Astronomers use different coordinate systems depending on the type of observation or research being conducted. Each system is optimized for a specific reference frame. Just as geographers use different map projections for different purposes, astronomers choose coordinate systems that make their particular work most convenient. The three systems supported by this tool — equatorial, galactic, and ecliptic — are the most widely used in modern astronomy.
The equatorial system is the most commonly used in observational astronomy. It is based on the projection of Earth's equator onto the celestial sphere. The two coordinates are:
Star catalogs, telescope goto systems, and planetarium software all use equatorial coordinates. When you look up a star's position, you will almost always find it given in RA and Dec.
The galactic coordinate system is centered on the Milky Way galaxy. The reference plane is the galactic plane — the disk of the Milky Way — and the reference direction is toward the galactic center in Sagittarius. The two coordinates are:
Galactic coordinates are used extensively in studies of the Milky Way's structure, interstellar medium, and the distribution of stars and gas clouds. Radio astronomers and researchers studying galactic structure routinely work in this system.
The ecliptic system is based on the plane of Earth's orbit around the Sun (the ecliptic plane). The two coordinates are:
The ecliptic system is particularly useful for studying solar system objects — planets, asteroids, and comets — because they all orbit near the ecliptic plane and therefore have small ecliptic latitudes. It is also used in astrological calculations.
Select the conversion direction using the tabs at the top of the tool. Enter the source coordinates and click "Convert Coordinates." The result shows both the decimal degree values and the equivalent degrees-minutes-seconds (DMS) or hours-minutes-seconds (HMS) notation.
For equatorial coordinates, RA is entered in decimal hours (e.g., 5.5753 for Orion's Betelgeuse, which is at 5h 55m 10s). Declination is entered in decimal degrees (positive for north, negative for south).
Different astronomical catalogs use different coordinate systems. The 2MASS infrared catalog uses equatorial coordinates, while many radio surveys use galactic coordinates. When cross-referencing objects between catalogs, coordinate conversion is essential.
Converting a star's equatorial coordinates to galactic coordinates immediately tells you its position relative to the Milky Way. A star with b close to 0° lies near the galactic plane; one with b close to ±90° is near a galactic pole, far from the disk. This is useful for understanding the stellar population and interstellar extinction toward any given object.
Some older telescope control systems or specialized instruments use ecliptic or galactic coordinates. Converting from a star catalog's equatorial coordinates to the required system allows you to input targets correctly.
| Object | RA (h) | Dec (°) | l (°) | b (°) |
|---|---|---|---|---|
| Galactic Center | 17.76 | -28.94 | 0.0 | 0.0 |
| Orion Nebula | 5.59 | -5.39 | 209.0 | -19.4 |
| Andromeda Galaxy | 0.71 | 41.27 | 121.2 | -21.6 |
| North Galactic Pole | 12.86 | 27.13 | 0.0 | 90.0 |