Rensselaer Astrophysical Society

When Astronomers want to talk about where in the sky a particular object is, they use a system of coordinates similar to latitude and longitude. Imagine, as the ancient astronomers believed, that all of the stars are located on a sort of two dimensional sheet a certain distance away from Earth, like flies on fly- paper. A star's actual distance from us doesn't enter into it's coordinates. This sheet surrounds the Earth, which, of course, is located in the center of the universe. Now imagine taking the lines of latitude and longitude (the astronomers who devised this system did understand the Earth was round), and extending them outward into space until they overlay the stars on the celestial sphere (the sheet). This, in essence, describes the celestial coordinates. Instead of latitude, we say declination, and instead of longitude, we say right ascension. You can remember these terms by understanding that declination defines the angle of declination from the north pole, and right ascension describes the direction of motion of the stars when facing north (east to west, or right to left).

Here's the tricky part. The Earth rotates around inside this celestial sphere once every 23 hours and 56 minutes. This means that while the coordinates of the stars don't change, the coordinates that you see overhead do. So how do you know what range of coordinates is visible at a given time? You could figure it out if you know the sidereal time. Sidereal time is the time is takes the stars to go around, as opposed to the time it takes the sun to go around (23h 56m as opposed to 24h) Let's say you want to know if you can see Vega, the brightest star in the constellation Lyra. It's coordinates are 18 hours 36 minutes right ascension, +38 degrees 47 minutes declination. The declination angles visible don't change with time, only with geographical position. Since we're around 35 or 40 degrees north latitude, Vega passes close to right overhead (the zero degree declination line lies directly over the equator, + corresponds to north, - corresponds to south). The question is when. At zero hours in sidereal time, zero hours right ascension is directly over the prime meridian. Here in New York, we're five hours earlier than Greenwich, England. So when it's 11:30 PM (23:30) sidereal time there, it's 6:30 PM (18:30) sidereal time here, and Vega should be right overhead. Unfortunately, the sidereal day is a different length then the solar day, and your watch is calibrated for a solar day. So it's probably easier to just go outside and look. After some time, you become accustomed to what parts of the sky are visible at what parts of the year.

You'll find, however, that celestial coordinates are a very convenient way to locate certain objects in a telescope. If you want to find a star that you can't simply recognize by looking, or if you want to find on object, a galaxy for example, that can't be seen with the naked eye, then you can look up the coordinates, then use the setting circles to aim the telescope. It's easy to see how setting circles work once you understand celestial coordinates.