A Question from a newbie

[Guest]

I have to admit; I did see a quick post from someone who tried, albeit briefly, to explain all the variables used to define a new planet/satellite.

Big picture: I want to recreate the various colonized planets from a vey popular sci-fi series, and I just need a little translation from what the variable names mean to meaningful English.

I.e., WTF does SemiMajorAxis represent?

Any help would be appreciated.

For example, I need to know how to format the data for a 'planet' vs. a planet's moon.

I have looked though the raw data that comes with Celestia, but I've got 4 girls and little time for my hobbies. Any help would be appreciated.

Thanks in advance,

[Guest]

In an elliptical orbit, the Semi Major Axis refers to the length of the shortest diameter (I think - can anyone verify this ?). The Major Axis is the longest possible diameter. The period is just how long the object takes to complete one orbit, the eccentricity is a measure of how elliptical the orbit is (as far as I know, 0 would make a perfectly circular orbit, 1 makes a parabolic orbit (i.e. the planet gets deflected a bit but ultimately shoots off into deep space) and larger values than one (not at all sure about this bit mind you) makes hyperbolic orbits. The inclination is the angle of the orbit from the horizontal. Afraid I have no idea what units any of these are supposed to be in.

(Mad Boris)

[alexis]

In an elliptical orbit, the Semi Major Axis refers to the length of the shortest diameter (I think - can anyone verify this ?).

That's the minor axis, the semi major axis is half the major axis.
(major = bigger, minor = smaller, semi = half)

/Alexis

[Hank R]

As noted above, the semi-major axis (a) is half the major axis. The major axis is the sum of the closest (q) and farthest (Q) distances from the primary. Thus, for a solar orbit, the major axis is the sum of the perihelion and aphelion distances. (For a circular orbit, these distances are the same, so the semi-major axis is just the radius of the orbit.) The eccentricity is determined by the ratio of the closest and farthest distances: e = (1-(q/Q))/(1+(q/Q)). So if you know the closest and farthest distances you can compute both the semi-major axis and the eccentricity.

Note that the semi-major axis is also related to the period of the orbit (P): P^2 = a^3*(4*PI^2)/(G*(M+m)) where M is the mass of the primary and m is the mass of the orbiting body and G is the gravitational constant (6.67259e-11 m^3/kg-s) and PI = 3.14159 (approx.)

Generally m (much smaller than M) can be ignored. Thus for solar system planets P = a^(3/2) for P in years and a in AU. And for extrasolar planets P = a^(3/2)/M^(1/2) for P in years, a in AU, and M in solar masses.

- Hank