The sun over the U.K. June 21, 2010, the moment when the sun reached its most northerly point in the sky. Ian O'Neill
On a clear night in the Northern Hemisphere, look up at the sky and, over a period of hours, you'll notice the stars rising in the east and setting in the west ... all except one, Polaris, the Pole Star.
It has the unique responsibility of marking, to within a few fractions of a degree, the north celestial pole. This grand title is the point in the sky toward which the Earth's axis of rotation is directed.
While Earth spins on its axis, which pops out at the north and south geographical poles (that are at different locations to the magnetic poles), it's hurtling through space in an annual orbit around the sun.
If we measure the angle of the rotation with respect to the plane of the orbit around the sun, we see it's tilted 23.5 degrees from vertical.
The tilt of the axis is responsible for the seasons we experience as the northern and southern hemispheres are alternately presented toward the sun throughout the year.
Today (June 20) the northern hemisphere is pointing directly toward the sun and so the Northern Hemisphere experiences the Summer Solstice -- or the 'longest day' -- whereas the Southern Hemisphere is plunged into winter and experiences the Winter Solstice and the 'shortest day.'
As a Northerner, I can be assured that the months of June/July will be my summer months and should, in theory, be nice and warm (although being British I could probably argue that point). However, this won't always be the case as the axis of Earth's rotation wobbles like a great celestial spinning top, completing one 'wobble' in 25,772 years.
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As the axis changes its orientation -- or to use its correct term, the 'precession of the axis' -- the role of pole star passes to another. If you could hop in a time machine and whiz forward a few thousand years Polaris will be moving around with the other stars and a different one will be standing sentry.
In AD 3000, the star Gamma Cephei will be the pole star followed by Deneb in AD 10000 and Vega in Lyra will be 4 degrees away in AD 14000.
In the Southern Hemisphere, there is currently no bright star marking the south celestial pole; its nearest is a rather faint star called Sigma Octantis. This will change though, in about 3700 years time Omega Carinae will become the South Pole star, followed by Delta Velorum in AD 9200 and in AD 14000 the bright star Canopus will be within a few degrees.
This constant changing position of the celestial poles actually scribes a circle in the sky which takes 25,772 years to complete, but the precession of the axis also effects the seasons.
If we were to mark the position of the Earth in its orbit at the precise moment of the Summer Solstice, i.e., when the north pole is pointing directly at the sun, then by the time the Earth has completed one orbit and returned to this point, its axis will have precessed a tiny amount and not be pointing directly at the sun. In effect, the moment of Summer Solstice will have occurred a little earlier by 20 minutes.
This isn't particularly noticeable from one year to the next, but over many years it causes the seasons to very slowly shift their position until in 25,772 years when they are back to the way they are today.