Right ascension

Ra and dec demo animation small

Ra and dec on celestial sphere

Old RA diagram

Stars and ra

Right ascension (RA) is one of the two coordinates of the equatorial coordinate system, the other being declination, used to specify the positions of celestial objects. It is comparable to longitude in the geographic coordinate system used on Earth's surface. Measured in hours, minutes, and seconds, right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle passing through the object.

Definition

Right ascension is the celestial equivalent of terrestrial longitude. However, while Earth's longitude is measured in degrees, minutes, and seconds of arc, right ascension is measured in time units: hours, minutes, and seconds. This is because the celestial sphere is imagined to rotate around the Earth in a period of 24 hours, parallel to the equator, making it convenient to express the position of celestial objects in time units. The complete circle of 360 degrees is therefore divided into 24 hours, with each hour representing 15 degrees of arc. One minute of right ascension equals 15 minutes of arc, and one second of right ascension equals 15 seconds of arc.

Usage

Right ascension is used in conjunction with declination to specify the location of objects on the celestial sphere. In the equatorial coordinate system, declination measures the angular distance of an object north or south of the celestial equator, analogous to latitude on Earth. Together, right ascension and declination allow astronomers to pinpoint the exact location of celestial objects.

Measurement

The point of zero right ascension is the vernal equinox, the place in the sky where the Sun crosses the celestial equator from south to north in March each year. As the Earth orbits the Sun, the vernal equinox point moves westward along the ecliptic, completing a full circle through the stars in about 26,000 years, a motion known as the precession of the equinoxes.

Right ascension is measured eastward from the vernal equinox. Because the Earth's rotation axis and orbit are stable over short time periods, the vernal equinox serves as a fixed point from which measurements can be made. However, due to precession, the vernal equinox slowly shifts its position relative to the background stars, requiring occasional adjustments to the celestial coordinate system.

Observational Techniques

To measure an object's right ascension, astronomers use a telescope equipped with a sidereal clock or a computerized system that can track celestial objects. By aligning the telescope with the celestial equator and measuring the time it takes for an object to cross the meridian (the line running from north to south through the zenith), astronomers can calculate the object's right ascension.

Importance in Astronomy

Right ascension and declination form the basis of the celestial coordinate system, which is essential for mapping the skies and tracking celestial objects. This system allows astronomers to share the positions of objects in the sky with precision and to predict where those objects will be at any given time.