Declination is the measure in degrees of separation between the celestial equator, and a point on the celestial sphere, and it is the celestial sphere’s corollary to terrestrial latitude. Like lines of latitude, lines of declination usually run horizontally, or east-west on a chart. Declination lines more distant from the celestial equator, or 0° declination, have higher numbers. North declination is identified as a positive number (+40°) while south declination is identified as a negative number(-40°).
To illustrate the relationship between declination and latitude, consider the terrestrial equator, which is 0deg latitude. The sum of all zenith points at the terrestrial equator define the celestial equator. Similarly, zenith for an observer in Ashton, Maryland (N39.15° latitude) is +39.15°. This can be observed on the sky chart below that prepared for Ashton, Maryland that was previously examined. Note that the zenith falls just south of the declination line that represents +40°.
© Jim Johnson, 2015.