July 8, 2025. Astronomy Concepts 4. Right Ascension and Declination

Right ascension (RA) and declination (Dec) are the celestial equivalents of longitude and latitude on Earth, used by astronomers to pinpoint objects in the night sky on the imaginary celestial sphere.

Declination (Dec) is like latitude. It measures an object's angular distance north or south of the celestial equator, which is Earth's equator projected into space. Declination is expressed in degrees, arcminutes, and arcseconds. Objects north of the celestial equator have positive (+) declination (up to +90° at the North Celestial Pole), while those south have negative (-) declination (down to -90° at the South Celestial Pole).

Right Ascension (RA) is like longitude. It measures an object's angular distance eastward along the celestial equator from a specific starting point called the vernal equinox (where the Sun crosses the celestial equator in spring). RA is measured in hours, minutes, and seconds (from 0 to 24 hours), because the sky appears to rotate 360 degrees in 24 hours. One hour of RA is the equivalent 15 degrees.

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