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I've recently been contacted by someone who has used Frank Reed's
useful lunar distance calculator, at-

http://www.clockwk.com/lunars/lunars_v4.html

He measured lunar distance to Jupiter, when it was close to the Moon,
only 5 degrees away or so, and when Jupiter happened to be well
out-of-line with the Moon's direction of travel. Normally, one chooses
to measure between the Moon and a body that's within a few degrees of
the Moon's path, but in this case the direction from Moon to Jupiter
was skewed by something like 65 or 70 degrees away from that path, to
judge by the direction of the Moon's shadow-terminator (that
terminator should usually be roughly at right angles to the motion of
the Moon, within a few degrees). So it wasn't the best choice, for a
lunar distance.

Nevertheless, his observation arrived at a cleared lunar distance that
differed by 1.8' from the predicted value. What surprised my was that
the lunar calculator then deduced a longitude error of 52.6', roughly
30 times greater; just the sort of number one would expect if the
skew-angle of the observation had been small.

So I wondered whether a skew-angle between the sight direction and the
Moon's travel is taken into account at all, when assessing longitude
error.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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