discover 10 or 12 new moons of Jupiter?

Per the Carnegie Science article that Magic Octopus Urn linked from NASA in the comments, a Carnegie Science team led by Scott S. Sheppard noticed something new in spring of 2017 (though some ...
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  • 6,084
20 votes
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At what distance from the Sun can planetary moons exist?

There are several factors determining the inner limit to moons. Perhaps the simplest is that it needs to stay inside the Hill sphere, the region around the planet where the planet's gravity dominates ...
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19 votes

Would an analogue of the definition for planets also work for moons?

In 2006 the IAU had a trilemma. They could decide that Eris was a planet, and potentially allow for future discoveries of tens of new planets. They could be inconsistent, declare that Pluto was a ...
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  • 88.7k
17 votes

Why don't the inner moons of Jupiter have tidally-induced volcanism?

There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Only one of those innermost moons (Thebe) has an ...
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15 votes

Do moons have moons?

It's theoretically possible for moons of moons to exist in stable orbits, gravitationally speaking. However, as far as I'm aware, no natural moons of moons have ever been observed as of yet. (...
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15 votes

Moon's orbit around the Sun

Not an answer, but I thought this was a good slice of a picture of the Moon's orbit around the sun. Source: http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/
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  • 22.9k
15 votes

Do celestial objects need to be big to have liquid water on their surfaces?

Do celestial objects need to be big to have liquid water on their surfaces? Yes. In a nutshell: liquid surface water needs an atmosphere. To sustain an atmosphere, a planet must be sufficiently ...
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  • 1,903
14 votes
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Are all satellites thought to be the result of collisions with the planets?

There are three main formation scenarios for planetary moons. The giant impact hypothesis: The satellite forms as a consecuence of an impact between the planet an a large planetesimal. The Moon is an ...
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  • 3,556
13 votes
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How do moons get captured?

How can a planet capture a moon? There are 178 moons in the Solar System, according to the NASA Planetary Fact Sheet, so it seems to be a common event. The following sections will show that moon ...
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13 votes
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How many satellites orbit their planet faster than the planet rotates?

Uranus rotates once every 0.718 days. 11 of its satellites have a shorter orbital period. These are inner satellites of Uranus which are roughly in the equatorial plane of Uranus. I don't quite ...
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13 votes
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Is there a name for a planet and its moons/satellites?

This question has been asked before on the Space Exploration page. In summary, the term used is system, e.g. the 'Jupiter system'.
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13 votes

Why don't we find planetoids at L4/L5?

In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of ...
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12 votes
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Can you explain the pattern of Hill sphere sizes of the objects of the Solar system?

Hill sphere is the region of space around a satellite where the satellite wins the gravitational tug-of-war with its primary. If the mass of the primary object is $M$, mass of the satellite is $m$, ...
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12 votes
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How can tidal heating lower Io's orbit?

How can tidal heating lower Io's orbit? It doesn't, at least not to first order. The first order effect is that tidal heating acts to circularize Io's orbit. Counter to that, orbital resonances with ...
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11 votes
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Moon's orbit around the Sun

What is the reason for this difference between assumed and actual path variation? Even your second image isn't correct. Imagine zooming in on a small portion of the Moon's orbit about the Sun, for ...
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11 votes

Will Saturn's rings become a moon?

The currently leading answer is correct to say that moon formation inside the Roche limit is unlikely. However, the disk is evolving due to viscosity between the particles, and as a consequence it "...
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  • 115k
11 votes
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What exactly is a "moon"?

Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?...
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  • 88.7k
11 votes
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Moons with curlicue paths around our Sun?

A moon will describe a path like this if its orbital speed relative to its parent planet is greater than the parent planet's orbital speed about the Sun. (Assuming the moon's orbit about the planet ...
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11 votes
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What is the longest natural bound orbit chain observed?

Certainly 4 (perhaps 5): The longest certainly-known chain is "3": Sun-Earth-Moon. Rhea (a moon of Saturn) has been observed to have a ring of orbiting material: That gives Sun-Saturn-Rhea-...
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  • 88.7k
11 votes
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Are satellites of trans-Neptunian objects classified as trans-Neptunian objects?

Yes/No as needed. The MPC, which is fairly authoritative, lists only (134340) Pluto, and doesn't include Charon and the other satellites of Pluto. Formally, binary asteroids are given a single ...
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