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Alex Eremenko asked-

>Here is a question to the Lunars experts:
>Why do we need a sextant at all??
>Just watch for the moment when the moon
>"collides with a star" on its normal way
>(there are so many stars around!)
>and notice this moment on your watch.
>It is easy to modify your lunar reduction programs
>then to find GMT of this "collisoon".
>(The "true distance" at this moment equals to
>the corrected "semidiameter". Then take Moon's parallax
>into account and that's it!)
>I mean, of course the version when no altutudes are measured.
>So you have the whole night to look for these collisions.
>
>
>Few days after this came to my mind, I found that
>I was not the first:-)
>The method was proposed centuries ago and it is called
>"ocultations of the stars by the Moon".
>
>Then why was not this practiced at least as much as
>the Lunars were?
>
>I sort of remember someone saying that Lunars give bad
>results when the distance is too small... In the case
>of occultation the distance is zero.
>
>But why is this so? I mean what is wrong with the method,
>and why Lunars are not recommended when the distance is small?
>
>Alex.

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

The first navigator to use lunars in a practical way at sea was Edmond
Halley (famous for Halley's comet), who made two scientific voyages in
command of HM sloop "Paramore", to survey magnetic variation in the North
and South Atlantic. His journals are in "The three voyages of Edmond Halley
in the Paramore, 1698-1701" edited by Norman J W Thrower, and published by
the Hakluyt Society in 1980.

You will find some nav-l correspondence about Halley if you search under
"Halley" from 5th May 04 and for a few days thereafter.

Godfrey and Hadley had not then invented their quadrants, and the only
instrument of use to Halley was a good telescope with a crosswire at its
internal focus.

He did not measure occultations, or collisions of stars with the Moon, as
Alex vividly describes them. Instead, he timed "near-misses", or close
conjunctions, in which the star passed close to the Moon. He set his
cross-wire along the line joining the Moon's two horns, and timed the
moment when the star crossed that line, presuming that to be the moment
when the star and the Moon had the same ecliptic longitude. This was nearly
true, but the line joining the horns could differ by a few degrees from
being at right-angles to a line of equal ecliptic longitude, so it didn't
work well unless the star came really close.

Halley was a professional astronomer who had spent years surveying and
recording the stars of the Northern and Southern skies, of which he had an
intimate knowledge. There's no shortage of stars, in the wide band that the
Moon can pass over, but some are dim and little-known, so Halley's
knowledge was certainly needed. He would have no difficulty in converting
star positions (in dec and right-ascension) to give the star's ecliptic
longitude that he needed. However, predictions of the Moon's ecliptic
longitude in 1700 were not good enough for his requirements.

One trouble with occultations is that you had to know precisely which part
of the Moon's disc the star would "strike", because it affects the timing
so greatly. Also, against the bright limb, the star tends to vanish.
Halley's technique, timing from a straight-line that sweeps across the sky,
avoids both difficulties. Though the rate of change of the distance between
Moon-star centres will fall to zero at closest approach, Halley doesn't use
this: instead, his line though the horns continues sweeping through the
stars at constant speed. Refraction differences present no problem, because
Moon and star are at nearly the same altitude. Even parallax matters little
if a star can be chosen which passes the horn-line near the time of Moon's
meridian passage.

Halley was an astronomer-navigator who was well ahead of his time.

George.



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