Celestial Coordinate Systems

Celestial Coordinates refer to a mathematically logical and physically convenient system of coordinates for locating an object on the celestial sphere. A particular celestial coordinate system is usually characterized by the point chosen for observation (the origin) and a reference plane used. For example, geocentric coordinates use the center of Earth as the origin, while heliocentric coordinates use the Sun as the origin.

Geocentric equatorial coordinates use the celestial equator as a convenient reference plane. The equatorial coordinate system is the most commonly used coordinate system in

Astronomy. The coordinates commonly used in this system are the right ascension (RA), which is roughly the equivalent of longitude on Earth, and the declination (δ), which is roughly the equivalent of latitude on Earth. The ecliptic coordinate system uses the ecliptic as the fundamental reference plane. Within this system astronomers define celestial latitude (β) as the angular distance of a celestial body from 0° to 90° north (considered positive) or south (considered negative) of the ecliptic and celestial longitude (λ) as the angular distance of a celestial body from 0° to 360° measured eastward along the ecliptic to the intersection of the body’s circle of celestial longitude. The vernal equinox is taken as 0°. Sometimes astronomers call these coordinates ecliptic latitude and ecliptic longitude.

In a topocentric coordinate system astronomers use a point on Earth’s surface as the origin. A horizontal coordinate system represents one type of topocentric coordinate system. The horizontal system typically specifies the angular position of a celestial object relative to an observer’s horizon at a given time. Often horizontal coordinates are expressed in terms of altitude (angular distance above the horizon) and azimuth (the clockwise bearing of an object from north).

In a heliocentric coordinate system the center of the Sun serves as the origin, and the ecliptic generally serves as the reference plane. Astronomers often use heliocentric latitude and heliocentric longitude to express the relative positions of celestial objects within the solar system. Heliocentric latitude (b) describes the angular position (0° to 90°) of an object north (considered positive) or south (considered negative) of the ecliptic when viewed from the center of the Sun. Heliocentric longitude (l) describes the angular position (0° to 360°) measured clockwise around the ecliptic starting at the vernal equinox.

Finally, astronomers develop galactic coordinates with reference to the plane of the Milky Way galaxy - that is, they describe the position of a celestial object in terms of its galactic latitude and longitude with respect to the galactic equator. Galactic latitude (b) gives the angular position (0° to 90°) of a celestial object north (considered positive) or south (considered negative) of the galactic equator . The galactic equator is the great circle on the celestial sphere that denotes the plane of the Milky Way galaxy. The galactic longitude (l) provides a measure of the angular position (0° to 360°) of a celestial object when measured clockwise along the galactic equator, starting from the point that marks the galactic center (or nucleus) of our galaxy (as seen from Earth), which is some 26,000 light-years away in the constellation Sagittarius.

Questions to Ponder

• Why do we need so many different Celestial Coordinate systems?
• What coordinate system should we use when we consider the Entire Solar System?