Celestia Forums

I'm trying to see how the parameters for orbits of planets are calculated for Celestia and I am using Grant Hutchison's spreadsheet available here to perform the calculation.

The orbit data I am using are from The Extrasolar Planets Encyclopaedia. However I am not sure what the parameter "PA of node" represents or what value is used to create the SSC for Celestia - the value is not provided in the Encyclopaedia (presumably because it is unknown).

Any help appreciated, thanks.

Exactly where is "P.A. of Node" used?
I didn't see that term in the small number of planet descriptions that I looked at.

Maybe it's a poor translation into English of a French phrase.

Never mind... I discovered that you mean "position angle of ascending node", one of the inputs that Grant's spreadsheet can use.

Remember that his spreadsheet was intended for calculating STC star orbits, not SSC planet orbits. This ascending node parameter is one often calculated by astronomers who observe double stars visually. It simply isn't available from the limited data about exoplanet orbits.

selden wrote:Never mind... I discovered that you mean "position angle of ascending node", one of the inputs that Grant's spreadsheet can use.

Remember that his spreadsheet was intended for calculating STC star orbits, not SSC planet orbits. This ascending node parameter is one often calculated by astronomers who observe double stars visually. It simply isn't available from the limited data about exoplanet orbits.

Would that be the same thing as inclination in the exosolar planet tables?

No.

selden wrote:Remember that his spreadsheet was intended for calculating STC star orbits, not SSC planet orbits. This ascending node parameter is one often calculated by astronomers who observe double stars visually. It simply isn't available from the limited data about exoplanet orbits.

However some coordinate transformation is being performed on the argument of pericenter. For example, the .ssc file information for HD 28185 b is:



The Extrasolar Planets Encyclopaedia gives omega=351. I don't understand how the ArgOfPericenter=144 is calculated from this value.

One of the complications in Celestia is that all orbital planes are referenced to the Ecliptic (the plane of orbits in our solar system).

However, planetary and stellar orbits that astronomers measure are referenced to the "plane of the sky." That local plane tangent to the "celestial sphere" has different orientations relative to the plane of the Ecliptic depending on the position of the star on the sky. So the parameters describing the orbits have to be adjusted if they're to be drawn accurately by Celestia.

selden wrote:One of the complications in Celestia is that all orbital planes are referenced to the Ecliptic (the plane of orbits in our solar system).

However, planetary and stellar orbits that astronomers measure are referenced to the "plane of the sky." That local plane tangent to the "celestial sphere" has different orientations relative to the plane of the Ecliptic depending on the position of the star on the sky. So the parameters describing the orbits have to be adjusted if they're to be drawn accurately by Celestia.

So what values should I put into the spreadsheet to perform the conversion?

damifino.
Sorry: Grant Hutchison is (was) the 3d trig specialist. He's no longer directly involved in Celestia development, but you might try sending him an e'mail message.