The longitudinal (east-west) coordinate in the astronomical 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.

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