RA/Dec to Alt/Az Converter

Convert right ascension and declination into local altitude and azimuth for an observer location and UTC date and time.

Right ascension (decimal hours)For example, Betelgeuse is near 5.59 h.

Declination (degrees)North is positive. South is negative.

Observer latitude

Observer longitudeEast is positive. West is negative.

UTC date and time

NESW

From catalog coordinates to your horizon

Right ascension and declination describe a direction on the celestial sphere. To find where that direction appears for an observer, the calculation first determines Greenwich Mean Sidereal Time, adjusts it for longitude to obtain Local Sidereal Time, and derives the object's hour angle.

Altitude measures the angle above or below the horizon. Azimuth is measured clockwise from true north. The calculation uses the standard spherical-trigonometry transformation from equatorial to horizontal coordinates.

This tool uses mean sidereal time and does not apply atmospheric refraction, precession to another epoch, nutation or proper motion. It is appropriate for learning and ordinary planning, while precision work should use a current astrometric solution.

Reference

The coordinate definitions and transformation approach follow the U.S. Naval Observatory guidance for computing altitude and azimuth from sidereal time.