Right Ascension
Right Ascension is similar to longitude on the Earth, however it is numbered differently. Longitude starts at 0 degrees at the Prime Meridian and goes to 180 degree East or 180 degrees West. Since the Earth is a sphere, 180 E is equal to 180 W. Right Ascension, on the other hand, is measured in hours instead of degrees, for historical reasons. It starts at 0 hours at the "First Point of Aries" and goes all the way around to 24 hours which is at the same place as 0 hours. So a star that has an RA of 12 hours is on the opposite side of the sky from one that is at 0 hours. The tricky part about RA is that the First Point of Aries doesn't stay still like the Prime Meridian does. As the Earth rotates, the First Point of Aries moves. To figure out where it is at a given time and date, you need to use some trigonometry.