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Michael Dorl wrote, on 21 Jan-

This AM, I noticed Antares very close to the moon being 46' from the
center of the moon.

So, I wondered if it's possible to do something similar to a lunar by
estimating the difference in elevation between the Moon and a star. For
example, one could wait until Antares was at the same altitude as the
lower limb of the moon, wouldn't that give you a good estimate of GMT?

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

When a bright star, such as Antares, is actually blotted out by the Moon
(i.e. "occulted"), it's a naked-eye event that can give a lot of
information, because it happens instantaneously, and its time can be
precisely determined. There's a big contrast between a star being occulted,
which is just like a light switching off (indeed, even more sudden than an
electric light switching off) and a planet, because a planet's disc has a
finite diameter, and takes time to vanish or reappear. I've watched Saturn
occult behind the Moon, and it's a process that takes many seconds. For a
star, one moment it's there: next moment, it's gone. That provided the best
evidence, to early astronomers, of how infinitesimal is the disc of any
star.

The best occultations, and those easiest to see with the naked eye, and to
time, are those in which the star or planet is obscured by the dark limb of
the Moon, because when a star or planet is close against the bright Moon
disc, it can be much harder to make out. It's also easiest to time a star
disappearing, when you can watch it until it vanishes, rather than
reappearing, when you have to watch for a new spot arriving at what may be
an unexpected point on the limb. In those respects, a waxing Moon gives the
best evening viewing of an occultation.

Timing occultations was by far the most precise way of determining the orbit
of the Moon, in the days before well-calibrated telescopic instruments. It
depends on the position of the star, and the position of the observer, being
precisely known.

What Michael Dorl saw wasn't an occultation, but a close passing of the Moon
near to a star Altares, without obscuring it. This was known as an
"appulse". I doubt whether that night's event was visible as an occultation
from anywhere on Earth, but could be wrong.

Appulse events such as that seen by Michael Dorl were used by Edmund Halley
(the astronomer famous for predicting the reappearance of Halley's Comet,
and later Astronomer Royal.) He was given command of a naval vessel ,
Paramore, in 1699 and 1700, to survey the Atlantic Ocean for magnetic
variation. I think he was the only civilian ever to have commanded a naval
vessel, and this seems to have been the first scientific expedition
supported in such a way. Halley needed to know his mid-ocean positions, in
latitude and longitude, well enough to allow him to plot in his measured
values of magnetic variation.

Halley needed a way to determine his longitude from at sea, and was, as far
as I know, the first mariner to do so successfully, by observing the
position of the Moon among the stars. Remember, this was more than half a
century before the "lunar distance" method had become feasible.

Nowhere does Halley give a clear account of his methods, so we have to apply
some deduction to his results. He used no instrument for measuring lunar
distance, just a good telescope with a cross-hair in the eyepiece, in the
use of which, even at sea,  Halley was very skilled. He seems to have relied
on the fact that the Moon's path in the sky, always within a few degrees of
the Ecliptic, is close to the great-circle line between Moon and Sun, which
line bisects the illumination of the Moon by the Sun. That means that the
line joining the horns, extended either way, is a line of equal ecliptic
longitude, or very nearly so. Halley would presumably time the moment when
the Moon, in passing close to the star, placed the star exactly in that line
of the horns, on which he had placed his crosshair. Then the Moon would have
the same ecliptic longitude as the star, and he could time that moment and
compare it with predictions of the Moon's orbit, some of which he could have
made himself.

Refraction corrections are no problem, because the two objects are at nearly
the same altitude, so refraction is the same for both. Parallax is another
matter, greatly affecting the Moon's altitude, by up to a whole degree, and
not the star. How Halley managed to allow for parallax, I have no idea.

Ted Gerrard's "Astronomical Minds" provides good reading about Halley's
observations.

So, to come back to Michael Dorl's suggestion, what he could time is not the
moment when Moon and star had the same altitude, but the moment when the
star passes through the (extended) line through the Moon's horns.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.








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