Recall that latitude and longitude are the coordinates used to locate objects on earth’s surface. Any point on earth can be identified by these coordinates.
Astronomers use a similar system to map the sky, as follows:
Declination and right ascension are the astronomical equivalents of latitude and longitude.
Dec (Declination) = Latitude
RA (right ascension) = Longitude
Dec is measured in degrees, just as latitude is measured in degrees north and south of the equator, which starts at 0 degrees.
RA is measured in hours and minutes, not in degrees. This is due to the practical nature of how the earth rotates.
The earth rotates 360 degrees in 24 hours. This means it rotates 15 degrees every hour (360 degrees / 24 hours = 15 degrees per hour).
Every hour of RA corresponds to 15 degrees of “celestial longitude.” Another way to put this is, everything in the night sky moves from east to west and an angular speed of 15 degrees per hour.
Every hour of RA is subdivided into 60 minutes and each minute is subdivided into 60 seconds.
The line of 0 hours RA passes through the vernal equinox.
The vernal equinox is the intersection of the celestial equator and the ecliptic.
The ecliptic is the path of the sun through the sky. When the ecliptic intersects the celestial equator, you have vernal equinox.