Star Altitude and Azimuth Calculator (RA/Dec to Horizon Coordinates)

What this tool does

Star catalogues give a position fixed on the sky, right ascension and declination, which does not depend on where or when you look; your telescope needs the position on your own sky, altitude above the horizon and azimuth. The link is the hour angle, the local sidereal time minus the right ascension: zero when the object crosses the meridian and at its highest, negative before, positive after. An object never rises above 90° − |latitude − declination|, and if that is below zero it never rises at all from your latitude. The sidereal time of your location can be found with the sidereal-time calculator.

Formula

H = LST − RA · sin h = sin φ sin δ + cos φ cos δ cos H · tan A = −cos δ sin H ÷ (sin δ cos φ − cos δ cos H sin φ)

Variables

Worked example

Limitations

• Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
• The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.