Our Earth, the Spinning Top?

An animation illustrating how the Earth precesses, or “wobbles,” on its axis. This gif was created by me using footage from a Youtube video published by Steven Sanders.

What if I told you that in a couple thousand years from now, your Zodiac sign would no longer be your Zodiac sign? It may be devastating to devout followers of astrology, but the relative positions of the Zodiac constellations are changing very, very slowly, at least from our viewpoint. This is due to a process called precession, the continuous, gradual wobble that changes Earth’s axial orientation in space. The Earth really is like a spinning top – just an extremely slow one. In fact, this top only makes a complete spin every 26,000 years.

Because Earth protrudes at its equator, the planet is not quite a perfect sphere. The equator is also tilted with respect to the ecliptic plane, and as a result, the gravitational attractions of the Sun and the Moon attempt to draw the equatorial bulge into the ecliptic plane. In simpler words, gravity from the Sun and the Moon tries to pull the Earth into straightness. However, because Earth tends to keep rotating, gravity fails to straighten out the Earth and instead causes the axis to precess.

It is not likely that we will see any major changes during our lifetimes, but the night sky will look a lot different thousands of years from now. The North Celestial Pole is pointed toward the star Polaris now, but in 3,000 BC, the North Star was actually Thuban, a star in the constellation Draco. In roughly 12,000 years, Vega, a star in the constellation Lyra, will be the new North Star. Precession also alters the points in Earth’s orbit at which equinoxes and solstices occur; this means that in 13,000 years, the seasons on Earth will have switched times of year.

An animation that illustrates how the North Celestial Pole (NCP) moves in relation to the North Ecliptic Pole (NEP) over a 26,000 year period. This gif was created by me using footage from a Youtube video published by Steven Sanders.