Expert: Ed - 8/14/2010

Question
Hi,

I am writing a science fiction novel for which I have created a fictitious planet which has three moons.  If it is in the realm of the science-fact possible, I would like the three moons to have very different orbits so that their pairings in the sky change.  Are there any types of orbits in which this would be possible?  It would be particularly helpful to the story if one of the moons were to be less frequently visible than the other two.

Also, is there any reading material on planet/moon orbits/interactions that you would recommend?  I'm recovering from a brain injury but I'm able to read at an undergraduate level.  My reading comprehension unfortunately is not up to reading higher levels at this time.  I have a library with interlibrary loan, so I can get access to quite a lot of books.

Thanks,
Jo

Answer
Jo, you are in luck!  Moons can have many types of orbits.  The orbits can be highly elliptical, or nearly round.  You could probably even have a little moon do loop-de-loops around other moons... hey, it's a science fiction story.  Read about the 63 moons of Jupiter, or the many moons of Saturn, if you want to start thinking realistically about the variety of orbits that a moon can have.
http://en.wikipedia.org/wiki/Moons_of_Jupiter
http://en.wikipedia.org/wiki/Moons_of_Saturn

As a rule of thumb, the smaller a moon is, the more opportunities you will have as a writer, to do neat things with its orbit.  Larger moons are stuck in fairly circular orbits.  If you want to have large moons do freaky things in the sky, then you had better be prepared to have your characters on the ground of the main planet suffer through terrible tidal forces, catastrophic magma eruptions when the planet's crust ruptures as the moon does funky things, etc.

If you want any of your planet's moons to have REALLY elliptical orbits, like a comet, then you will need to start thinking of the planet's Hill Sphere.  If the moon goes beyond your planet's Hill Sphere... it won't be returning.
http://en.wikipedia.org/wiki/Hill_sphere

Two ideas that you might want to read more about are "resonance," and "syzygy."
http://en.wikipedia.org/wiki/Orbital_resonance
http://en.wikipedia.org/wiki/Syzygy#Astronomy

Here is an entertaining short story from the 1950s, about a fictional third moon of Mars.  Sorry, I am destroying all the suspense in the story by telling you that, but there's no other way I can introduce the story in the context of answering your question.
http://www.gutenberg.org/catalog/world/readfile?pageno=1&fk_files=1675880

If you are recovering from a terrible head injury, why don't you spend some time browsing through a good astronomy dictionary?  That way you can make whatever mental associations you want, at your own speed.  Here is a good one, in my opinion:
http://www.amazon.com/Dictionary-Astronomy-Oxford-Paperback-Reference/dp/0192115...

Also, ask your local reference librarian to show you their copy of Magazines for Libraries.  I write the astronomy chapter for that book.  There are several astronomy magazines that you could look at, and look for helpful articles, and then you can e-mail the authors of the articles, with your technical questions, as you write your story.

I hope this helps you get started.  

Keep Looking Up!
-Ed