The North Star is not the constant we think – skywatching

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’s the star that doesn’t seem to move while the Earth’s rotation carries all the other stars in a circle around it.

This star is located over the North Pole, which is why it is called the North Star or Polaris, which is its “star name”. It’s also called the North Star because for anyone in the northern hemisphere who isn’t at the pole, looking in its direction means we’re looking north. In addition, measuring the elevation angle from the northern horizon tells us our latitude.

Centuries ago, before we could accurately determine our longitude (our east-west location), we used Polaris to navigate the oceans. We sailed north or south until we reached the latitude of our destination, and then sailed east or west depending on where we wanted to go, keeping North Star 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 that it is a constant. It is not.

At some point we must all have played with spinning tops. If we made them spin fast enough, they stayed upright and balanced on their tips. We found that it is very difficult to get a spinning top upright exactly, and if we failed to do this, the top wobbles slowly in a circular motion, but does not fall over. This wobbling process is called precession and is due to the interaction between the spinning motion and gravity trying to pull this non-erect top up. Earth is in a similar situation.

Our planet rotates on its own axis once a day. However, consistent, solid spin is made impossible by the fact that the earth is not perfectly spherical. It bulges at the equator, and the Sun and Moon gravitationally pull on that bulge. The result is that our spinning planet wobbles like spinning tops; it precesses. So Polaris was not always the North Star and will not remain so in the future.

If we stand at the North Pole and look up at the zenith – the point just above us – we will look 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 orbit it. As the Earth’s rotation precesses, the axis of rotation describes a circle between the stars.

It takes 26,000 years to complete each round. In the days of the ancient Egyptians, the axis pointed to Thuban in the constellation Draco, “The Dragon”. Since then it moved until it pointed near Polaris. Now it is slowly moving away and towards the star Alderamin in the constellation Cepheus (a constellation named after Queen Cassiopeia’s husband), which will be our North Star in the year 7500 AD.

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

Around 10,000 AD the pole position will be in the constellation Cygnus, the swan. In AD 13,700, the North Star will be the bright, bluish star Vega, which is currently almost overhead in the evening. By AD 23,000, Thuban will again be the Pole Star, and by AD 27,000, Polaris will be back in action.

An interesting consequence of the precession is that the signs of the zodiac are slowly slipping backwards.

The first sign of the zodiac is the constellation that sits where the sun crosses the celestial equator in spring. It used to be Aries. The first sign is now Pisces. But in just under 26,000 years, when the earth begins the next wobble, it will be Aries again.

•••

• On September 23, the sun crosses the equator south, marking the autumnal equinox. It will be in front of the Virgo constellation.

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

• The moon is new on September 25th.

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