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 Why is your horoscope hopelessly out of date?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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