Precession of the Equinoxes

Wikipedia has a good article on this

But here are some more pictures...

In these pictures, the white star-shape near the top is the location of the true north pole in the sky - As the night goes on, the stars circle around this point, while a star at this point would appear be motionless.

There happens to be a star that is very close to this point in the sky; it is the 'North Star', more properly called by it's name 'Polaris'.  It is not a particularly bright star, but it is bright enough to see from a city.  It is not bright, but it is special, if you look  for it from a certain place, such as you backyard, it is always at the same point in the sky (even during the day, but you can't see it).  It is even more special because  it can tell you which way is north (as long as you are in the Northern Hemisphere!).

However, the star named Polaris  has not always lined up with the Earth's North Pole.  The Earth's axis wobbles very slowly (it takes 25,700 years to complete one wobble).  As it wobbles, the point that the stars appear to revolve around moves through the sky.  Right now this point is very close to Polaris, it is not exactly at Polaris, but it is to close to notice the difference.  The 21st century happens to the time when Polaris is as close to true North as it can get.

The picture below shows the situation as it is in 2006, the red arrow points to the position of Polaris, at the tip of the handle of the Little Dipper.  The Yellow arrow points to the star named 'Vega', it is a bright star that rises very high in the late Summer sky.


In the year 14,000, AD,  Vega will be close to true north.  There will not be any star where the 'white star' that represents true north is on this picture, but Vega will be making small circles in the sky close to north.  Meanwhile, Polaris, the relatively unimpressive star at the tip of the Little Dipper, will not be important at all.

Just for interest, here is a picture of the sky at about the time Jesus was born, Polaris was near the North Pole, but not quite.

The Wikipedia articles says that the point of 'True North' moves the distance of one degree through the sky in about 72
years.  The width of the full moon, as seen from earth, is about one-half a degree, so it has moved that far, or farther, since your scout leaders were born!

Why is this called Precession of the Equinoxes?

Well there's a related effect.  The equinoxes are the dates when days and nights are equal.  There are two each year, the first day of Spring (the Vernal equinox) and the first day of Fall (The Autumnal equinoxes).  Ancient people who studied the stars started counting their years from the date of the Vernal Equinox.  They believed that the stars influenced people (they were actually Astrologers) and were very interested in where the Sun was against the background constellations (even if you couldn't see them).  Because the earth was wobbling, this place were the Sun was on the day of the Vernal Equinox, slowly changed from year to year.  This slow movement is called 'precession', and thus we have 'Precession of the Equinoxes'.  Somewhat interestingly, the point of the equinox started in the constellation Aries when astrology was invented, it has slowly moved across the constellation named Pisces, and is now moving into the constellation Aquarius (hence the dawning of the Age of Aquarius).