Right Ascension - Explanation

Explanation

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. For right ascension, the primary direction is known as the vernal equinox or 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. Right ascension is measured continuously in a full circle towards the east.

Any units of angular measure can be used for right ascension, but it is customarily measured in hours ( h ), minutes ( m ), and seconds ( s ), with 24h being equivalent to a full circle. This is a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. Since a complete circle has 360°, 1h of right ascension is equal to 1⁄₂₄ of this, or 15 degrees of arc, 1m of right ascension is equal to 15 minutes of arc, and 1s of right ascension is equal to 15 seconds of arc.

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 = 01h 30m 00s is on the meridian, then a star with RA = 20h 00m 00s will be on the meridian 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 angular distance of an object westward from the local meridian.