Coordinate converter

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This page presents an interactive tool to convert between equatorial coordinates and altitude and azimuth. Follow the three steps below to enter the necessary information and then click on the convert button.
1. Enter the object's coordinates
Enter the coordiantes to the object, Right ascension in hours and Declination in degrees, both as decimals:
Right Ascension: Declination:
2. The observer's location
Next, specify the geographic latitude and longitude of your observatory, again using decimals. Northern latitudes are positive, East longitudes are positive:
Latitude: Longitude:
3. The time of observation
Finally specify the date and time of observation, the date should be in the form DD/MM/YYYY. The time should be entered as HH:MM in the 24 hour clock. As you have already entered your longitude, you need to enter Greenwich time:
Date: Time:

About the conversion procedure
The conversion procedure used is based on standard formulae that relate equatorial coordinates with altitude and azimuth. These formulae can be found in many observing manuals and handbooks, for example the section B61 of the Astronomical almanac. No attempt has been made to apply any correction for diurnal parallax or aberration, which typically amounts to a fraction of a second of arc.

Likewise, the conversion is not compensated for refraction caused by the atmosphere, which would additionally require figures for the temperature and barometric pressure at the time of observation. If the altitude of the object you are observing is less than 15 degrees then it is worthwhile to perform this correction.

As an intermediate step in the coordinate conversion, it is necessary to find the sidereal time for any given date and time. the routine for finding this has been based on that in "Astronomy on the personal computer", by Montenbruck and Pfleger. Use has also been made of a procedure to find the Julian day number for a given date in the Gregorian calendar, which is based on that in "Calendrical calculations" by Dershowitz and Reingold.