Precession of the Equinoxes

Currently, Polaris is the “North Star”, which basically means if you walk toward it then you will be heading north. Because the Earth rotates, no other star has this property, as where they are in the sky depends on the time of day. Polaris is special because the Earth’s axis of rotation conveniently points nearly directly at this star in the sky.

That won’t always be the case; Earth’s axis of rotation itself rotates, or more accurately, precesses. The best illustration of this is to imagine a spinning top: as the top spins, where it points also seems to rotate.

As a top spins down, its tip changes direction in a well-defined circle. — A spinning top demonstrates the effect of precession. From Tenor.

This is called Axial Precession, or the Precession of the Equinoxes.

Why the Earth Precesses

The Earth’s precession is caused by an unequal tug of gravity by the Sun. Because the Earth is spinning, it actually isn’t a perfect sphere; instead, it bulges out at the equator.

This means one part of the bulge is closer to the Sun than the other. The Sun pulls on the closer bulge more strongly, resulting in a net force tugging on the Earth’s axis of rotation.

The Effects of Precession

As the Earth precesses, where the axis points in the night sky changes over time, which has a number of interesting consequences:

In just a couple thousand years, Polaris will no longer be the northiest North Star. It will gradually drift away from the northern position until other stars become closer to true north.
Constellations will drift from when and where they currently show up. Some constellations that are exclusive to the southern hemisphere might start peeking into the northern hemisphere thousands of years from now.
When summer and winter happen during the year will flip flop. Since seasons are determined by Earth’s axial tilt, if the tilt changes its orientation, then so will the seasons change their timing.
- Halfway through the precession cycle, summer in the north will happen in January instead of June.
- Although, our modern calendars actually take this drift into account, so people 13,000 years from now will still have summer in June.

And by the way, the precession won’t cause any catastrophes! In fact, the Earth completes a cycle of precession every 26,000 years, and hence has gone through many thousands of these cycles throughout its history.