Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.

Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east. As seen from Earth (except at the poles), objects noted to have 12h RA are longest visible (appear throughout the night) at the March equinox; those with 0h RA (apart from the sun) do so at the September equinox. On those dates at midnight, such objects will reach ("culminate" at) their highest point (their meridian).

How high depends on their declination; if 0° declination (i.e. on the celestial equator) then at Earth's equator they are directly overhead (at zenith). Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours (h), minutes (m), and seconds (s), with 24h being equivalent to a full circle.

Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates. The line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360° (degrees of arc), 1/24 of a circle is measured as 1h of right ascension, or 15°; 1/1440 of a circle is measured as 1m of right ascension, or 15 minutes of arc (also written as 15′); and 1/86400 of a circle contains 1s of right ascension, or 15 seconds of arc (also written as 15″).

full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400s, or 24 × 60 = 1440m, or 24h. Because right ascensions are measured in hours (of rotation of the Earth), they can be used to time the positions of objects in the sky. For example, if a star with RA = 1h 30m 00s is at its meridian, then a star with RA = 20h 00m 00s will be on the/at its meridian (at its apparent highest point) 18.5 sidereal hours later.

Sidereal hour angle, used in celestial navigation, is similar to right ascension but increases westward rather than eastward. Usually measured in degrees (°), it is the complement of right ascension with respect to 24h. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures the angular distance of an object westward from the local meridian.