Ken, the reason for precessing back to 1875 is because it is much easier to determine to which constellation the DSO's coordinates belong. Specifically, the DSO's coordinates are precessed back to 1875, which is much simpler to determine the constellation. I believe that is the difference between ya'll and me, my project goes a little farther than yours. So far, my program is doing quite well but is still under development and is in need of some bug fixing. Another of the reasons for this exercise is to have something to write a program as there are many web sites that will do this conversion for you although I haven't found one to identify the constellation. The next question is why is this being done? Best answer I can come up with is that it is to satisfy an itch that just can't be, otherwise, scratched. Hope this helps. AJ Crayon Phoenix, AZ ----- Original Message ----- From: "Ken Sikes" <kengsikes@xxxxxxx> To: <az-observing@xxxxxxxxxxxxx> Sent: Saturday, May 22, 2010 11:02 AM Subject: [AZ-Observing] Re: Rigorous Precession AJ, I am familiar with the program you are talking about, Bill Anderson and I once talked about the 1875 version. At that time the constellation boundaries followed simpler lines. Over time the boundary lines have changed and that program is not as accurate. There is a newer program, but it is difficult to use. When you say "objects" are you referring to DSO's or stars? If stars one has to take into account the proper motion of the star as well when calculating the new location and constellation. Bill and I had a very long and deep conversation about this one time at Hovatter and he concluded that the new program would be more work that gain. If one was going to precess coordinates to a specific date say Aug 20, 2010 then the primary constants would remain the same and the only change would be the spherical coordinates. The book I mentioned to Stan is excellent for this and you are welcome to borrow it if you like. Ken -- 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.