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Frank Reed wrote-

If you measure the Moon's altitude, it's simple enough: you
>need more accurate observations than you would lower down. And high altitudes
>are always dificult to measure so this is a real concern --measure the
>altitude of the Moon wrong by a quarter of a degree and you'll throw off your
>longitude by as much as an eight of degree.

I agree with Frank's numbers here. However...

There are two main contributions to the true, cleared, lunar distance. One
is the error in the clearing process, and Frank has assessed the main
component of that correctly.

The other is the contribution from the measurement of the lunar distance
itself. If we were aiming for a contribution to the overall error in
longitude from that source to be no more than an eighth of a degree, then
the angle between the Moon and the other body would have to be measured to
a quarter of a minute (this factor of 30 arises because the Moon moves past
the stars at only 1/30 of the speed that the Earth spins).

So there we have the comparison: for the same accuracy in the result, the
lunar distance must be measured to a quarter of a minute (very difficult),
whereas the Moon altitude only needs to be measured to a quarter of a
degree (very easy). That's why, in my simplified picture, I have completely
ignored any such inaccuracy in the clearing process itself, and presumed
that all the error resides in the lunar-distance measurement. It's not
exactly true (approximations never are) but it's quite good enough for the
purposes of our discussion.

It's a poor navigator who could measure Moon altitudes only to a
quarter-degree, though special circumstances may arise, such as measuring a
Moon's altitude at night above a shimmering horizon that's lit only by that
Moonlight.

So when Frank says-

>The rapidly changing parallax of the Moon at high altitudes does have an
>effect on the accuracy of lunars. It's not a huge issue, but there's
>something to
>it.

I can't disagree. It isn't a huge issue, at all. It's a VERY small one. But
as Frank says, there is something in it, if not much. It's pleasing to note
that Frank's views and mine appear to me converging, about the general
principles involved.

George.

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