The Earth spins on its "axis". This axis is an imaginary line running through the Earth. If you were to be high above the Earth, looking straight down along the axis, all the points on Earth would appear to move in circles around the axis. If you followed this axis out into space from the northern hemisphere on Earth, it would point toward a particular star in the sky. We call that star the "North Star" since it sits in the direction that the spin axis from the northern hemisphere of Earth points. At present, the star known as Polaris is the North Star. However, Polaris has not always been the North Star and will not always be the North Star. To understand that, we need to look at how the Earth spins on its axis. The spin axis of the Earth undergoes a motion called precession. If you have ever watched a spinning top, you know that its spin axis tends to stay pointed in the same direction. However, if you give it a slight nudge, the axis will start to change its direction, and its motion traces out a cone. This changing of direction of the spin axis is called precession. So what gave the Earth the "nudge" it needed to start precessing? The Earth bulges out at its equator, and the gravitational attraction of the Moon and Sun on the bulge provided the "nudge" which made the Earth precess. It was the ancient Greek astronomer and mathematician Hipparchus who first estimated the precession of the Earth's axis around 130 B.C. The period of precession is about 26,000 years. In other words, it takes 26,000 years for the axis to trace out the cone one complete time.



You can see precession of the spin axis in a spinning top

