Why compasses dont point north

The setup procedures for all computerized telescopes assume that you know which way is north. In altazimuth mode, any error that you make will be corrected as soon as you align on a star. In equatorial mode, however, the polar axis must point exactly north for smooth, accurate tracking; we'll return to this in the next chapter.

The proper way to find true north is to sight on the star Polaris. Sometimes, however, you have to use a compass for initial orientation, and in parts of the world - especially Canada and the American West - compasses are surprisingly inaccurate.

The reason, of course, is that a compass points toward the north magnetic pole, not the pole of the Earth's axis. The two poles are not in the same place. What's worse, the north magnetic pole is a complex structure, not a single point, and moves around significantly from year to year.

The discrepancy between magnetic north and true north is called magnetic deviation, magnetic declination,or compass correction and is shown (for North America and Britain) in Figure 3.5. Clearly, if you try to find Polaris with a compass in Seattle, and you are not aware that your compass points 20° east of true north, your attempts to match the compass with the sky will be frustrating.

The good news is that if you can find Polaris, you need not bother with any of this. Polaris is always within 0.8° of true north.

U.S. Geological Survey (adapted)

U.S. Geological Survey (adapted)

Compass points directly north

Compass points EAST of north

Compass points directly north

Compass points WEST of north

Compass points WEST of north

Figure 3.5. Magnetic deviation maps of North America and Britain. Data are for 1995 and can change 1° or more per decade.

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