Tuesday, April 5, 2011

Precession of the Equinoxes

Ptolemy and Hipparchus before him estimated* that the time between the vernal equinox of one year and that of the next was differed from 365 1/4 days by one day in 300 years. This shift has now been more accurately measured over a longer period of time* and is the reason why we skip a leap day* every 400 years. Since the Equinox is defined as the point where the Ecliptic crosses the Equator the direction of the Earth's axis of rotation changes slightly each year. This change is known as precession. The effect is that the direction of the Earth's rotational axis slowly circles the pole of the Ecliptic at the angular distance of the Inclination of the Ecliptic with a period of about 26,000 years. The effect is due to the gravitational attraction on the nonspherical cross section of the Earth caused by the Moon, Sun and the planets.

The significance of this change on global warming can be seen by what happen when the Earth is at perihelion and at aphelion. Currently perihelion occurs on about Jan 3 each year and aphelion is on about Jul 4 and the Earth is closer to the Sun in winter that it is in summer. But the date of perihelion and aphelion do not change with the date of the Equinox. The northern hemisphere of the Earth gets slightly more warming during winter with this orientation than it does when the pole is pointed away from the Sun. The opposite is true for summers. The difference between the amount of sunlight received by a point on the Earth directly beneath the Sun at perihelion and at aphelion is abou 6.7% and while this may not change the average heating of the Earth for the year it will probably affect the extremes in temperature*.

*Edit (5 Apr): changes made to text

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