NavList:

This is a continuation of the thread, previously Re: The *&^%$#@ Moon


I have been thinking a bit more about my previous posting (No 1084),
which commented on what
Lars Bergman had written-

He wrote-

| Regarding Robert's trouble with the moon,
|
| ... 2. Sometimes it can be very difficult, or actually impossible,
to
| know
| whether the upper or lower limb of the moon is the one to shoot.
Then
| you'll have to wait an hour or so until the moon has tilted
suffiently
| to show a clear limb but near full moon it is easy to make mistakes.
A
| false LL gives a too large altitude, a false UL gives a too small
| altitude.
|
| ========================
And I replied-

| I agree with Lars that it's important to choose the appropriate limb
| of the Moon. But in the circumstances he describes, in which the
| choice isn't obvious, would one expect to see a significant error if
| the wrong one was chosen? Robert Eno was looking for discrepancies
of
| a few arc-minutes.
|
| =========================

I still consider that it would be unlikely for such a limb-ambiguity
to give rise to the large Moon discrepancies that Robert Eno reports.
However, I think I was too quick to dismiss the difficulty to which
Lars drew attention, near the moment of full Moon. There are
circumstances, particularly in low latitudes, when the Moon
shadow-divisor is nearly horizontal. Then, near full, when it's hard
to choose the appropriate limb, choosing the wrong one could cause a
significant
error, though I think discrepancies as big as Robert reports are
unlikely. It might, as Lars suggests, be better to wait until the
correct, sharp, Moon-edge limb became obviously different from the
shadow-limb. But I think that would call for a wait of more than Lars'
"hour or so". Perhaps it would be better to take the moment of full
Moon from the almanac, and deduce from that what should be the right
edge to use. Because Robert Eno gave no details, we don't know whether
or not his problems occurred at or near full Moon.

It would be useful to look at the full details of Robert's observation
log of Moon and Vega on that night. There are corrections that have to
be made to the tabulated values that are far more important in the
case of the Moon than for other bodies, because of its fast motion. If
he is getting by with short-cuts that are acceptable for other bodies,
but not for the Moon, that might explain his problems.

There's one type of observation that you simply can't make with the
Moon, that you can with other bodies. With anything else, you can do
without any timepiece, and simply observe the highest altitude at the
moment of culmination, whenever that may be, as it crosses your
meridian. However, the Moon's declination can be changing so fast (at
up to 17' per hour, a North-South speed of 17 knots!) that its maximum
altitude can occur at a very different time from meridian passage.
There is, then, no alternative but to note the time of the
observation, whenever that was, and base the corrections and
calculations on that moment.

The Sun's greatest rate of change of declination, at the equinoxes, is
no more that 1' per hour, or a North-South velocity of 1 knot, so
there's little discrepancy between meridian passage and maximum
altitude. Planets travel slower still: the stars not at all.

Lecky, in "Wrinkles", in the late 1800s, had already dismissed using
the Moon for lunars, as obsolete technology. As for Moon altitude
observations, he first goes into some detail about correcting for
parallax, and then continues-

"From the foregoing we deduce the practical resuts that in
observations of the stars Parallax is totally insensible, and  that in
observations of the Sun or Planets it is so small that it may be left
"out in the cold" without detriment to Navigation. The Moon, then, is
the only body which is seriously affected by it, and as for this and
other reasons we decided long ago to send the Moon to Coventry (except
when required for Azimuths), Parallax may as well pack up and go with
her."

Non-British readers may need to know that "sending to Coventry" means
to ignore. Lecky doesn't go into those "other reasons", which is a
pity. Yes, parallax is an important correction for Moon observations,
and has to be got right, but it's not a difficult thing to do, from
the correction tables in the almanac. Parallax for the Sun is never
much more that 0.1', and for planets within 0.5', so Lecky's
assessment, that for those bodies parallax can be neglected (for
altitude observations) is reasonable. For the Moon, however, parallax
can reach one whole degree, so that it has to be got just right.

I can't claim anywhere near such experience of observing the Moon (or
anything else) as can Robert Eno. And I have no experience at all with
his bubble attachment. But it would surprice me if there was any
inherent difficulty about Moon observations (other than that of false
horizon at night) to give rise to erroneous results, and first, we
should eliminate any possible source of error in the data corrections.

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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