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Bill Murdoch wrote, about an online calculator for magnetic variation-

>I entered my location (35.0N 85.0W) the variation was -3.797 degrees.   I
>entered a point on my latitude but one degree father west (35.0N 86.0W)
>and  the
>variation was -3.797.  On land I think I could measure the magnetic  azimuth
>of a setting star (or at lest one low in the sky) to  maybe 0.25 degree without
>too much trouble and determine my local  variation.  That would give me my
>latitude to 20 minutes.
>
>Does Harrison owe me a couple of pounds (with interest)?

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I'm puzzled. Bill implies that there was no change in variation with
longitude? (Both locations showing variation of -3.797 deg, though 1 degree
apart in longitude?). Was that what he meant to write?

Halley's hope was to measure his longitude by magnetic variation changes.
Latitude was of no interest, because that was so easy to measure anyway.

In some parts of the world, variation does change rather rapidly with
longitude, and in those circumstances it would be just about practical to
use variation to get a rough idea of longitude. But the shape of the
equal-variation lines is so complex, and so inconstant over time, that it
was never going to be a worldwide method, as Halley had hoped. I have come
across just one account of its practical use, at sea.

Bill thinks he could measure local variation to 0.25 degrees, using the
azimuth of a low star, and on land perhaps he could, though it's asking a
lot of a magnetic compass if there's a large angle of dip. Not so easy at
sea, though.

George.

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