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Thousands of years ago, ancient Greek astronomers calculated that the track of the Earth's axis is constantly, even if in a very slow way, shifting in a uniform pattern. The variation is very similar to the manner a spinning top slowly leans one way and then another as it slows down. It is a wobble that happens as its axis alters direction.

This odd movement of the planet is due to a couple of factors, the most important of which is something called 'precession'. Precession arises from the fact that the Earth is not a perfect sphere. It is in fact about twenty-seven miles longer around the Equator that it is around the Poles. The Earth then is oblate, or fat around the middle like middle-aged spread, but it is due to the rotation not to its age.

If you picture the Earth with its Poles off centre. Then rotate that image and you will find that any point, except the very centre of the axis, will travel in a circle. But very, very gradually. So slowly that it takes 26,000 years to go full circle and get back to where it started from.

This point then, any point you want, is very slowly shifting its location in relation to the stars because the axis is gyrating too. The result of this is that, what we call the North Star (officially known as Polaris, which is actually one degree off true North) will not be above our North Pole one day. In fact, by about 15,000 AD, Vega will be almost over the North Pole, although it will be about four degrees off true North. But even this will not last, and by 28,000 AD, Polaris will be back above where it is nowadays.

One of the effects of the precession is that seasons vary. They change the dates that they occur, so that Summer could come earlier or later. The remarkable thing about our calendar is that it is corrected for that (with the leap year). If it were not, the vernal or Spring equinox would shift over 13,000 years from March 21st to September 21st., which is the date of the autumnal equinox, precisely half a year later.

It is for this reason that the precession of the Earth is generally referred to as the "Precession of the Equinoxes". Although the precession of the equinoxes is very slow, it can be readily observed. The correct year of 325.25 days is the length of time from one vernal equinox to the next vernal equinox, however, it takes 20 minutes and 24 seconds longer for the Sun to appear in exactly the same place in relation to the stars behind it over the same period. It is for this reason that accurate star maps have to be stamped with the exact time and date to which they relate.