This path is also the path followed by the Moon and all the planets. They exert a gravitational pull on Earth, specifically at the bulge at its equator, which has the effect of trying to tug the Earth 'upright'. These dynamics cause the Earth's axis of rotation (the axis of its daily spin) to wobble in space. This wobble is called precession.

The Earth as an hourglass
From the point of view of someone here on Earth observing with the naked eye, precession causes our planet's orientation within the fixed sphere of stars to change very slowly: it takes the Earth 26,000 years to complete a single precessional wobble on its axis. For example, when it was built, the Great Pyramid at Giza was aligned to the pole star of its era, alpha Draconis, or Thuban. Today, some 5,000 years later, Polaris is our pole star. Precessional motion has caused this change.

Likewise, in the southern skies, the location of the southern pole in the stars also changes over time. Thus, at both poles, the Earth's axis of rotation wobbles, or precesses, through two circles, each some 47 degrees wide (twice 23.5 degrees), traced through the stars, and turning around two centres that can be thought of as points marking the location of the north and south poles of the Sun, or ecliptic plane.

The precessing Earth can be imagined as creating an hourglass figure, with the narrowest point (where the sand runs through) at the centre of the Earth and with the two circular ends, or bases, of the hourglass representing the circles at the poles made by both ends of the Earth's precessing axis.

The rising of the Sun
To understand how the ancients could have perceived this magisterially slow motion, one further fact needs to be understood.

As long as the angle of the Earth's tilt in relation to its annual path (the ecliptic) remains constant - and it does, within very small margins - its precessional wobble has no effect whatsoever on the spot on the horizon where the Sun rises - for example, at the June solstice. Were it not for the background of stars rising at dawn on a given solar date - such as solstices and equinoxes - there would be no way that an astronomer scanning the skies with his naked eye would have known that precession was occurring.

To put this another way, suppose our entire solar system were like a bird cage. Because of its tilt, the Earth is precessing, but if each night some celestial 'keeper' put a hood over the cage - that is, blocked our view of the fixed sphere of stars outside the 'cage' - then none of us 'birds' would have any way of recognising the existence of precessional motion. The solstice and equinox Suns would continue to rise at their familiar stations on the horizon after the correct number of days.

Believing our grandfathers
The visible effect of precession is that, over time, it causes a given star or constellation to rise 'late'. The rate of change is about one day every 72 years. For people to 'observe' precession over this length of time, some sort of a 'database' - for example, a collection of myths - would be required.

As Professor Phillip Morrison of MIT put it, 'All you would need was an old tree and faith in your grandfather's veracity.' In other words, suppose your grandfather had told you that, on the morning of the spring equinox, the Sun rises in the 'Eye of the Lion', the name of a star. Then suppose that you went out and stood at the correct viewing spot and looked out over the tree marking the equinox sunrise point on the horizon and failed to observe the 'Eye of the Lion' twinkling low on the horizon. If you believed your grandfather, you'd think that something was not quite right. The next morning, when the Sun no longer rose precisely over the tree (or perhaps over a heelstone, such as at Stonehenge), you would catch a glimpse of the 'Eye of the Lion' twinkling on the horizon. If you believed your grandfather, you would have made an observation of precession.

Ancient myths
According to William Sullivan, ancient myths recorded precessional observations by referring to a particular solar date (such as solstice or equinox) and naming a star just rising in the gathering dawn. These two pieces of information are sufficient to generate a precessional date, accurate to about 50 years (plus or minus). That is, precessional information encrypted in myth provides dates more accurate than radio-carbon dates.

Dr Sullivan, in his book The Secret of the Incas, asks if we have faith in the veracity of our ancestors, or will we continue to view them as 'howling savages' and their immensely sophisticated mythology merely the undisciplined musings of the savage imagination?