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PostHeaderIconOrbital Dynamics

I'm working on the next Hoister Family book, Children of the Eighth Day. (I'm on page 148 and hope to have the book done for Balticon 2014, thank you for asking.)

Anyway, I've reached a scene set on the planet Leikeis, fifth planet of Altair. So I look up "Leikeis" on this site, and I find that I have precious little information about the world: it was colonized in 2088, it became a great financial and banking center in the Terran Empire, the planet is well-known for the quality fo its seafood, that sort of thing. There's a rudimentary map with the names of three cities.

However, I don't have anything about what it's like on Leikeis. The problem is, I've never written anything set there.

In order to describe the planet, there are certain things I need to know. How hot (or cold) is it? What does the sun look like (Altair is a good deal hotter than our sun)? What's the gravity like, how long is the day, how long does the year last, what's in the sky, etc.?

Some of these are arbitrary -- I can make the day any length I want, although I usually stick to between 20 and 30 hours. But some of this stuff is interdependent: the planet's distance from its star determines the length of the year and the average temperature, for example.

I used to have a nice spreadsheet that calculated all these kinds of variables, throwing in random-but-plausible values. It would even construct a whole planetary system. However, that spreadsheet is in Appleworks 5 format, and nothing I have handy will read it (I'm at work, and my emulators are all installed on the iMac at home).

No matter, there are websites that can help. The Planet Termperature Calculator let me plug in Altair's known luminosity and various other assumptions; I kept fiddling with the distance until I hit a figure that made a planet slightly warmer than Earth (2.43 times Earth's distance from the sun, if you're interested).

Then it's over to Orbital Period of a Planet to calculate the revolutionary period. I gave Leikeis a mass of 1.06 times Earth and plugged in Altar's known mass along with the distance 2.43 AUs. Badda-bing, badda-boom, I have a planet with a year nearly three times the length of Earth's (2.8308 Earth years, or 1033.97 Earth days).

Now this gives me something to work with. Leikeis is a warmer world than Earth, it's far enough from Altair that its sun would look a bit smaller than Earth's but would be brighter. Altair is a younger star, rotating in about 9 hours, which gives it a distinct equatorial bulge -- according to my scratchpad calculations, it would appear as a flattened circle about a third of a degree wide. (Earth's sun appears as a circle about half a degree wide.) That small sun would be even more uncomfortable to look at as Earth's.

Now, of course, all this work will get stirred into a sentence or two in the finished book....

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