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Subject: Re: Annual Increase of Variation
From: Steven Wepster (wepster@XXX.XXX)
Date: Tue Feb 12 2002 - 08:11:14 EST
>What is the cause of the "annual" portion of the magnetic variation on
>the compass rose?
The earth's magnetic field is not constant but subject to ... well,
variation. The annual correction is a local linear approximation to the
change. As such it is not valid for ever; its value should be revised after
some time on the basis of regular determinations of the variation.
If the annual change is given as 1' -- 3' then after 20 to 60 years the
variation has changed 1 degree. I don't know the policy of the hydrographic
office: whether they update the compass rose only at new editions of a
chart or also at reprints. If the former applies, and no new edition has
occured in 20 years, then you don't see a change in variation on the chart.
_Steven.
-----------------------------------------------------------
Steven Wepster wepster@XXX.XXX
tel +31 30 253 1186
Mathematisch Instituut
Universiteit Utrecht
PO Box 80.010
3508 TA Utrecht
The Netherlands
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