Precession of the Equinoxes

Last post, I mentioned how the constellations of the zodiac do not match what you see in your horoscope because of something called precession. As I explained then, precession is just saying that the Earth wobbles as it rotates on its axis, much like a top does when it slows down.

 



The amount of wobble for the Earth is not as extreme as it is for a slowing down top, i.e. we don't have to worry about rotation of the Earth ending any time soon.

Because of the wobble of the Earth, two things occur that can be measured.

1. The location of the North Celestial Pole will change. Right now, the north star is Polaris in Ursa Minor. In about 12,000 years, the north star will be Vega, in the constellation Lyra.

   

   http://people.timezone.com

2. The location of the equinoxes and solstices have changed over time. When the vernal equinox occurred 2000 years ago, the Sun was in Aries. The summer solstice occurred when the Sun was in Cancer (hence, the tropic of Cancer). The autumn solstice occurred when the Sun was in Virgo. And the winter solstice happened when the Sun was in Capricorn (and from where the name for the tropic of Capricorn). Now, the Sun is in Pisces when the northern hemisphere spring begins. This is where we get the term "Precession of the Equinoxes".

   

   Original artwork by me (yes, I know I can't draw)

 

The period of the "wobble" of Earth is about 26,000 years. So while Vega will become the north star in about 12,000 years, by the year 28,000, Polaris will be back at the celestial north pole.

Precession of the North Celestial Pole, (Note at +2000 the NCP is near Polaris. The bright star near +10000 is Deneb in Cygnus. At +14000, the NCP will be near the bright star Vega)

Image Credit:

"Precession N" by Tauʻolunga - self, 4 bit GIF.

 

 

If you are planning on using the stars to find north in the next 10000 years, you won't be able to use Polaris any longer in about 1000 years.