ASTR 1230 (O'Connell) Lecture Notes



  • The gravity of the Moon and Sun act on the "bulge" at Earth's equator, causing a gradual cyclical change in the direction of the Earth's spin axis called precession. This is shown schematically in the animation above.

  • Projected on the celestial sphere, the poles slowly trace out large circles at a rate of 0.5 degree per century. It takes 26,000 years for the poles to complete one cycle. See figure below. Though subtle, precession was first detected in 150 BC by the Greek astronomer Hipparchus.

  • Polaris is a convenient "North Pole star" now, lying about 1 degree from the true North Celestial Pole. However, it will not be as useful in a few 1000 years. Vega will be close to the pole 14,000 years from now, but most of the time there is NO useful pole star. The animation below shows the pole position as a function of date (Note: the point labeled "zenith" in the drawing is actually the "North Pole".)

  • Precession changes the location of the equinoxes as well as the celestial poles. The vernal equinox moves from one constellation of the Zodiac to the next in about 2000 years. Thus, precession changes the RA,DEC coordinates of all astronomical objects. The maximum annual change is about 10 seconds of time in RA and 20 seconds of arc in DEC.

  • Because of precession, all listings of RA,DEC must have the "epoch"---i.e. the date for which they are valid---specified. Most listings now give epoch 2000 coordinates, though some still use 1950.

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    Last modified September 2015 by rwo

    Precession drawings copyright © by Nick Strobel and Scott Anderson. Text copyright © 2000-2015 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.