Stan and Ken, I am just now in the middle of working on a program to take an objects position and determine its constellation. In the process the coordinates are precessed back to 1875, which is the date constellation boundaries were agreed upon. That said I have a program, from Project Pluto, that precess coordinates from one epoch to another. During my research I did check out Sky Catalog 2000.0. The only, seemingly difference, seems to be amongst the constants in use. So it appears those constants change from time to time and I don't know how to find the most current ones, but I would believe the ones from Project Pluto to be more current than 1970 or 2000. Any idea how to determine to most current constants? AJ Crayon Phoenix, AZ ----- Original Message ----- From: "Ken Sikes" <kengsikes@xxxxxxx> To: <az-observing@xxxxxxxxxxxxx> Sent: Saturday, May 22, 2010 10:28 AM Subject: [AZ-Observing] Re: Rigorous Precession Stan, I am quite familar with what you are talking about in rigourus precession methods. Back in the 1970's I wrote a reduction for Richard Lines to reduce his photoelectric readings. To precess to epoc of date I needed to precess from Jan 0 1900 to current date using Simon Newcombs constants. Bill Andersen and I went to Lowell and got Art Hoag's permission to us e the library to research Newcomb's constants even farther. I also have an excellant book "Astronomical Algorithms" by Jean Meeus published by Willmann-Bell that has the constants for both the last and current centries. Ken Sikes -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list. -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.