Viking Navigation

Paul Constantine & Søren Thurslund

An Outline of the Sun Compass story

A Danish archaeologist C.L. Vebaek was excavating the ruins of an ancient Norse,  Benedictine convent in Uunartoq fjord, Greenland in 1948. There were many interesting finds, some of which could not be identified, including a broken wooden disc with carved edges.

Eventually, in 1953 the Illustrated London News published an article in which Mr. Vebaek described the excavation and a picture of the broken disc was included. Following this, Vebaek’s office at the Danish National Museum was contacted by Capt. Carl V. Solver who thought that the broken disc could be part of an early compass card. After Capt. Solver examined the disc, he had a complete reconstruction made. Other people became interested including Commander W.E. May, Deputy Director of the National Maritime Museum, Greenwich, who suggested further studies should be undertaken. Much later in 1978, following publications, a Swedish astronomer Dr. Curt Roslund saw in a photograph of the broken disc, a straight and a curved line scratched into the face that looked like gnomon lines. He went to the museum and after close examination was convinced that the disc had been used as a sun compass.

Making a Sun Compass                      Information from Viking Navigation. Søren Thurslund

Place a circular wooden disc about 120 diameter, horizontal outside, where the Sun can shine on it during the full day. In the centre should be placed a gnomon of 10 to 20mm, a nail, but even better, a cone. This gnomon should cast a shadow on the horizontal disc during the day.

When, in the morning the Sun begins to cast a shadow on the disc, mark the tip of the shadow and continue doing so, during the day. If the marks are placed at every full hour, you will also have a Sun-dial, telling the local time for the season. In the evening, draw a curve through all the marks and you have the gnomon curve for that particular day, but in practice also for some days ahead, depending on the time of year.

When the curve is closest to the centre, the shadow is shortest and the Sun has its highest altitude. Here the shadow is pointing North or South depending on your geographical latitude and the season. From this North/South-line, you can divide your compass-card in points or degrees, and now you have a true Sun Compass.

To use the compass, place yourself in the Sun and hold the compass level. Then turn it until the point of the shadow is exactly on the curve in the mornings over the western half and in the afternoons over the eastern half. Then, the compass is showing true directions.    S. Thurslund

Understanding the Sun Compass

To understand how it works first understand how it can be made (above) with two basic components, A disc with a central point called a gnomon – but also – the Sun.

This image shows a plastic lid and a roofing-felt tack casting a shadow of the sun. Simple!

It must be done OUTSIDE.

Record the tip of the moving shadow of the point across the disc for a whole day.

How it Works

The DAILY Movement of the Sun

As we know, it is not the sun that is moving. It is we who are spinning on the earth, to make day and night (right) The earth is also spinning in an orbit around the Sun.

BUT

You can forget this for a while and for the sake of simplicity, think of it as the sun that moves.

We are thinking of a day in May or September.

The sun comes up over the Eastern horizon. It is low down, so the gnomon point casts a longish shadow on the Western half of the disk.

The sun does not stay low down. It rises in the sky, gradually shortening the shadow.

In the Northern half of the earth the Sun moves around to the South.

By midday it has reached its zenith and will begin to descend through the afternoon.

At noon the Sun is due South, the shadow is at its shortest and pointing exactly North.

The shadow will slowly extend across the Eastern half of the disc in the afternoon.

At dusk the Sun will be setting in the West, creating a long shadow.

The Gnomonic line

The point is called a Gnomon.

If a line is drawn linking all the shadow points recorded during the day it is called the gnomonic line.

For most of the year the line will be curved, but NOT equidistant from the gnomon like this diagram.

A proper line will be more like the diagram below, because of the different heights of the sun.

As already mentioned by Søren, the shadow can be used as a sun dial to tell the time.

It can also be used to find direction.

Marking direction

At Noon the sun is due South and the shadow is due North.

Mark the North point by extending the N/S line to the edge of the disc.

A line at 90° to North/South drawn across the disc above the gnomon will be West – East.

The rest of the remaining disc can be divided to get the points of the compass.

Example. Halfway between North and East on the disc will be North-East or 45° etc. 

The illustration right shows the genuine curve of the gnomon line joining the shadows that have been cast during the day.

Next to it is the original broken wooden disc correctly aligned with the compass rose.

The first full point at the top of the disc was marked with a series of short cuts (16) to indicate it was the North point on the disc.

Finding direction

You can easily find direction from the shadow at noon, but what about other times of day?

The disc must be horizontal. The tip of the shadow may not be on the line.

Turn the disc so the point is exactly on the gnomon line in the Western half in the morning or the East during the afternoon.

The North point on the compass will be pointing True North when the shadow in on the line.

East, West, & South and other compass points will also be accurate relative to North.

Experimenting with the Sun compass

You can do this at home and get a better understanding of what is happening before you begin to deal with some of the more complex aspects of how it all works.

The Problem

The ANNUAL movement of the Sun.

Remembering that it is actually the earth that is doing all the moving, but that’s not how it appears to us; look at the Northern hemisphere.

The earth is tipped on its axis, so each year as it moves around the Sun, the sun appears to move up and down the earth. That’s why we get Summer and Winter. When the North is tipped towards the Sun (June) it is warm, but at the same time the South of the earth is facing away from it and it is having its cold Winter. This is why we have the Seasons.

