NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Compute suns position
From: J Cora
Date: 2006 Apr 4, 22:44 -0700
From: J Cora
Date: 2006 Apr 4, 22:44 -0700
Without much experience with a sextant but an interest in the mathematics,
I found this web site some time ago and am just getting around to working through
the tutorial "Computing planetary positions - a tutorial with worked examples"
http://stjarnhimlen.se/comp/ppcomp.html
I am part way through the tutorial and have worked up a python program for the
Suns RA and declination. To verify the program and examine the results of the computation, I am using various programs such as winephem, xephem and the
online ephemeris at ephermis.com which I am assuming is an accurate reference.
http://www.geocities.com/tmarkjames/WinEphem.html
http://www.clearskyinstitute.com/xephem/
http://www.ephemeris.com/
The listed values for RA and dec vary among the various programs by as much
as 5 minutes of arc, which seems to be the average change over a 24 hour day.
The computations from the tutorial aim for an accuracy of 1 minute or less as
compared to the values in the astronomical almanac. I have not gotten to the
equations for GHA or LHA yet but hope to include that soon.
Here is my question to the list, Although these simplified equations can be used
to predict the position of the sun or other bodies, at what level of accuracy does
the exercise become useless? As an example If I were off by a day the RA
would be off by approx 5 minutes assuming the times were the same. Please
correct my guess here but if a 4 sec error corresponds to 1 nautical mile
then 5 minutes represents about 60 miles of error?
I found this web site some time ago and am just getting around to working through
the tutorial "Computing planetary positions - a tutorial with worked examples"
http://stjarnhimlen.se/comp/ppcomp.html
I am part way through the tutorial and have worked up a python program for the
Suns RA and declination. To verify the program and examine the results of the computation, I am using various programs such as winephem, xephem and the
online ephemeris at ephermis.com which I am assuming is an accurate reference.
http://www.geocities.com/tmarkjames/WinEphem.html
http://www.clearskyinstitute.com/xephem/
http://www.ephemeris.com/
The listed values for RA and dec vary among the various programs by as much
as 5 minutes of arc, which seems to be the average change over a 24 hour day.
The computations from the tutorial aim for an accuracy of 1 minute or less as
compared to the values in the astronomical almanac. I have not gotten to the
equations for GHA or LHA yet but hope to include that soon.
Here is my question to the list, Although these simplified equations can be used
to predict the position of the sun or other bodies, at what level of accuracy does
the exercise become useless? As an example If I were off by a day the RA
would be off by approx 5 minutes assuming the times were the same. Please
correct my guess here but if a 4 sec error corresponds to 1 nautical mile
then 5 minutes represents about 60 miles of error?
