Blawgg

15 sec exposure, with ISO 200 on a 4 Megapixel non SLR, Olympus c770uz

Right Ascension or RA, and Declination or Dec, refer to the coordinate systems for stars and other heavenly bodies. They are similar to the concept of  latitude/longitude on earth and provide a “constant”  reference for locating objects in the night sky. However, for an observer on earth, as the sky moves through the night, a given RA and Dec doesn’t always fall in the same spot, but instead, depends on the local latitude/longitude and date/time.

In order to make practical use of the RA/Dec tables for objects, you also need to be able to convert them to a simple “degrees” format that shows how high from your horizon (technically not horizon but the celestial equator) and how far west or east you need to point your telescope.  The former is the Altitude, and the latter the Azimuth coordinate.

After looking at a dozen explanations of the conversion method, I created this excel sheet where you input certain parameters and get the Altitude and Azimuth outputs. The whole thing is based on the great explanation at http://www.stargazing.net/kepler/altaz.html.

Download the excel

Note that there are progams such as “Stellarium” which automatically do this for you. The excel sheet is probably a handy tool in certain cases if you don’t want a full fledged application running, and is likely to be heplful to understand the conversion method.

Bug reports are welcome. Next version should contain things like automatic time input (using your PC’s system clock) and possibly a version for mobile phones!