Precession Index

| Basic Celestial Phenomena |

1. Precession: North Pole drift
   • Is Polaris always the north star?
   • When will Polaris be closest to true north?
   • What was the north star when the Great Pyramid of Khufu (Cheops) was built?
   • What will be the north star 2000 years from now?
   • What is the period of precession?
   • Sky chart showing north pole drift during precession.
     
2. Precession: Equatorial slip
   • What happens to the right ascension of Orion during precession?
   • How does the celestial equator appear to slide along the ecliptic?
   • Imagine that a star is located exactly at the location of the September equinox.
     • Where will that star be located at the next September equinox?
     • At a September equinox a hundred or a thousand years later?
   • Imagine that you are riding the sun in its course around the ecliptic. Which is longer, a sidereal year or a tropical year?
   • In what way does an equinox "precede" the stars it was associated with before? (This is "precession of the equinoxes.")
   • Does the angle between the ecliptic and celestial equator remain at 23.5 degrees, or does it change with precession?
     
3. Why is your horoscope hopelessly out of date?
   • Hipparchos of Nicaea discovered precession around 150 B.C. using Babylonian records and some ingenious mathematics.
     
4. Further study:
   • Examine Crowe's modern explanation of the cause of precession on pp. 8&endash;10, Theories of the World from Antiquity to Copernicus.
   • See also Bob Berman, Secrets of the Night Sky, "Puzzles of the Pole Star," pp. 80-89.

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