One element of the astronomical coordinate system on the sky that is used by astronomers. Declination, which can be thought of as latitude on the earth projected onto the sky, is usually denoted by the lower-case Greek letter delta and is measured north (+) and south (-) of the celestial equator in degrees, minutes, and seconds of arc. The celestial equator is defined as being at declination zero (0) degrees; the north and south celestial poles are defined as being at +90 and -90 degrees, respectively. When specifying a comet's location on the sky, one must state the right ascension and declination (with equinox), along with date and time (since a comet moves with respect to the background stars). For examples of how right ascension (R. A. ) and declination (Decl. ) appear on a star atlas, see the Millennium Atlas.