The Compass

I had a fascination for magnets and compasses all my life. My first respectable job was to look after a geomagnetic observatory on Crozet Islands for a year in 1977. Destiny maybe? Anyway, I just loved it and I still do!

I live in a house in Piora. It was built in 1912 and when I looked at it for the first time, I saw its main veranda was facing North. Looking more closely on a sunny day at noon, I saw that indeed it was exactly facing the sun. The roof post shadows were square with the floorboards edges. How on Earth could they build something with such precision in 1912 still beats me, given that there is not one square angle in the whole structure!

Last week I got my old hiking compass and put it on that floor. I aligned its edge to a board edge. Then I took the picture. Well, the compass needle is not quite square to the boards, is it? In fact it is about 12 degrees to the right, or to the East as navigators say. Is that normal?

Yes, it is. What we are observing here is the magnetic variation, or rather declination. The angle between Geographic (or true) North and the Magnetic North, as given by my trusty old compass.

You can find magnetic declination maps all over the Internet. By convention, declination to the East as observed here is positive. For Casino, the map says between +11 and +12 degrees. Fairly consistent with what we saw.

Next week I will explain what causes the declination.

Map reference: https://geomagnetism.ga.gov.au/agrf-calculations/agrf-model