Precession of the Equinoxes
- by Bernie Reim -
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Bernie Reim is an astronomy professor at the University of Southern Maine. Bernie publishes a monthly astronomy column in the Portland Press Herald called
"What’s Up in [month]". HERE is the March 2021 edition. Bernie also has a weekly
half-hour radio show called "Scientifically Speaking" at 1:00 PM on Fridays on
WMPG, 90.9 FM in southern Maine. Bernie and I share an interest in protecting high quality
darkness at night, so we have collaborated on that issue. He wrote the text on this page specifically for this web site. Thanks, Bernie!
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The ”Precession of the Equinoxes” is a very interesting phenomenon that is based on a couple of subtle physical facts about how the earth spins and its exact shape. The earth is 27 miles wider at the equator than at the poles, so it is technically an oblate spheroid and not a perfect sphere. That is not much of a difference, but it is enough to cause it to wobble a little like a top as it spins continually, with the surface moving at over 1000 miles per hour at the equator. This causes the imaginary axis through the earth that connects the poles to continuously shift against the fixed background of stars. Polaris will not always be our North Star!
The shift is only one degree in just under 72 years, so one complete cycle takes 25,800 years. In just 2000 years there will be another fairly bright star on this circle that our axis traces onto the sky. That will be Gamma Cephei in Cepheus the King. Then in 13,000 years our North Star will be Vega in Lyra, the 5th brightest star in our sky. By comparison, Polaris is only the 48th brightest star. As this circle continues, we will encounter some other bright stars like Thuban in Draco the Dragon, which was our North Star in 3000 BC and Kochab, the Guardian of the Pole, in Ursa Minor, which was our North Star just 2500 years ago. Then we will be back to Polaris and the cycle will just continue until the sun runs out of fuel in about 5 billion years.
The other reason that our axis wobbles a little as we spin fairly rapidly is that our spin rate is slowing down a little due to friction with our oceans and atmosphere and gravitational drag due to our interaction with the moon. This is only about a millisecond per century, but it adds up since the earth is about 4.6 billion years old. When Earth was first formed, one day was only 6 hours long.
Our axis continuously migrates against the fixed background of stars along the zodiac in a clockwise fashion. On the first day of spring 2000 years ago, this point was in Aries the Ram, then it was in Pisces the Fish for 2000 years, and it is just now moving into Aquarius, hence the “dawning of the Age of Aquarius”. It will spend just over 2000 years in each of the 12 zodiac constellations. The sun is in Pisces in March when the Vernal Equinox happens, but that will not always be the case, due to this precession of our equinoxes, due to the shape of the earth and our spin rate.
Our sky will look very different in 13,000 years at this latitude based on that fact, which was known as early as 2000 years ago by Hipparchus, who carefully compared the sky then to much earlier star maps to figure that out, which was really quite brilliant of him. We will be able to see the Southern Cross right here in New England (Now you have to be down around 30 degrees to see it.) and we will lose half of Orion and all the constellations below it. In turn we will gain Carina (the keel), Ara (the altar), and several others.
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