# Equatorial Coordinates

We can make maps of Earth from lists of the latitude and longitude of cities and other features.  Similarly, we can make maps of the sky if we have a list of the coordinates of the stars and other objects.  The coordinates that we use for this purpose are called "equatorial coordinates."  They take advantage of the fact that the stars' positions are nearly fixed with respect to a plane drawn through the earth's equator because the earth's axis points at the same part of the sky, the North Star Polaris, all year long.  Just as altitude is measured from the horizon and latitude is measured from Earth's equator, we measure declination from the "celestial equator" defined this way.

Just as latitude is measured from the Earth's equator, declination is measured North (+) and South (-) from the celestial equator.

Just as longitude is measured from the Greenwich meridian, right ascension is measured from the Vernal Equinox.  This is the point in the sky where the sun crosses the Celestial Equator going north.  Although it can be a little confusing, we also call the moment in time this happens, the Vernal Equinox.

Just as longitude is related to time zones, right ascension is related to sidereal time.  For convenience, right ascension is computed in hours, minutes and seconds instead of degrees.

1) Once around the sky is ________ hours of right ascension.

24 Hours
23 Hours, 56 Minutes
360 Degrees
12 Hours

2) A special line in the equatorial system is the celestial equator which has a declination of what?

3) A special point is the vernal equinox which has a right ascension of what?

4) Two other special points are the North Celestial Pole which has a declination of what?  And the South Celestial Pole with a declination of what?

This is the diagram to the last 3 questions.

The Vernal Equinox moves very slowly with respect to the stars (due to precession - the wobble of the Earth as it spins on its axis) and stars also move in space, so the right ascension and declination of a given star will not always stay the same.  Still, these changes are so slow on the human time scale that we use the equatorial coordinates of stars to give their positions (in star catalogues) and to plot up star maps.  It takes 26,000 years for precession to move the Vernal Equinox around the sky once; this means that precession changes the coordinates of stars by about 0.014 degrees/year or 8.4 arcminutes per century -- this fact is important only for pointing moderately large telescopes or identifying a really faint star.

Star catalogues list the right ascension (sometimes written as "RA" or the Greek symbol Alpha) and the declination (sometimes written as "dec" or the Greek symbol delta).  These values are used for making star maps and star charts.  The RA and dec will usually be listed with a date -- 1900, 1950, or 2000 -- indicating that they have been corrected for stellar motions and for precession to be most accurate in that year.