[AZ-Observing] Rigorous Precession

  • From: Stan Gorodenski <stanlep@xxxxxxxxxxxxx>
  • To: AZ-Observing <az-observing@xxxxxxxxxxxxx>
  • Date: Sat, 22 May 2010 09:51:51 -0700

Years ago, before the dominance of the internet, I bought S&Ts Sky 
Catalogue 2000.0, vol 1 and 2. In volume 1 in the introduction, pages 
xiii and xiv, they give the the formulas for rigorous precession. It 
uses Newcomb's expressions for some constants. With Newcomb's 
expressions and the other formulas given, I can calculate what the RA 
and DEC was for Polaris, for example, way back to 1755 to almost exact 
accuracy to what was observed by J Bradley. I searched the internet for 
precession correction formulas but could not find any, perhaps because I 
have dial up and so I cannot take the time to do a real thorough search. 
Then I remembered the Catalogue I just mentioned had the formulas. I 
wrote my program and it works very nice. However, I did run into one 
problem others may have encountered if they attempted to program the 
same S&T precession formulas. The Newcomb expressions have all the 
coefficient values in seconds, but they need to be in degrees. I divided 
each coefficient value by 3600 to make the conversion. When I did this, 
I reproduced exactly the book example, the one going back to year 1755. 
This was confusing because the quick and dirty method they had given in 
an in a preceding page worked with coefficients in seconds and trig 
values in degrees.
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