What is Precession?

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  • 0:01 Hipparchus & His Observations
  • 0:49 Rotation and Revolution
  • 1:28 Precession
  • 2:21 The Consequences of Precession
  • 4:06 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

You've probably heard of the term precession. But what is this really? And what significance does it have to our night sky? This lesson will explain what it is and what it causes.

Hipparchus and His Observations

A long time ago lived the famous astronomer Hipparchus. We don't know too much about him, but we are aware that in 134 B.C., he discovered a new star in the sky.

This discovery led Hipparchus to catalog around 850 stars, their positions, and an indication of their brightness. Although the original catalog is now long gone, it is believed a representation of it is found on the celestial globe located on the shoulders of a statue of Atlas in Naples, Italy, from the second century A.D.

Anyways, when Hipparchus finished his catalog, he noticed that the positions he noted were about two degrees off from the ones noted by astronomers that came almost two centuries before him. This lesson will tell you why this was so.

Rotation and Revolution

To help explain why this is the case, I hope you have a spinning top at home. If you spin the top, you can clearly see that it rotates. Rotation is the movement of a body on its axis. In astronomy, don't get this confused with revolution, which is the orbital motion of a body around another body or point in space, and is more correctly referred to as orbital revolution or simply orbit.

As the top rotates, you should be able to see in front of you that it slowly moves its axis around in a circle or cone-like shape. This is more evident when the top is slowing down than when it's spinning really fast, but it occurs in either scenario.


Like the spinning top on your desk, Earth also spins like a giant top. The change in direction of Earth's axis of rotation, this kind of spinning top-like motion, is known as precession. Sometimes you may hear it being referred to as the Earth wobbling on its axis.

Unlike a spinning top right as you launch it seems to do, the Earth doesn't spin entirely upright; it's actually tipped 23.4 degrees from the vertical, but it doesn't eventually fall over like a spinning top either.

While gravity makes the spinning top fall over on Earth, the gravitational influences of the sun and moon actually tug and twist at our planet to keep it 'upright' in its orbit. This twisting is actually what's mainly responsible for producing this motion of Earth's axis over time (i.e. precession).

The Consequences of Precession

Earth's axis precesses in a 26,000-year cycle. Since this cycle is very long, you're not going to notice any changes in the night sky from night to night.

However, over time, the location of the celestial poles and celestial equator will change as a result of precession since they depend on Earth's rotational axis. Thus, another way to restate precession is as a slow circular motion of the celestial poles among the stars. As the poles move, so will the equator.

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