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Skywatching

The North Star is not the constant we think is

The 'temporary' North Star

Most of us know how to find the Pole, or North Star.

We find the Dipper and follow the line indicated by the pair of stars opposite the handle. It is the star that does not appear to move, while the rotation of the Earth carries all the other stars in circles around it.

This star is overhead at the North Pole, which is why it is called the Pole Star, or Polaris, which is its "star name". It is also called the North Star, because for anyone in the Northern Hemisphere, not standing at the pole, looking in its direction means we are looking North. In addition, measuring its angle of elevation from the northern horizon tells us our latitude.

A few centuries ago, when we had no accurate way of determining our longitude (our position east-west), we would navigate our way across the oceans by using Polaris. We sailed north or south until we were at the latitude of our destination, and then sailed east or west, depending on where we were going, keeping Polaris at the same angle above the horizon. The Pole, or North Star, has been so fundamental, for so long that it is easy to assume it is a constant thing. It is not.

At some point, we must all have played with spinning tops. If we set them spinning fast enough, they would stay upright, balancing on their tips. We found that it is very hard to set a top spinning exactly upright, and that when we failed to do so, the top would describe slow, circular wobbles, but not fall over. This wobbling process is known as precession and is due to the interplay between the spinning motion and gravity trying to pull that non-upright top over. The Earth is in a similar situation.

Our planet spins on its axis once a day. However, a uniform, fixed spin is made impossible by the Earth not being perfectly spherical. It bulges at the equator, and has the Sun and Moon gravitationally tugging at that bulge. The result is that, like the top, our spinning planet wobbles; it precesses. This means Polaris has not always been the Pole Star and won't remain so into the future.

If we stand at the North Pole and look upwards to the zenith—the point exactly overhead—we will be looking along the line of the Earth's axis of rotation, and see that the axis points very close to Polaris. This means that as the Earth rotates, that star stays where it is, and all the other stars appear to circle around it. As the Earth's rotation precesses, the axis of rotation will describe a circle among the stars.

It takes 26,000 years to complete each lap. Back in the days of the ancient Egyptians, the axis pointed at Thuban, in the constellation of Draco, "The Dragon". Since then it moved until it pointed close to Polaris. Now it is slowly moving away and in the direction of the star Alderamin, in the constellation of Cepheus (a constellation named after the husband of Queen Cassiopeia), so in 7500 AD that will be our North Star.

The constellation of Cepheus looks rather like a house, and none of the stars in it, including Alderamin are very bright.

Around 10,000 AD the pole position will lie in the constellation of Cygnus, the Swan. In 13,700 AD the North Star will be the bright, bluish star Vega, which at the moment lies almost overhead in the evening. Around 23,000 AD Thuban will be the Pole Star again, and by 27,000 AD Polaris will be back on the job.

One interesting consequence of precession is that the signs of the Zodiac are slowly slipping backwards.

The first sign of the Zodiac is the constellation sitting at the point where the Sun crosses the celestial equator in the spring. That used to be Aries. The first sign is now Pisces. However, in just under 26,000 years, when the Earth starts its next wobble, it will be Aries again.

•••

• On Sept. 23, the Sun will cross the equator, heading south, marking the autumn equinox. It will be in front of the constellation of Virgo.

•Saturn and Jupiter are in the sky after sunset. Mars rises three hours later.

• The Moon will be new on the Sept. 25.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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