Positional Astronomy:
Coordinate systems:
the second equatorial or "RA-dec." system

The first equatorial system (HA and declination) is still tied to the observer's here-and-now. Now we change the zero-point for our coordinates. We choose a fixed point on the celestial equator, called the vernal equinox, or the First Point of Aries.

The declination of object X measured in the same way as before. The Right Ascension or RA of object X is the angle along the celestial equator measured eastwards from the vernal equinox to the meridian of X. Like HA, RA is measured in hours 0-24h, but it goes in the opposite direction.

Comparison of these celestial coordinate systems with the terrestrial system:
terrestrial                 alt-az                  HA/dec.                    RA/dec.
equator                             horizon                   celestial equator             celestial equator
North Pole                      zenith                       North Celestial Pole    North Celestial Pole
South Pole                      nadir                          South Celestial Pole   South Celestial Pole
latitude                             altitude                     declination                   declination
parallel of latitude        parallel of altitude parallel of declination parallel of declination
co-latitude                     zenith distance        North Polar distance  North Polar distance
meridian of longitude  vertical circle           meridian                        meridian
Greenwich Meridian    Principal Vertical   celestial meridian        celestial meridian
longitude                          azimuth                     Hour Angle                  Right Ascension

The Right Ascension and declination of an object do not normally change over short periods of time; the Hour Angle changes constantly with time. Consequently we have to find a way of defining the time.

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