That intersection where the Sun appears to cross the celestial equator from south to north is chosen as the reference point and is known as the vernal equinox. Right ascension , designated alpha , is measured along the celestial equator eastward from the vernal equinox. For convenience, right ascension is normally measured in hours and ranges from 0h to 24h. Note that, in the northern hemisphere, right ascension increases clockwise when you are facing north. Declination , designated delta , is measured perpendicular to the celestial equator.
Negative declinations indicate objects south of the celestial equator.
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