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Astronomy Coordinate Systems

It is helpful (though incorrect) to imagine all the stars to be fixed onto the inside of a vast crystalline dome - the celestial sphere.
Absolute distances between stars are of less importance than their angular separation in the sky. Adopting the Babylonian system of angular measure, the distance from horizon to zenith (directly overhead) is 90°. From North to South along the horizon is 180°.

The constant proportions of the human body give fairly accurate references that you can use to estimate angular separations in the night sky:

  • Full Moon/Sun's disk = 0.5°
  • Separation of "pointers" in Plough = 5°
  • Width of the Plough = 20°

(all of these are held at arms length)

The Altitude/Azimuth System (Altazimuth)

The Azimuthal angle is the number of degrees measured clockwise from True North to the star's position projected onto the horizon (ranging from 0 to 360°).
The Altitude of the star is its elevation in degrees from the horizon (ranging from -90° to +90°).

This co-ordinate system can only be used if the precise time is also stated, since stars move about the observer.

The Right Ascension/Declination System

The celestial sphere has two poles, both on a line with the Earth's axis - the North Celestial Pole (Marked approximately by Polaris), and the South Celestial Pole. The Earth rotates once every 23 hrs 56 mins within this celestial sphere, which is deemed to be fixed.
The sphere has lines of "celestial latitude" or declination measured in degrees above or below the celestial equator. Declination, ranges from -90° to +90°.
The equivalent to lines of longitude on the Earth are a series of lines of Right Ascension. These all meet at the celestial poles. Position in right ascension is measured eastwards in hours, minutes and seconds from the first point of Aries (the position of the Spring or Vernal equinox on the celestial equator).


Page last modified on 2004-11-09 © S.R.Parkinson