3.True North

True North

True North can be determined using the simplest of instruments to a pointwithin a mm at a distance of 100m or 1000m.

The apparatus is simple. A vertical pole set on a small hill is the firstfixed point of the alignment. This can be set vertical with great precisionusing a plumb line.

It helps if the pole has a sharp straight edge, such as a strip of copperor bronze.

A cross bar is set on this pole, again with a sharp, straight edge. Itshould be close to horizontal, but this is not essential, as it is the junctionof the vertical and horizontal members that makes a corner which is usedto occlude a star.

Over 80% of the worlds population live today with a sky polluted by light,and can never see the stars rotating in the sky through the night, but getaway from the city lights, and spend your nights in the open, and it is easyto see the stars turning in the sky, and to select the star nearest to thecenter, that turns with the smallest circle.

Stand so that this northern star just fits into the arms of the cross- at the junction of vertical and horizontal.

An animation - rather large 2MB -show this process(The stars shown are not a representation of the real polar sky).

At this point where you stand, set up a strong framework to hold a clothfacing the northern star.

Look through the weave of the cloth at the cross and the northern star.

Now as the stars turn, move your head to keep the northern star in theangle of the cross. Mark the path it appears to trace on the cloth.(justuse one eye!)

What you have drawn on the cloth through the night is part of a perfectcircle whose center is exactly south of the angle of the cross on the hill.

Now in daylight you can have your ceremony to mark the two chords on thecloth that when bissected give you the exact centre of the circle.

Now draw two circles with the radius of the chord and the centres wherethe chord cuts the circle.

Join the two points where these two circles cross with a straight line.

Now repeat this for the other chord, drawing circles centered on the chordends.

Join the two points where these two circles cross with a straight line.

These two lines cross at the exact center of the circle, exactly southof the angle of the arms of the cross on the hill!

Use a plumb line to drop this point to the ground.

Your point should be accurately due south of the angle of the cross onthe hill to the nearest mm!

Perhaps this is the origin of the religious significance of a metal edgedcross set on a hill: it was used to determine true north by determining theexact centre about which the heavens rotated. A point of certainty and significancein an ever changing world.

This information is copyright Peter Thomson 2001-2004