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AFAIK, there is no precise definition, but IMO there is room for the smaller irregular-shaped moons to be called moons. Probably the biggest criteria to be called a moon is that it is an object in a stable orbit around a planet, which brings us to another definitional connundrum. What is a planet?
I would argue that Titan is more a planet than Pluto or Mercury. |
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Tomba: I don't think there would be a stable orbit like that. |
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But he that sows lies in the end shall not lack of a harvest, and soon he may rest from toil indeed while others reap and sow in his stead. -J.R.R. Tolkien, The Silmarillion |
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Everything I need to know I learned through Googling. |
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I guess that apart from a stable orbit aruond a planet, it also has to 'fly' on its own, not in a larger group in approximately the same orbit. Thus you exclude rings. As for minimum size, that's harder to decide...
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Knowledge is a curse, but ignorance is worse |
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http://www.solarviews.com/eng/janus.htm Take Janus and Epimethius for example.
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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all. |
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Russ: The Dictionary is certainly source to consider, when determining how a word is used in common parlance. However, as anyone with any science or technical background can verify, it is not an authoritative source for professional jargon.
So, if we are applying the common man's definitions, you are quite correct. However, since this is an Astronomy forum, I assumed we were talking in terms of the jargon of Astronomers. Astronomers have no formal definition of planet. It is a topic that is actually in hot debate in recent years with the discoveries on Titan and extra-solar planets, particularly the planets that roam free from any star. I am not sure if there is a formal definition of "Moon", but my suspicion is that it is as nebulous as "planet". I will have to look it up. |
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Knowledge is a curse, but ignorance is worse |
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I suppose my own personal take on it would be, any object in orbit around a planet which is individually identifiable as separate and distinct from other bodies orbiting a planet.
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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all. |
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Honestly, I doubt any universal definition will ever be satisfactory to everyone involved. People interested in celestial mechanics will always insit that a moon orbits a planet, and a planet orbits a star. However, people more interested in the composition of the planet will look at Titan, and be hard pressed not to call it a planet.
Edit: As to the size limit for a moon, I propose that we set the minimum at a size where a human could walk/jump on the surface without risk of flying off into space, and without noticably altering the orbit of the "moon". I am not sure whether all of the currently identified moons would meet this size limit. |
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There must therefore be a region of space around the moon where its gravitational pull is greater than that of the Earth. If there was not, then the rockets we fired to the moon would never have been able to land there, and would have fallen back to the Earth instead. This region of space is likely rather small, and the chances of our Moon capturing a satellite that would orbit within this critical distance are equally small, nevertheless there do exist stable orbits around the Moon (and thus any moon of any planet), its just that a configuration happening naturally is unlikely. |
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A better word (and often used by astronomers) may be satellite, for which size is irrelevant. Quote:
But you won't ever see moon and planet being interchanged. |
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