# Software to convert RA and DEC into ALT and AZ

I have implemented the formulas to convert AR and Dec into altitude and Azimuth in C++ following the book Practical Astronomy with your Calculator or Spreadsheet 4th Edition.

In the book there is an example to check if I have do it fine. This example works perfectly but I want to check if my formulas are correct.

I have use this page to check it but I get different minutes values in both altitude and azimuth values.

I have checked on that page that I get the same LST value, but ALT and AZ have different values. This data I have tested:

Altitude: -15° 27' 40"
Azimuth:  273° 12' 34"


This is the data get it from that page:

Do you know if there is another web page or program to check if the result of my formulas are correct?

UPDATE:
I have tried to get Azimuth and Altitude using a date in year 2000 in web page and my program and this is the result (you can find my program's result at the bottom of the image):

Is this the same problem like the previous one (I'm not taking account for precession)?

• I haven't seen the spreadsheet, but my wild mass guess is that you are not accounting for precession. – user21 Nov 28 '15 at 16:30
• Yes, I'm not accounting for precession. – VansFannel Nov 28 '15 at 17:12

Based on stellarium, I think you're not accounting for precession (more after the picture):

As you can see, Rigel's J2000 coordinates and Rigel's current coordinates are different by about the same amount as your calculation.

• I have updated my question with a calculation for year 2000 to check precession problem. But I'm not sure what is precession. – VansFannel Nov 29 '15 at 7:55
• Read en.wikipedia.org/wiki/Axial_precession and ask here if you still have questions. – user21 Nov 29 '15 at 19:41

Minutes of a degree are "minúte". There are a few things that could cause errors. One is atmospheric refraction: it can cause an error of about 1 minute at the an alt of 45degrees, and over 30minutes on the horizon. When observing objects close to the horizon, this is a sigificant source of error

Its pretty much impossible to check without access to the source code. However There is plenty of astronomical software that can calulate Az-Alt positions. Pyephem, xephem, stellarium come to mind, and you can ask them to calculate with or without atmospheric distortion.

• Thanks but I don't asking about if you can check my software, I want to use another implementations to check if my software is generating the right value. – VansFannel Nov 28 '15 at 7:24
• Ok then pyephem or stellarium. (depending on whether you want a programming or graphical interface) – James K Nov 28 '15 at 8:07
• The example (Rigel?) could be better chosen, as it is below the horizon at the time given, and it is not clear how to deal with atmospheric distortion for points below the horizon. Test with stars that are near the zenith – James K Nov 28 '15 at 8:24
• Yes, it's Rigel. I will check it with stars near zenith. – VansFannel Nov 28 '15 at 10:30
• I have updated my question with a calculation for year 2000 to check precession problem. But I'm not sure what is precession. – VansFannel Nov 29 '15 at 7:55