Version 0.70 PAGE 5 The Equatorial Coordinate System: Declination and Right Ascension Positions are always measured with respect to something.  For instance, latitude and longitude are measured with respect to the earth’s equator and the Greenwich meridian.  Coordinates on a piece of graph paper are measured with respect to the corner or to the origin of the graphs.  The coordinates that are commonly used to specify star positions in astronomy indicate the star’s position with respect to the celestial equator, an imaginary line in the sky that runs above the earth’s equator, and are therefore called the equatorial coordinate system.  The two coordinates in the equatorial system are called Declination and Right Ascension. The lines of declination are like lines of latitude on the earth, and are designated by their angular distance north or south of the celestial equator, measured in degrees (°), arcminutes ('), and arcseconds (").  There are 360 degrees in a circle, 60 minutes in a degree, and 60 seconds in a minute.  A star with a declination of +45° 30' lies 45 degrees, 30 minutes north of the celestial equator; negative declinations are used for an object south of the equator. Right ascension lines are like lines of longitude on the earth, running through the north and south celestial poles perpen- dicular to the lines of declination.  They designate angular distance east of a line through the vernal equinox, the position of the sun when it crosses the celestial equator on the first day of spring.  Right ascension is measured in hours (^H), minutes (^m) and seconds (^s).  This may sound strange, but an hour of right ascension is defined as 1/24 of a circle, so an hour of right ascension is equal to 15 degrees.  There are 60 minutes in an hour, and 60 seconds in a minute of right ascension.  A star with a right ascension of 5 hours would be 50 hours, or 75 degrees, east of the line of right ascension (0^H) that runs through the vernal equinox. There are many catalogs of objects in the heavens which list their right ascensions and declinations.  It’s impossible to list all the stars in the sky, so a catalog usually contains stars that are selected by astrono- mers for a particular purpose.  One of the most important catalogs is called the FK5 Catalog, because it is one of the funda- mental catalogs used as a reference for measuring the positions to other stars in the sky (see the next section of this manual).  The FK5 Catalog only contains 3522 stars, all of them rather bright.  The right ascensions and declinations of the stars in the FK5 Catalog have been especially carefully measured and re- measured so that they can be relied upon as standard reference points for the measurement of the positions of other objects in the sky. Another useful catalog which we shall use in this exercise is the Hubble Space Telescope Guide Star Catalog, (GSC). The GSC lists almost all the stars in the sky that are brighter than apparent magnitude 16, which is almost ten thousand times fainter than the faintest star you can see with your naked eye.  There are coordinates of almost 20 million stars in the GSC, so many that the full catalog requires two CD-ROMS to hold it.  The GSC has been one of the most useful catalogs for astronomers in recent years. There are so many stars in it, scattered all over the sky, that you can practically count on having several GSC stars with known coordinates anywhere you look in the sky.  On the other hand, there are so few stars in the FK5 Catalog, that it’s rare that a FK5 star will be in the same direction as an object of interest. Figure  2 The  Equatorial  Coordinates