Right ascension

An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from a given latitude on Earth, where the celestial equator intersects the horizon at an oblique angle.

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.

In astronomy, the equinox refers to a direction in space that is coincident with the intersection of the celestial equator and the plane of Earth's orbit. To measure right ascension, 0 hours is in the direction of the Sun from Earth at the March equinox. This is currently located in the constellation Pisces. (Though defined according to the position of Earth and Sun at the March equinox, nevertheless the direction 0 hours persists throughout the year. Meanwhile, the right ascension of the Sun advances 6 hours per season, at 0 hours at the March equinox, at 6 hours at the June solstice, etc.) Right ascension is given as a number of hours after the equinox crosses the observer's meridian that the celestial body will cross the observer's meridian.

The Sun crosses the observer's meridian at local noon. At the March equinox, local noon coincides with 0h right ascension. Celestial bodies with right ascensions greater than 6 hours and less than 18 hours will be widely visible, and a star that is crossing the meridian at midnight that same night will have a right ascension of 12 hours.

At the June solstice, the Sun has a right ascension of 6^h, and so bodies with right ascensions greater than 12 hours and less than 24 hours will be widely visible. At the September equinox, the sun has a right ascension of 12^h, and so on.

For a specific star example, η Psc is at RA = 01^h 31^m 29.01026^s. At the March equinox, this star crosses a meridian on earth approximately 1.5 hours after the sun crosses the same meridian, so observers on Earth will not be seeing it. At the September equinox, the star continues to cross the meridian at approximately 1.5 hours after the equinox, but now the Sun is at 12 hours after the equinox. This means η Psc will now cross the meridian approximately 13.5 hours after local noon (so about 1.5 hours after local midnight) and will be visible.

Any angular unit could have been chosen for right ascension, but it is customarily measured in hours (^h), minutes (^m), and seconds (^s), with 24^h 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 24^h of right ascension or 360° (degrees of arc), ⁠1/24⁠ of a circle is measured as 1^h of right ascension, or 15°; ⁠1/1440⁠ of a circle is measured as 1^m of right ascension, or 15 minutes of arc (also written as 15′); and ⁠1/86400⁠ of a circle contains 1^s of right ascension, or 15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400^s, or 24 × 60 = 1440^m, or 24^h.

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 = 1^h 30^m 00^s is at its meridian, then a star with RA = 20^h 00^m 00^s 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 24^h. 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.