# Right Ascension (RA)

by on December 2, 2010

Want to stay on top of all the space news? Follow @universetoday on Twitter

Equatorial Coordinates

Right ascension, often abbreviated RA, is one of the two astronomical coordinates of a point on the celestial sphere. The other is declination.

Right ascension is the celestial equivalent of longitude. Both right ascension and longitude measure an angle that increases toward the east as measured from a zero point on an equator. For right ascension, the zero point is known as the first point of Aries. The first point of Aries is the place in the sky where the Sun crosses the celestial equator during the March equinox.

Right ascension is customarily measured in hours, minutes, and seconds, with 24 hours being equivalent to a full circle. The method was chosen because the Earth rotates at an approximately constant rate. Since a complete circle has 360 degrees, an hour of right ascension is equal to 1/24 of a circle, or 15 degrees of arc. A single minute of right ascension is equal to 15 minutes of arc and a second of right ascension equal to 15 arcseconds. Right ascension can be used to determine a star’s location and how long it will take for a star to reach a certain point in the sky.

The tilt of the Earth’s axis gradually rotates over time(precession). This effect causes the right ascension and declination of a celestial object to change noticeably over time(usually only decades). This makes coordinates relative to the year that they are measured, so astronomers specify equatorial coordinates with reference to a particular epoch. An astronomer comparing coordinates from different epochs must mathematically rotate one of the coordinate systems to match the other, or rotate both to match a common epoch. The current standard epoch is J2000.0(January 1, 2000 at 12:00 TT). The J means the Julian epoch.

Without right ascension and declination astronomers would not be able to pinpoint an object in space. That would mean constant duplication of research and endless headaches. Thankfully, that is not something modern scientists have to go through.

We have written many articles about right ascension for Universe Today. Here’s an article about conjunction, and here’s an article about declination.