[AR] Re: [OT] Convention for describing elliptical orbits?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 23 May 2016 16:17:52 -0400 (EDT)
But looking around it seems more often altitude is used instead of distance
from center. Should I be calling this a (300 km x 35786 km) ellipse?
Yes, that is the normal practice for orbits around sizable planetary bodies.
Addendum: the usual convention is to take orbit radius and subtract
Earth's *equatorial* radius, ignoring the fact that Earth is not exactly
spherical. (Polar radius is about 21km less than equatorial.)
(To add further complexity, the equator is not exactly a circle, and there
are several slightly-different values for equatorial radius. Sometimes
you can find more than one in the same piece of old and much-modified
software...! The choice is unimportant for just quoting orbital
altitudes, but can be significant for calculating nonspherical
perturbations, where the radius figures in the equations along with the
spherical-harmonic coefficients -- a set of those coefficients should
always be accompanied by the radius value used to compute them, and that's
the value you should use with them.)
Henry
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