Astronomy Online

Final Event - The Polar Star.

Here follow detailed instructions about the measurement of the alitude of the Polar Star (also known as the North Star and Polaris) above the horizon - the first method within the Astronomy On-Line Final Event that you may use to determine your geographical latitude.

Where do you live ?

It is fairly easy to find the North-Star - if you do not know it, click here to see a finding chart

The North Star has the advantage it stays fixed in the sky - all night long - as illustrated here :

Figure 1

On these Figures you will see the well-known constellation The Big Dipper - in the opposite corner of the last Figure, you may find Cassiopeia. The star Polaris is right in between.

This North-star has two advantages, known since ancient times.

Note that the Geographical Latitude is 0 deg at Equator, and 90 deg at the North Pole.

Figure 2

An easy exercise

Some history

During the Middle Ages, every Christian man with self-respect should visit Jerusalem at least once during his life.

Bishop Nicolas from Iceland published a method to determine the altitude of the North Star and thereby to determine the geographical latitude in 1150:

Figure 4. Click to obtain larger figure (JPG, 85k)

This method works the following way: Lie down on the ground and put your right hand above the knee, as shown on the picture. When the North Star is right above your thumb, then you have arrived at the latitude of Jerusalem.

Another exercise

Check Bishop Nicolas' method with your school-fellows.

Measure the altitude of the Polar Star

Now we are ready to measure the height of our polar star ourselves.

There are several ways to do this, first of all by means of the astrolabium - an instrument applied by Columbus during his voyages to America:

Figure 5: A simple astrolabium - based on what you may find in any school

Measure the height of Polaris applying the equipment above. You merely have to point the long side of the triangle towards the Northstar. Having done this, put a finger on the seewing thread and move into your house. Standing in clear light, read the number and write it down.

Please observe - if you apply a protractor-device like above - it will not show the altitude directly, but instead it will show z = (90 - alt).

So, if the measured angle above is say 32 deg, you live on 90 - 32 = 58 deg latitude.

Not all explorers applied the astrolabium, some applied the Jacobian Stick:

Figure 6: The Jacobian Stick

Construct such a device and calibrate it. Ask your math-teacher how to do so.

The Jacobian device had the disadvantage of being a bit clumsy - it is difficult to keep an eye on both a star and the horizon. If you construct this instrument - beware - don't fall while you are aiming it out in the dark !

A much more handsome device was presented to Vasco da Gama (1469-1524) by native sailors outside India. This device is called a "Kamal".

Figure 7

The Kamal is far more easy and handsome compared to the Jacobian Stick. Notice, you only need a piece of wood - and a tiny rope. Pull the rope - until the wooden piece fills your field of vision from the horizon up to the North Star.

Sailors off the shores of India marked the different cities simply by tieing a knot.

Today, sailors apply a sextant or similar device. Mostly they do not look for the North Star, but instead look for the Sun during daytime.

However - the Danish sailor and author Troels Klovedal once rescued an American shipwrecked sailor off the coast of Bermuda. This sailor had navigated his lifeboat for weeks - by means of the North Star. Keeping an ordinary pencil in outright arms distance - he tried to keep the same latitude all time. This made him stay within the ordinary shipping routes - were he was later picked up by Troels Klovedal.

So, once again, stay awake during the astronomy lessons - it may actually save your life!

Have a nice Hunt! - and don't forget to report your results!


Figure 4 is courtesy of Soren Thirslund - Naval Museum, Helsingor - permission to reproduce only if AOL and Soren Thirslund are mentioned. Soren has written several interesting books on navigational history - some of them are to be published by the Conway Maritime Press - London. Soren Thirslund would appreciate a feedback.

Author: Mogens Winther
Amtsgymnasiet Sønderborg
Gruntvigs Allé 66
DK-6400 Sønderborg

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