**** Ulrich Bretscher's Pocket Watch Page ****

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Compass Declination
and the determination of the age of a sundial

The declination of the compass needle was discovered independently in 1535 by the English mathematician H. Gellibrand and 1544 by the German mathematician Georg Hartmann, who reported his discovery in a letter addressed to his lord Albrecht of Prussia I, he had found a deviation of the compass needle of 9° east when in Rome in 1544.
Later, Hartmann settled in Nuremberg where he opened a shop as compass maker. (Portable sundials were then called "compasses" then). He was the first who engraved his compasses with an arrow, indicating the magnetic declination. That allowed the exact aligning of the sundial north-south.

In the last 400 years the magnetic pole wandered on a narrow loop going towards and away America. Seen from Europe the pole moved east and west. So the apparent movement of the magnetic pole was far greater seen from Europe than from USA and Canada.

Fig 1: The wandering of the magnetic pole since 1600

The era of pocket sun dials went from 1550 to 1840. Luckily, during all this time the magnetic pole moved from east to west. Hence any compass declinations of a sun dial can be assigned to a specified date. Since the magnetic field of the earth doesn't run straight from north to south but is meandering, the declination can't be computed from two recorded positions of the magnetic north- and south pole at a given time. We have to rely on recorded data.

The declinations of the compass needle were first recorded in London and Paris, both since 1544. Most countries started to record and publish it in the early 1800s, but Switzerland only started recording it in1903. Sadly there are no data available from the Swiss survey authorities nor from Swiss universities prior to 1995! To approximate the declination in Switzerland, I take the declination of Paris minus 1 degree. Declinations from 1955 and later can be taken from Swiss maps, where the declination is given at the year of their issue. On earlier maps the declination is missing.
Fig 2: Compass declinations for different major places in Europe