NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Paul Brewer
Date: 2010 Mar 2, 18:07 -0800
I have an old Booklet from Watts the instrument makers regarding compasses entitled " How to use a Prismatic Compass" from which I have extracted the following..
"CHANGES IN MAGNETIC VARIATION.
It has already been explained that the angle between the Magnetic and True North is called the Magnetic Variation. This angle is termed East or West variation according to whether the needle points to the East or West respectively of the True North. The magnetic variation is subject to several changes :
(1) Annual. At Greenwich the variation was 11° E. in 1576. It then gradually decreased until in about 1660 the magnetic needle pointed to True North. The needle continued to move westward until in about 1816 it reached its greatest westerly variation (24° 30' W.). Since then the needle has been moving eastwards, and the variation has been decreasing at the rate of about 7' annually, but this rate is not constant; it is now (October, 1917) 15° 0' W. at Greenwich. The complete cycle of movement occupies a period of about 400 years, i.e., the needle will again point True North about the year 2060.
(2) Positional. The variation also changes according to the position on the earth's surface. See Figs. 3, 4, and also 41 (end of book).
(3) Seasonal. The variation changes slightly with the seasons, being at its greatest between Spring and Summer, and smallest between Autumn and Winter. The difference seldom exceeds 16'.
(4) Diurnal. The needle occupies its mean position about 10 a.m. and again about 6 p.m. throughout the year. It reaches its most westerly position about 2 p.m., and its most easterly position during the night or early morning, according to the season of the year. The difference is very slight. Recent observations at Woolwich show an average increase of 10' between 10 a.m. and 2 p.m.
(5) Secular. Accidental perturbations, due to magnetic storms, affect the needle. These variations in the position of the magnet occur with great suddenness, deflecting the needle right and left with great rapidity, and are, generally speaking, coincident with the passage of great outbursts of sun-spots across the sun's central meridian. The duration of these changes is not accurately known.
(6) Local Magnetic Attraction. This local variation is due to the presence of masses of magnetic iron ore, or of iron or other magnetic substances in the neighbourhood of the needle. For instance, in certain hills in Central Africa a movement of a few feet will cause a movement of the needle of 10° or more. There is no way to allow for local variation except by carefully comparing the compass reading along a line of which the true bearing is known.
The annual, positional, and local variations are the only ones of any real importance.
This publication was dated 1917! but the principle effects must be still with us.
Paul.
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