Astronomical Coordinates

The Celestial Sphere

Ancient astronomers perceived the sky as a large sphere with the Earth at its center. While we know the stars are at vast distances, this celestial sphere concept remains useful for describing positions in the sky. Astronomers have created a grid of reference lines and points on the celestial sphere to describe the position of each star, planet, and galaxy.

Altazimuth Coordinates

Altazimuth coordinates describe the position of an object with reference to the local horizon:

• Azimuth is measured along the horizon, starting at 0° in the north, through 90° in the east, 180° at south, and 270° in the west.
• Altitude is measured from 0° at the horizon to 90° at the zenith directly overhead.

Altazimuth coordinates are local - they change as the Earth rotates and depend on your location.

Equatorial Coordinates

Equatorial coordinates are based on the Earth's poles and equator, projected onto the celestial sphere:

• Right Ascension (RA) - analogous to longitude, measured eastward in hours from the vernal equinox (0h to 24h).
• Declination (Dec) - analogous to latitude, measured in degrees from the celestial equator (+90° at the north celestial pole to -90° at the south celestial pole).

Unlike altazimuth coordinates, an object's RA and Dec remain fixed as the Earth rotates.

Ecliptic Coordinates

The ecliptic is the Sun's apparent annual path against the background stars. Ecliptic coordinates are defined by the plane of Earth's orbit rather than the equator. The planets are always found within about 18 degrees of the ecliptic. The twelve constellations along the ecliptic form the zodiac.