right ascension

Right ascension, one of the coordinates of the equatorial coordinate system, is the celestial sphere's analog to longitude. It is the angle to an hour circle (which is like a meridian), which is an angle measured along the celestial equator eastward from the vernal equinox. It and declination (the analog of latitude) together specify a direction, i.e., a point in the celestial sphere.

Right ascension is generally cited in hours (angular hours), a unit equal to 15 degrees, which is the angular distance the Earth rotates over an hour and is the angle over which a telescope on an equatorial mount must be shifted over the course of an hour to track a star. The angular hours are subdivided into angular minutes and seconds that are 1/60 and 1/3600 of an angular hour. (A degree is similarly divided into the smaller arcminutes and arcseconds, i.e., an angular second is 15 arcseconds.) The hours (or degrees) are counted from a line from the Sun through the Earth's position at the time of the vernal equinox (March equinox), counting to a higher number of hours as you go eastward. For example, within the constellation Orion, Rigel has a right ascension of about 5 hours and 15 minutes and Betelgeuse's is about 5 hours and 55 minutes.

It is significant that the angle measured in hours is between planes through Earth's axis, or equivalently, an angular distance along the celestial equator: two objects differing by an hour are not necessarily separated by an angular distance of 15 degrees; for example, a pair of stars within a degree of the celestial north pole are within two degrees of each other, even if their right ascensions differ by multiple hours.

Hour angle is another type of measure of the placement of an hour circle: there is more than one kind of hour angle, but a common type counts angular hours from the observer's zenith, and counts them westward, i.e., the opposite direction around the celestial sphere as does right ascension. Like the zenith and horizontal coordinates, a star's hour angle is constantly changing through the course of the day.