(Discovering the Universe, 5th ed., §1-7)
- The Earth is not a perfect sphere; instead, it is slightly oblate, i.e. it bulges in the middle along the equator.
This bulge is due to its rotation and the fact that it is not completely rigid.
As a result, the equatorial diameter is 43 Km greater than the polar diameter, a difference of 0.34%.
- Because the force of gravity weakens with distance, the Sun and Moon have non-uniform gravitational forces on the Earth, pulling harder on the near side of the bulge than on the far side.
This differential gravitational force is called a tidal force.
Basically, the Sun and Moon try to "straighten" the rotation axis to bring it in line with the orbital axis.
- However, instead of straightening, the Earth's rotation axis precesses, i.e. it exhibits a slow, conical motion around the orbital axis.
Precession is the same effect you see with a top.
As long as it is spinning, the top does not fall over, and likewise the Earth's axis won't straighten.
- The Earth's precession is a very small effect; it takes 26,000 years for the axis to make one full circle!
Currently the Earth's axis points within a degree of the star Polaris, and it will slowly get closer until around the year 2100, when it reaches a minimum separation of 27 minutes of arc.
Almost 5000 years ago the Earth's axis pointed towards the star Thuban in the constellation of Draco, and this star was used by the ancient Egyptians as their pole star.
In 6,000 years the Earth's axis will point towards the star Alderamin in Cepheus, and in 12,000 years it will be near Vega in Lyra.
The Earth's precession can be easily observed by standing at the Earth's north pole, where the north celestial pole is at the zenith, and watching how that point changes over time (note the year in the lower-left corner):
- The circle traced out by the north celestial pole can also be observed on the celestial sphere.
Two different positions, now and 13,000 years in the future, are noted in the picture at the right.
- As the precession of the axis occurs, the orientation of the celestial equator will also change, since it is necessarily perpendicular to the axis.
- However, the ecliptic is fixed on the sphere.
As a result, the intersections between the celestial equator and the ecliptic, the equinoxes, move along the ecliptic 50 arc sec (3.3 sec R.A.) per year.
The vernal equinox moves in the direction shown.
Currently, the vernal equinox is in Pisces; 2000 years ago it was in Aries; in another 1000 years, it will move into Aquarius.
- This movement of the celestial equator relative to the ecliptic is called precession of the equinoxes.
Obviously, this effect is so slow that it is only observable over many years.
Precession was discovered by the Greek astronomer Hipparchus, who had access to several centuries of Greek and Babylonian records.