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 seem to move, while the rotation of the Earth carries all the other stars in circles around it. This star is above the North Pole, hence it is called Pole Star, or Polaris, which is the “name of the star”. It is also called the North Star, because for anyone in the northern hemisphere, unless they are standing at the pole, looking in its direction means looking north. Additionally, measuring its elevation angle from the northern horizon tells us our latitude. A few centuries ago, when we didn’t have an accurate way of determining our longitude (our east-west position), we would navigate the oceans using Polaris. We sailed north or south until we reached the latitude of our destination, and then we 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’s easy to assume it’s a fixed thing. Is not. At some point we all must have played with the spinners. If we made them spin fast enough, they would stay upright, balancing on their edges. We found that it was very difficult to set a top to spin exactly upright, and that when we failed, the top would describe slow, circular wobbles, but would not fall. This oscillating process is known as precession and is due to the interaction between the spinning motion and gravity trying to pull this non-standing top. Earth is in a similar situation. Our planet rotates on its axis once a day. However, a uniform, stable rotation becomes impossible because the Earth is not perfectly spherical. It bulges at the equator and has the Sun and Moon gravitationally pulling on that bulge. The result is that, like the top, our spinning planet wobbles. presupposes. This means that Polaris has not always been the Pole Star and will not remain so in the future. If we stand at the North Pole and look up at the zenith – the point directly above – we will look along the line of the Earth’s axis of rotation and see that the axis is very close to the Pole. This means that as the Earth rotates, this star stays where it is and all the other stars appear to circle around it. As the Earth’s rotation proceeds, the axis of rotation will describe a circle between the stars. It takes 26,000 years to complete each round. Back in the time of the ancient Egyptians, the shaft pointed to Thuban, the constellation Draco, “The Dragon.” From then it moved until it pointed near Polaris. Now it is slowly moving away in the direction of the star Alderamin, in the constellation Cepheus (a constellation named after the husband of Queen Cassiope), so in 7500 AD. will be our North Star. The constellation 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 be in the constellation Cygnus, the Cygnus. In 13,700 AD the North Star will be the bright, bluish star Vega, which is currently almost overhead in the evening. About 23,000 AD Thuban will again be the Pole Star and by 27,000 AD. Polaris will be back to work. An interesting consequence of precession is that the zodiac signs slowly slide backwards. The first sign of the Zodiac is the constellation that sits where the Sun crosses the celestial equator in spring. This was Aries. The first sign is now Pisces. However, in just under 26,000 years, when the Earth begins its next swing, it will be Aries again. ••• • On September 23, the Sun will cross the equator, heading south, marking the autumnal equinox. It will be in front of the constellation Virgo. •Saturn and Jupiter are in the sky after sunset. Mars rises three hours later. • The Moon will be 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.
