What a coincidence. Just this morning I picked out of my pile of "read
one day" books, a book called "Electricity and Magnetism", fifth
edition, by S. G. Starling, dated 1929. I happened to be browsing the
chapter on Terrestrial Magnetism ... and then your email popped up. The
chapter has a prediction on passage of the agonic line (0 magnetic
declination) through London which turns out to be a bit wide of the
mark, understandably.
The section reads:
Secular variation - The declination at all points is undergoing long
period change. Records of the declination do not go far enough back for
us to compute with accuracy the periodic time of secular variation, but
it is of the order of magnitude of 960 years. In 1580 the declination at
London was 11d 15' E ; in 1600 5.5d E. According to an observation in
1633 it was still 4d 5' E.; and in 1659 it was zero, the compass at
London pointing due north. Later observations show a westerly variation,
10.5d in 1709, to 24.5d in 1820, when it reached its maximum and has
since been diminishing. At the present time (1911) it is 15d W., and it
is *probable that in 2139 it will again be zero*. It was pointed out by
Lord Kelvin that the magnetic system is slowly rotating from east to
west, making a revolution on 960 years, so that in 960 years the
magnetisation lags behind the Earth by one rotation. The magnetic north
pole describes a small circle of about 17d radius, and the affect of
this rotation upon the declination at any fixed point may be seen from
Fig. 30.
On 31/8/19 8:50 am, [email protected] wrote:
This might be of interest to dialists.
https://www.theguardian.com/science/2019/aug/30/compasses-to-point-true-north-for-first-time-in-360-years
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