Bill Gottesman contributed:
>
>Sorry to be a kill-joy, but I fail to see the utility of scribing hour lines
>near the limit of human resolution, when the shadow's edge cast by a solid
>(non-wire) gnomon is blurred by the 0.5 degree width of the sun.
This brought to mind an informal experiment I carried out this morning with an
enlarged version of my Meridian Alidade recently completed for a US client.
The instrument consists of pairs of vertical cylindrical metal posts spaced
0.4mm apart. (see my webpage for a picture of the standard model) The posts
stand astride a fine centreline on a brass protractor plate with one pair at
each end of the line.
This one-off custom version has extra high posts for use by a client at low
latitudes. In use, a sunray passing through the gap is aligned with the
central line and the other pair of posts is used as a sight to lay out a
meridian line after certain calculations are performed. Because of the
height-extended posts and low winter sun the sunray, which diverges slightly
for the reasons Bill has mentioned, passed right across the protractor plate
and between the second pair of posts. Result?...a non-diverging line of light
that crossed the whole table in a very distinct line.
There's a useful principle for a heliochronometer here I think.
Tony (re-inventor of old wheels) Moss
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or phosphor bronze. Professional-quality Dialling Scales,
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