ALTHOUGH the Earth revolves around the Sun, from our point of view we see the Sun moving around the sky along an imaginary circle called the ecliptic once every 365.256 days. This is known as the sidereal year. The ecliptic crosses the equator twice a year. On March 20, known as the vernal (spring) equinox, the Sun starts its northward journey from the equator toward the Tropic of Cancer. The interval of time from one vernal equinox to the next, known as tropical year, is 365.242 days. The southward motion of the Sun from the equator starts on September 22 and the day is called autumnal equinox. On these dates, day and night become equal at all latitudes.
Earth is not a perfect sphere; it is an oblate spheroid, with an equatorial diameter 43 kilometers larger than the polar diameter. The gravitational effects of the Sun, Moon, and other planets on the Earth's equatorial bulge cause the Earth's rotational axis to sway clockwise in a slow circular motion similar to that of a spinning top. Historically, this motion of the axis is called “precession of the equinoxes” because the equinoxes move westward along the ecliptic relative to the fixed stars, opposite to the motion of the Sun. The time taken by the axis to complete one swing is about 26,000 years. If the Earth were perfectly spherical, precession would not occur.
Precession is of minor importance in day-to-day life, but over long periods of time, its effects are observable. Because of the precession, the apparent position of the Sun relative to the backdrop of the stars at some seasonally fixed time, e.g. the vernal equinox, moves from one zodiacal constellation into a new one every 2,166 years. This is known as the “astrological age.” It takes the Sun approximately 26,000 years to move through all 12 constellations of the zodiac.
A major consequence of precession is that the zodiac signs do not now correspond with the constellations of the same name. For example, astrologically I am an Aquarius, but astronomically I am a Capricorn.
In 3000 B.C., the Sun appeared in Taurus at the vernal equinox and entered Aries in 1900 B.C. Presently the vernal equinox is in the constellation Pisces and is slowly approaching Aquarius. This is the origin of the so-called "Age of Aquarius" when, according to astrologers, there will be unusual harmony and understanding in the world.
Due to precession, the Sun's apparent position relative to the vernal equinox slowly regresses about 50 arc seconds (0.014 degree) every year, or approximately one degree every 72.2 years. This results in the difference of 20 minutes between the sidereal and tropical years.
Precession also causes the North Pole star to move in circles against the backdrop of stars. While today it is Polaris, 5000 years ago it was Thuban in the constellation Draco. In about 14,000 A.D., it will be Vega in the constellation Lyra. In 27,000 A.D., it will again be Polaris.
Precession may also cause climate changes. The Earth is currently nearest the Sun when it is winter in the Northern Hemisphere. In 13,000 years, the Northern Hemisphere will be farthest from the Sun during the winter months. This will make winters much colder in the Northern Hemisphere.

The writer is a Professor of Physics at Fordham University, New York.