What are RA and DEC?

RA (right ascension) and Dec (declination) are the coordinates on the sky that correspond to longitude and latitude on Earth. RA measures east and west on the celestial sphere and is like longitude on the Earth. Dec measures north and south on the celestial sphere and is like latitude on the Earth.

RA is measured in hours minutes and seconds of time. The reason for this is the sky turns once a day to the west as the earth rotates to the east. The celestial sphere moves one hour of RA west per hour of time and 24 hours of RA during the course of the whole day. Since this is a 360 degree rotation, one hour of RA is equal to 15 degrees of turning (360/24 = 15). Just like lines of longitude, RA lines are also great circles converging on the north and south celestial poles.

Longitude has the Greenwich meridian as the zero line dividing east and west. On the sky, the zero meridian in RA is labeled 00h 00m 00s. It intersects the celestial equator at a point called the vernal equinox (where the Sun crosses the celestial equator in late March of each year).

Measurements north and south on the sky are called declinations (commonly abbreviated as Dec, DEC or dec). Just like latitude, declination is measured in degrees, minutes and seconds north (positive) and south (negative), with 60 minutes in each degree and 60 seconds in each minute of declination.

The celestial equator is 0 degrees declination, and the north and south celestial poles are +90 and -90 degrees. The North Star Polaris is nearly at the north celestial pole at +89.2 degrees. The celestial equator is a full 360 degree circle splitting the celestial sphere into the northern and southern celestial hemispheres or simply the northern and southern sky. It’s the projection of our equator in space. It is directly overhead at the Earth’s equator.

You can use a star’s declination to figure out how high it will get in the sky. The star Vega has a declination of +39 degrees, so it passes directly overhead at north latitude 39 degrees on the Earth (approximately the latitude of Denver). At 47 degrees north latitude (approximately the same latitude of Seattle or Vancouver), Vega will never reach 90 degrees altitude, but will peak out eight (47-39) degrees south of the zenith.