NavList:

There is  a practical problem with observing the upper limb of the moon
with a sextant which if the conditions are unhelpful will make it almost
impossible to get a good alignment measurement.
The problem is that unless the sea state is very calm the glow of the sea
surface beneath the moon will be the same colour and will appear almost as
bright as the moon. The exact contact can be imagined if you take a disk of
white paper and lay it on a page of virgin paper and try and align its upper
edge with the edge of the paper. Almost impossible in poor lighting.
Filters are not a great deal of use unless the horizon filters are a
different colour to the index filter. Some older sextants have this feature
but it is quite rare. All the modern sextants I have seen use the same
colour for both light paths.

Clive.

----- Original Message -----
From: "George Huxtable" <george@huxtable.u-net.com>
To: <NavList@fer3.com>
Sent: Saturday, August 19, 2006 11:24 PM
Subject: [NavList 1090] Re: Moon altitude problems.


>
> This relates to Jim Van Zandt's recent posting under threadname "The
> *&^%$#@ Moon".
>
> I'm overwhelmed, and rather impressed, by Jim Van Zandt's mathematical
> exercise, for testing which limb of the Moon, upper or lower, is the
> sharp one, to observe when it's nearly full. It's far beyond me to
> confirm or refute it. However, I can propose a different test,
> simpler, in my view, and rather more intuitive. Whether they
> correspond to the same thing, Jim may be able to deduce. If not, one
> (perhaps even both) must be wrong.
>
> This is how I see it.
>
> At full Moon, the Sun and Moon are at almost exactly opposite points
> in the sky. Their GHA will differ by 180 deg, and the Moon's
> declination will be the same as that of the Sun but with opposite
> sign. At least, that would be the case if the Moon and Sun followed
> identical paths through the sky, around the ecliptic, but they don't,
> not quite. The Moon can diverge from that path by up to 5 degrees,
> because of the tilt of its orbit to the ecliptic. However, at full
> Moon, a tilt of the sunlight falling on its surface by 5 degrees will
> pull in the shadow-edge from part of the limb by no more that .06', so
> from now on I will neglect that tilt, and presume that the Moon
> follows the same path as the Sun.
>
> Before full Moon, it's always the Western edge of the Moon that is
> lit, and crisp. The Eastern edge is shadowed and fuzzy, and would give
> the wrong answer if used for an observation. This will be true, no
> matter where on Earth the Moon is viewed from. And after full Moon, of
> course, the contrary applies; it's the Eastern edge that must be used.
> Within a few (say 4) hours of full Moon, it doesn't matter which, the
> Moon's edge is lit all round.
>
> The situation we are considering is an observation made away from the
> moment of full Moon, so it becomes important to choose the right limb;
> but not so far from full that it's immediately obvious which limb is
> the sharp one.
>
> Whatever the phase of the Moon, if we observe it near the moment of
> meridian passage, its shadow-pattern will always be symmetrical about
> a horizontal line through its centre. In that case, there's always a
> sharp line round 180 deg of its edge, from its upper limb to its lower
> limb, on one side or the other, with a fuzzy edge on the other side.
> In that case, it doesn't matter which limb is used, because there will
> always be a crisp edge or cusp extending to both upper and lower
> limbs.
>
> What about observations before meridian passage, and after? The
> following rule will apply, as I see it.
>
> Before full Moon-
>
> Before meridian passage, use upper limb, and vice versa.
>
> After full Moon-
>
> Before meridian passage, use lower limb, and vice versa.
>
> Near full Moon, meridian passage will always occur near local
> midnight.
>
> I put this forward with some diffidence, being by no means certain
> that I have everything right. What do others think?
>
> 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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