I'm writing an application that draws maps of night sky that can be visible from other planets (ultimately they are to be exoplanets). Mostly I do it out of curiosity :).
I's thinking a lot where I should put my question and I've choose the physics SO, because I think my problem is more physical in nature than a purely software design.
My program takes into consideration below orbital parameters:
- Axial tilt
- Longitude of ascending node
- Ascending node of the planet's equator
I specifically have chosen a star data source so it has got these properties:
- its X axis points at the Earth's vernal equinox point,
- its Z axis points at Polaris (I mean the north pole on the celestial sphere)
The algorithm makes a sky by doing a series of rotations of the star data. I assumed it would be enough to rotate using above orbital parameters and put the planet in the center of the star system (from mathematical stand point it doesn't matter where a planet is but its orientation). These rotations are done like this:
- nivelate the Earth's axial tilt by rotating around X axis by 23°
- make an A vector by rotating a vector pointing at +X axis around +Z axis by the planet's "Equator asc. node"
- rotate around the A vector by the planet's axial tilt
- make a B vector by rotating a vector pointing at (new) +X axis by the planet's "Long. of asc. node"
- rotate around the B vector by the planet's Inclination
My program works in a way, but yet it is still not perfect. Let's take for example the Uranus night sky:
According to the Wikipedia Uranus's north pole star is
35Eta Oph which on my map is a quite above the center.
I believe I didn't consider yet another orbital parameter in my calculations, but I'm not sure what it is.
(For those who want there is my app: https://github.com/felix-leg/ExPlanetNightSkyGenerator )
During my research I found an IAU paper. It hasn't gave my the answer I'm was looking for, but it left me with another question: what's the invariable plane?
In my program I assumed the Invariable plane is just the plane where the Earth orbit lies and all I have to do to make my data's Z axis pointing in the same direction like the IP "up" vector is to nivelate Earth axial tilt.
But now I'm not sure. The IAU paper mentions there are two standards: old B1950 and new J2000. It also mentions that some data about Solar system planets was made when the old one has been applied and some other data when the new one had been taking effect.
So, I reformulate my question: In relation to WHAT are my orbital parameters (axial tilt, longitude of ascending node, inclination, and equator's ascending node) written in astronomical sources?