The longitudinal (east-west) coordinate in the
equatorial coordinate system. Right ascension, denoted by the
Greek letter alpha, is measured in units of hours,
minutes, and seconds. There are 60 seconds in one minute and 60 minutes
in one hour. Its origin (0h) is at the vernal equinox,
i.e. the Sun is at 0h 0m 0s at the moment of the vernal equinox, on or
about March 21. A full
east-west circle around the sky takes up 24h in right ascension, with each
hour consisting spatially of 15 degrees (note that 24h * 15 degrees =
360 degrees). The
coordinate system increases eastwards, so that the Sun
is at 6h 0m 0s at the summer solstice, at 12h 0m 0s at the autumnal equinox,and at 18h 0m 0s at the winter solstice. Due to the earth's
precession, the Sun is now located in Pisces at the vernal equinox
rather than in Aries as it was in antiquity.
The right ascension of the north-south line -- the meridian -- drawn
directly overhead at any given moment is the local sidereal time.
A convenient way of remembering what constellations are in the sky at
night is simply to add 12h to whichever solstice or equinox you are closest
to. This tells you what right ascension will be overhead at midnight.
For example, if you are stargazing in June or July, near the
summer solstice, 6h + 12h yields 18h.
The latitudinal (north-south) coordinate in equatorial
coordinates is the declination.