Astroparticle Physics

401C Definition of Equatorialand Galactic Coordinates“In the technical language of astronomy, therichness of the star field depends mainly onthe galactic latitude, just as the Earth’s climatedepends mainly on the geographic latitude,and not to any great extent on the longitude.”Sir James JeansOptical astronomers mostly prefer equatorial coordinates,like right ascension and declination, while astrophysicistsoften use galactic coordinates.For equatorial coordinates, which are centered on theEarth, the plane of the Earth’s equator is chosen as the planeof reference. This plane is called the celestial equator. Thereis another plane which is defined by the motion of the Eartharound the Sun. This plane has an inclination with respect tothe celestial equator of 23.5 degrees. The plane of the Earth’sorbit is called the ecliptic. At the periphery these two planesintersect in two points. The one where the Sun crosses thecelestial equator from the south is the vernal equinox.The coordinate measured along the celestial equator eastwardfrom the vernal equinox is called right ascension,usuallynamed α. The distance perpendicular to the celestialequator is named declination, named δ. Right ascensionvaries from 0 to 360 degrees, or sometimes – for convenience– from 0 to 24 hours. Declination varies from −90to +90 degrees (see Fig. C.1).equatorial coordinateseclipticright ascensiondeclinationnorth celestial poledeclinationδequatoreclipticvernalequinoxright ascentionαFig. C.1Definition of the equatorialcoordinates right ascension anddeclination