RA & Dec: Driving the SDSS Telescope
Understanding Right Ascension and Declination
If you want to go someplace in a car, it is handy to have a map or a GPS system to assist you. The same is true when you drive a telescope to a position on the sky. Astronomers refer to this as slewing the telescope, and to do so you need to define the location. Astronomers use a system of numbers to define the location of any object on the dome of the sky. Here’s how it works:
Lines to Guide Us
Imagine a giant light bulb at the center of a transparent Earth that projects the lines of longitude and latitude onto the night sky. You now have a grid on the sky that you can use to define the location of any object.
Astronomers give these projected lines unique names. Lines of longitude are now called right ascension (RA) and the lines of latitude are called declination (Dec). Because lines of right ascension appear to cross the sky from east to west in time with the rotation of the earth, they are measured in divisions of hours, minutes, and seconds. There are 24 hours in a day, so there are 24 hours of right ascension on the sphere of the sky. Likewise, each hour is divided into 60 minutes of RA, and each minute is divided still further into 60 seconds.
Declination is a much easier measurement to visualize since the distance between these lines remains uniform as you move north and south across the sky. We measure lines of declination using degrees, minutes, and seconds as we would most curved surfaces. The equator projected onto the night sky is referred to as the celestial equator and given the declination of zero degrees. All lines of declination north of the celestial equator are positive measurements from zero at the equator to +90 at the celestial north pole. Lines of declination south of the celestial equator are negative measurements from zero to -90 at the celestial south pole.
Computers and RA and Dec
The Sloan Digital Sky Survey (SDSS) stores hundreds of pieces of information about more than four million objects in organized collections of tables, the SDSS database. The database is stored and accessed by computers. When programming computers to search and retrieve information, it is not convenient to have one measurement contain multiple units such as we find in traditional RA and Dec measurements. It is for this reason that RA and Dec measurement are reported in the SDSS as decimal degrees. Let’s see how this works.