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The Polar Museum: news blog

Friday fun (on a Monday): solar eclipse and solar compass

When I got into work on Friday, I was eagerly anticipating the first solar eclipse to be visible in the UK since 1999. Sadly, the sky above Cambridge was very overcast that morning, so the eclipse itself was a bit underwhelming. Here’s the view from my desk at 9.30am, when the eclipse was at its deepest:

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It didn’t look much darker than a normal, gloomy March day!

However, the eclipse did remind me about one of my favourite objects in the Polar Museum, so I’m going to write about that instead:

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This is a sun compass that was made and used on the British Antarctic Expedition of 1910-13 (Terra Nova).  A sun (or solar) compass was a useful instrument for navigation in situations where you can’t use a magnetic (needle) compass.

Magnetic compasses are of limited use in Antarctica for several reasons. Firstly, there are not one but three South Poles: the geographic South Pole, the geomagnetic South Pole, and the magnetic South Pole. The Geographic South Pole is what most people mean when they talk about the “South Pole” and was the point that Scott and Amundsen were racing to reach in 1912. There are two geographic poles, South and North, and they are the the northernmost and southernmost points on the earth’s surface (at 90°N and 90°S). If you imagine giant rod going right through the middle of the earth, and the earth spinning around this axis, the geographic poles would be located at the points where the rod went through the earth’s surface.

The magnetic and geomagnetic South Poles are not in the same location as the geographic South Pole – in fact, the magnetic South Pole is currently nearly 3000km away from the geographic South Pole! If you draw a line from your location to the geographic South Pole, and another one to the magnetic South Pole, the two lines will form an angle, known as the magnetic declination. In many parts of the world, the declination will be very small, but as you approach a geographic or magnetic pole, the difference can become very large. If you were standing halfway between the geographic and magnetic South Poles, the declination would be 180°, and your compass would be leading you in completely the wrong direction!

The magnetic South Pole is important for navigation because it is the point where the earth’s geomagnetic field is vertical: the magnetic poles are the points that the needle on a compass are attracted to. This can be useful for navigators, but also provides them with a bit of a headache: when you’re directly over a magnetic pole, the needle on your compass no longer spins round to point to the pole but instead tries to point straight up or down because the magnetic field is vertical at that point. If your compass doesn’t have an appropriate counterweight, the compass needle can break against the casing.

Another problem with trying to navigate by the magnetic poles is that they don’t stay still, but are constantly “wandering” relative to the earth’s surface. At the moment, the magnetic South Pole is drifting northwest by about 15 km per year. You might know where the it was last time it was surveyed, but it’s quite likely to have moved since then…

Finally, traditional navigation methods can be difficult in Antarctica because there is a lot of natural geomagnetic variation. On Ross Island, there is so much iron in the rock that magnetic compasses are virtually unusable and you have to use other, non-magnetic methods. This is where the sun compass comes in!

The sun compass above was made and used during the Terra Nova expedition, and was designed to be used in the vicinity of the the magnetic South Pole. It is made from wood, with a hand-written paper disc stuck to the top. The disc is divided into two sets of 12 hours (am and pm), and also has compass bearings marked in red around the outside. There is a hole in the middle of the circle that would originally have had a needle stuck in it – this casts a shadow on the face of the compass, a bit like the gnomon on a sundial. According to our collections database, “sun compasses or portable sundials  have been used in expeditions, particularly in the vicinity of the magnetic poles. This instrument affords a means of finding the time if the meridian is known, or the meridian if the time is known. It may be set with the help of a watch chronometer and held in the hand while marching. The error is no greater than that of a sluggish magnetic compass.”

The men on the Terra Nova expedition would have known the time accurately because they had chronometers with them. A meridian is a line of longitude – if you walk due south along any meridian, you will eventually end up at the geographic South Pole (which is what Scott’s men were aiming for!). This particular sun compass is marked “Cape Evans, Lat 78°S, Dec 1st” (click on the picture above to see a bigger version).

This isn’t the only sun compass in our collection – or even the only one from the Terra Nova expedition. The one below was made by Raymond Priestley and was used by Apsley Cherry-Garrard on the Terra Nova expedition:

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As you can see, it has a very similar design to the other one. We also have this sun compass, which was made by Edward Wilson and used on the southern sledge journey of the Discovery expedition (Scott’s first Antarctic expedition):

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I love these objects for their simplicity and homemade charm – they prove that, sometimes, simple methods beat high-tech ones, and that you can always find your way if you know where the sun is (except during an eclipse…).

Christina

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