document updated 21 years ago, on Aug 16, 2005

study THIS for a long long time. And this. And if you need lots of math, this.
various celestial coordinate system (category) are used, both for time and location. The ICRS seems to be primary among these.
- the main frame of reference is the equatorial coordinate system
- the main time coordinate seems to be J2000
- There are various astrometric catalogs available also, NOMAD seems to be primary among these (even simply studying the definition of the data columns can be very educational)
- and then there's all that sidereal stuff (s-time, s-year)
when doing orbital math, there are some simplifications that can make things much easier