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Similar to a spinning top, the earth's north pole wobbles in a circular motion once every 26,000 years. In 1905, scientists agreed on a fixed position for the celestial coordinate system. Thus, the instantaneous position of the earth's poles and equator are titlted with respect to the celestial coordinate system's pole and equator.
This VRML animation demonstrates the three rotations needed to align the actual position of the earth's north pole and equator with the agreed upon position of the celestial equator and north pole.
You will need a VRML 2.0 viewer to see the illustration below. Please download the Cortona vrml plug-in.
Click here for the VRML Animation Within the animation you will see three buttons:
Precessed versus Celestial Poles
Y axes not labelled
Unlike terrestrial coordinates, the axes of the celestial coordinate system, here called the Mean Right Ascension (MRA) system are as follows:
Because the celestial equator "moves" around the earth (due to wobble of the earth in orbit), the position of the XMRA axis changes depending on the time (signified as τ or Greek "tau") when observations are made.
To align the precessed pole and equatorial plane with the celestial poles and equator you will need to perform the following mathematical conversions to position and rotate the axes.