# [AZ-Observing] Re: Rigorous Precession

• From: "AJ Crayon" <acrayon@xxxxxxx>
• To: <az-observing@xxxxxxxxxxxxx>
• Date: Sat, 22 May 2010 11:36:38 -0700

```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
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

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