Coordinate Systems

Astronomers describe the position of objects in the sky by using coordinates. Although astronomical objects are at a wide range of distances from us, to locate them in space, we only need to know their direction. The easiest way to do this is to use coordinate systems which assume that all objects in space are at the same distance, on the inside of an imaginary celestial sphere. A sphere is a three-dimensional object, but its surface is two-dimensional. So, if we imagine that all celestial objects are on the inside surface of a sphere, we only need two measurements to locate an objects position - a latitude and longitude. This is similar to how we measure locations on the Earth's surface.

To make a spherical coordinate system work, we also need to define a line of zero latitude and zero longitude (like the Earth's equator and meridian) as seen from a central location. In astronomy three different coordinate systems are commonly used, each with a different central location in space. These three systems are the equatorial, ecliptic and galactic coordinate systems which use the Earth, the ecliptic plane (the path the Earth takes around the Sun), and the center of our galaxy, respectively, as their central locations.

Equatorial Coordinate System

In the equatorial system, the Earth's equator and poles are projected outward into space to form a celestial sphere with the Earth at the center. The projection of the Earth's equator onto this sphere is called the celestial equator. Positions of astronomical objects are described by their right ascension or RA (the angle eastward from the vernal equinox) and declination or DEC (the angle above or below the celestial equator). Right ascension is measured in hours, minutes and seconds rather than degrees (as we do with the Earth's longitude). One complete circle is 24 hours rather than 360 degrees. Declination is measured in the same way we measure the Earth's latitude and ranges between -90 and +90 degrees measured from the celestial equator. The equatorial system is the one most commonly used in astronomy.

Ecliptic Coordinate System

In the ecliptic system, latitude is measured with respect to the ecliptic plane and position is measured by ecliptic latitude (elat) and ecliptic longitude (elon). Both ecliptic latitude and longitude are measured in degrees. This system uses the plane of the Earth's orbit around the Sun as a base. Ecliptic coordinates are useful for describing the position of objects within our solar system. Most other objects in space are so far away that their positions in the sky do not change much on a human time scale. This is not true for close objects, like the planets, moons, the Sun, asteroids and comets. So ecliptic coordinates can be more useful than the equatorial system for solar system objects.

Galactic Coordinate System

The galactic system uses galactic latitude (glat) and galactic longitude (glon) to define the postion of objects in space relative to the galactic center (the center of our galaxy - the Milky Way). Galactic coordinates are measured in degrees. The galactic coordinate system is used to see how astronomical objects are distributed with respect to the galactic plane.