NavList:

I suspect we may be getting to the bottom of Robert Eno's difficulties
with the Moon, using the bubble attachment to his sextant.

I will quote below the dialogue between us on that matter.

================
| George wrote:
|
| When observing the Sun, as I understand
| > it, the aim is to match the disc of the Sun with the disc image of
the
| > bubble, and try to keep it there.
|
| Robert responds:
|
| That is pretty much the case, although it is better to have a bubble
which
| is slightly larger than the disc of the sun. Not too much larger but
just a
| couple of "hairs".
|
| George wrote:
| How does that work out with the
| > non-disc of the Moon, when only a part of that non-disc is the
crisp
| > edge of the appropriate limb? What do you centre, against what?
|
| Robert responds:
|
| This is where it gets tricky. Sometimes, you can see the faint
outline of
| the rest of the moon, while other times you have to almost "imagine"
the
| shape of the rest of the moon and try to align this, and the portion
which
| you can see, in the centre of the bubble. My observations of a few
nights
| ago, were of a gibbous moon, so there was plenty to work with.

=================

And there, I suspect, lies the problem, that doesn't occur when a real
horizon is used, offset by a precise semidiameter. Nor does that
problem occur with a bubble horizon, with any other body than the
Moon, because everything except the Moon is symmetrical about its
centre, and symmetry is easy to estimate precisely.

Robert's phrase, about a gibbous Moon, "there was plenty to work
with", is particularly revealing. No, there wasn't plenty to work
with, there was no more to work with than for the flimsiest crescent
Moon. The only part of the Moon's edge that has any significance for
such a bubble observation is the half-circle of its lit edge. The rest
of the Moon's non-disc is no more than a dangerous distraction that
has to be ignored, though I suspect that the observer's eye can't help
but to be influenced by its presence when assessing the centre. All
that's of relevance is the "D" shape, comprised by that lit
half-circle and the line between the cusps, ignoring everything else.
Then, Robert's technique implies that he must somehow estimate the
centre point of that half-circle, and put it in the centre of the
bubble. Although the cusps show up precisely when the Moon's disc is
about half-lit (at first and last quarter) their position is much
harder to estimate when the Moon is nearly gibbous, or when it's a
fine crescent. So taking the mid-point between the cusps would be an
unreliable method, when those cusps are very blunt or very fine..

How, otherwise, is the Moon's centre to be found, when estimating by
eye? Perhaps the best way would be to assess the total diameter from
the distance between the cusps, mentally halve it, then work in from
the lit edge by that halved amount, deliberately ignoring the rest of
the disc.

I suspect that Robert would probably admit that he assesses the Moon's
centre in a semi-intuitive manner, saying to himself "that looks about
right" when he thinks it is so, without applying any particular rule
to the job. And in that case, it seems plausible that there lies his
difficulty, that such a procedure may turn out to be roughly correct,
but may well be liable to errors of a few arc-minutes, in a Moon with
a diameter of about 30'.

Paul Hirose, always dependable on such matters, with his quote from a
bubble-sextant manual, hits the nail on the head. It suggests, for
Moon observations, that the appropriate limb, upper or lower, rather
than the Moon's centre, is centred in the bubble, and then corrected
accordingly.

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.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---