Gradually, as the earth moves in its orbit around the Sun (speed 30Km per second), the Sun appears to be overhead at different places up or down the earth. We have invented the names of the Tropics of Cancer and Capricorn to mark the limits of how far North and South the sun seems to go. When it is midway between them it is over the Equator. To go up, it must pass the Equator and then pass it again on the way down.

The sun is overhead at noon in different latitudes. If we look at the situation of the Sun in June (earth left) it will appear to be overhead, level with the tropic of Cancer. If we look at it again in December (earth right) it will appear to be overhead on the tropic of Capricorn. This is due to the tilt of the earth rather than the movement of the sun.

Here comes the important bit.

My latitude is the place that I am standing; my position up or down the earth.

The Sun casts a different shadow from the gnomon for different latitudes.

We have numbered the latitudes. 0° is the equator. 90° is the North Pole.

Land’s End, Cornwall, England might be about 50°North and Oslo, Norway might be about 60°N. Naples in Italy might be about 40°N. On any single day the shadow length will be different for each of these three locations.

Add to this the understanding that the sun continues to move up or down the earth constantly from one tropic to the other.

If the shadow is recorded on a sun compass one day, it will be slightly inaccurate the day after, and a little more inaccurate the day after that as the track of the Sun has changed marginally, going up or down the earth.

The daily changes are so small as to be almost imperceptible, but they accumulate.

This means that the sun compass gnomon line will be usable for direction finding for a couple of weeks or so, but then it needs to be recorded again. The accuracy of the compass for any given latitude relates to the time of year when it is drawn up.

It is important to realise that places on the same latitude as each other will produce the same resulting daily Sun Compass.

Example. Vancouver, Luxemburg, Prague will all be about 50 degrees North.

Sun Compass Users

The Sun Compass found at Uunartoq was unearthed from a layer relating to about 1000AD.  This was within the time of the Viking colonisation of Greenland. At this time the latitudes of various Viking destinations were known to them. Norse navigators knew that they could make records of the shadows cast onto a sun compass disc at similar latitudes in Norway to the latitudes of their destinations, then use that compass to sail West along the chosen latitude to arrive at their selected destination.

Example. It was known that the 61°N latitude of Hernam in Norway was approximately on the same latitude as the southern area of Greenland. This can be seen on the cover of Søren Thurslund's book Viking Navigation at the top of this page.

They supplemented the sun compass’ ability to check latitude with pilotage observations of landmarks and voyage duration such as this one for that same voyage.

From Hernam (Norway) to Hvarf (Cape Farewell, Greenland)

Steer due west passing north of Shetland close enough to see it clearly in clear weather and south of Faroes so the mountain is half sunk below the horizon and a day’s sail to the south of Iceland so you may have birds and whales from there.

Discovering New Land

The ships used by Scandinavian people were seaworthy, but in open ocean conditions they could have been blown off course, especially in adverse weather. This was responsible for the discovery of new lands. The classic description of this can be found in The Vinland Saga which tells of Bjarni Herjolfson in 985/6AD who was going to Greenland with a cargo of cattle. A gale lasting several days brought him in sight of land to the West. In sailing back northwards he saw more land and reported what he had seen, so that 15 years later Leif Ericsson bought Bjarni’s ship and with a crew of 35 went to explore the new land to see if it could be exploited. They were in North America 500 years before Columbus.

Course Correction. If Bjarni had used a sun compass he would have understood after the gale that he was South of his intended latitude, because he could check to see if the shadow was falling on the gnomonic line at noon.

If the shadow was short of the line he was on a more southerly latitude and should sail North.

If the shadow was over the line, he had to sail South.

If a sun compass is getting a little elderly, because a period of time has passed since it was made, or because the ship has strayed from its chosen latitude, it has been found that it can still be useful. Following the sun compass in the morning, the ship will sail a few degrees to one side of the course, but in the afternoon, it will be brought back onto the desired course.

... and there's more ...

There is much more information about the Sun Compass for you to search for and find. Navigators and famous seafarers such as Robin Knox-Johnson have investigated its properties and used it on voyages. One important aspect of all this, is that it proves that ocean sailors many hundreds of years ago had knowledge and understandings equal to, or better than yours or mine, when it came to observing the sky. They had detailed understandings not only of the Sun, but also the movement of the stars, the seasons, the wildlife in both sky and sea and the tides. They learned and remembered coastlines, landmarks and seamarks. They accumulated this knowledge and passed it down the generations. In many respects they had knowledge and navigation systems similar to the Polynesian people.

On occasion, today’s ‘experts’ may speculate and wonder how sailors managed to find their way across the North Sea without the use of a magnetic compass, which was first used in China and later, somewhere around 1200AD, becoming used in Western Europe waters. I am sure that seafarers who lived virtually their whole lives afloat, prior to these times, would simply have laughed at such questions – like the Polynesians did. They knew where they were relative to the land, almost all of the time – AND – the magnetic compass doesn’t even consistently point to True North. 

Make a Sun compass and play with it. It will teach you a great deal.