I promise I'll finish this entry one day...

The Earth spins on its axis, the axis through the North and South poles, in an effect we experience as days. As it spins different parts of the Earth's surface face the Sun, causing daytime on the side facing, and nighttime on the other side.

Suppose the Earth was on its own in the universe, without the Sun and Moon there. It could keep spinning on its axis forever, and have its axis keep pointing in the same direction. There would be nothing to stop it. However, the Earth isn't alone. The Sun and the Moon both exert pulls of gravity on Earth, more specifically on the Earth's equatorial bulges.¹ This has the effect of twisting the Earth, making it precess like a gyroscope or spinning top.

Discovery

Precession was observed by the Ancient Greek astronomer Hipparchus. He used eclipses of the moon to work out when the equinoxes fell- although the ancients didn't have accurate clocks which could tell them the exact time, they could work it out by the fact that at the equinox the sun rises exactly in the east and sets exactly in the west. Hipparchus compared his results in around 130 BC with ancient Babylonian data. He concluded that the axis around which the heavens rotate had shifted since the time of the Babylonian records.

But what is Precession?

Imagine the Earth is a spinning top. When you set it spinning on the ground it doesn't just spin, it wobbles slightly too. This wobble is precession- the axis the top is spinning round describes a circle in the air, but much slower than the top is spinning. The Earth spins much slower than a top, and it takes the polar axis around 26000 years to describe their circle. This number is also known as the period of precession.

Pole Stars

Because the Earth is precessing, the North Pole doesn't always point to the same place in the sky. This means that although at the moment the pole star is Polaris, in 14000 AD the pole will no longer be pointing at Polaris- the pole star then will be Vega, in the constellation of Lyra. As the cycle continues other stars will be the pole star, and eventually it will get back to Polaris again.

Other Precessions

The pull of gravity of the other planets in the solar system causes the Earth's orbit around the Sun to precess. This effect is very much smaller than that caused by the Sun and Moon on the Earth, however. It has a much longer period of 71000 years.