Geology Reference
In-Depth Information
F IG . 5.5 'Steelhenge' on the campus of Strathclyde University, by Gerald
Laing, 1974 (Photo by Linda Lunan, June 2010)
the lunar orbit, marking out four arcs of the horizon. Corrections
for parallax would have to be applied to the lunar alignments, of
course, because we were on Earth's surface instead of at its cen-
ter. The effect of parallax is that a heavenly body appears lower
in the sky than it really is, relative to the center of Earth, and the
displacement has its maximum value when the body is on the
horizon. The stars are so far away that even their horizontal paral-
lax is negligible; for the Sun the average figure is only 8.8 s of arc,
too small to affect my efforts. But for the Moon the value is a full
degree and could not be ignored.
I could neglect it meantime, however, because the effect of refrac-
tion is to bend light downwards as it enters the atmosphere, thus
making the heavenly body look higher in the sky than it otherwise
would. Furthermore, both parallax and refraction have maximum
effect at the horizon and diminish towards the zenith, so there would
be a ticklish job of multiple calculations to do once the actual height
of the horizon had been determined, relative to the true horizon at
 
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