Looking Up: Spinning Under Control

Nov 25, 2019

Precession of Earth's rotational axis due to the tidal force raised on Earth by the gravity of the Moon and Sun. Vectorized by Mysid in Inkscape after a NASA Earth Observatory image in Milutin Milankovitch Precession.
Credit NASA, Mysid / nasa.gov / public domain / wikimedia commons

This week on Looking Up Bruce Bookout sets his sights on the Earth's precession.

During a recent 80’s musical flashback, I wondered about a fundamental part of our movement in the universe.  It is well understood that our spherical earth spins on its axis providing the basic unit of time defined as a day.  That spin establishes an axis of rotation that we further define by having poles on the surface of the earth; in the north and south.  And like a top spinning in space, the Earth wobbles, or more precisely “precesses”.

The Earth’s precession was recognized by Greek and Chinese astronomers thousands of years ago and calculated to be about 1 degree every 72 years.  Sir Isaac Newton even explained why it should happen with his understanding of gravity in the late 1680s.

So, the impact of this precession is that the positions of the stars and the dates of equinoxes & solstices shift over time. The north pole axis will point to a different position over time.  Today it points in the direction of Polaris, but during the construction of the Egyptian Pyramids it pointed to the star Thuban.  A full wobble takes about 26,000 years. 

The path of the north celestial pole among the stars due to the precession. This file is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.
Credit Tauʻolunga / wikimedia commons

So, the 80’s musical question?  How much would you precess if “You spin me right 'round, baby, right 'round”?

If you’d like to take a closer look at precession, or any of the other wonderful and amazing things in the sky, please visit csastro.org for a link to information on our monthly meetings and our free public star parties.