Where's North?

"I swear I don't know where it is"

Everyone should be able to find the pole star, or Polaris to give it its proper name. It sits in the sky, due north of Diddlebury, at an altitude of 52 degrees.

The easiest way to find it (because its not very bright), is to find the Great Bear (Ursa Major), and use the two 'Pointers' like this:

But the 'celestial pole' - i.e. the point in the sky that the heavens seem to pivot on as they rotate, is about 0.7 degrees away from Polaris, so the pole star seems to move on a circle 1.4 degrees in diameter (nearly the width of your finger if you look at it with your arm outstretched). To be more accurate, its declination is 89 degrees 17.5 minutes in 2006.

The interesting thing is, that this distance is always changing slightly, and the gap is currently closing. But about 400 years ago, when Galileo was looking through his telescope for the first time and finding Jupiter's moons, the gap was about 3 degrees.

In 982 the Althing, the General assembly of Iceland, considered the case of an ill-tempered immigrant, Erik the Red, who had been expelled from Norway for murder. He ended up killing again and was exiled by the Althing for three years. Erik sailed west to explore a land he had heard about from sailors who had been blown off course. This 'green land' he decided would become his place to live. In 985 he returned to Iceland and enlisted a group of followers to help him establish a base at Brattahlid in Greenland. At the time of his voyages the gap between the celestial pole and Polaris was about 5.8 degrees, the width of three fingers at arm's length.

This is why the sailors at the time couldn't use it to find their way, or more precisely, their latitude (how far they were from the equator).

In fact the celestial pole moves in the sky, drawing an imaginary circle 47 degrees (2 x 23.5) in diameter, and returns to the same point once every (approx) 25,920 years. This is called a Platonic Year, and is due to the earth wobbling on its axis, much in the same way as a spinning top does when it slows down.

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