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First, I should apologise for a bit of finger-trouble which resulted in my
accidentally sending a completely empty response to a recent posting by
Brad. Please bin [7159].

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

Michael Dorl had proposed [7134]that he might be able to determine UT by
observing the moment when the Moon, and a nearby star, had the same
altitude. In [7154] I had suggested making a different observation for that
purpose.

Michael now asks-

"Does it make any difference what I measure as long as I can determine
the UT for that event from an almanac or some computational tool?"

Well, as long as you can do so, then yes. But...

There are many occasions and places when such observations will provide no
information, or only inaccurate information, about UT. For example, when the
Moon (and therefore also the nearby star) is near culmination, near due
South from the Northern hemisphere, then its altitude is hardly changing,
and any such relative changes in altitude will provide little information
about UT. Such changes would relate more to declination changes of the Moon,
which is even slower than the Moon's motion in celestial longitude.
Similarly, when observed from high latitudes, such relative altitudes, even
when well away from the meridian, change much more slowly than they do from
the tropics. This relates to Frank's qualification, in [7157] "as long as
the ecliptic is relatively vertical (within let's +/-30 degrees) at the time
of the observation", with which I agree..

Michael continued-

"I was thinking that there are two ready references, the vertical and
horizontal.  If knew roughly where I was and had some instrument with
horizontal and vertical cross hairs, I could mark when the star had the
same altitude or azimuth as a limb of the moon.  My almanac or
computational tool could then be used to determine the UT for that event."

Yes, if Michael had some instrument with a crosshair which he could rely on
to be pretty-well horizontal, then he could determine when a limb of the
Moon and a nearby star had the same altitude. But what instrument is
available at sea, for doing that job? I suggest that at sea it could be done
only by measuring up from the sea horizon, and most of the difficulties in
measuring altitudes relate to observing the horizon, rather than observing
the body itself. At least, Michael's proposol could null out a major
unknown, the effect on dip of refraction at the horizon.

The strength of the traditional lunar-distance technique is that it measures
the Moon's motion against the stars in a direction  which is near to the
direction in which the Moon is actually moving. So that motion is near
maximum and rather uniformly changing with time; and the measurement doesn't
involve the horizon at all.

There's a useful description of a similar method, in John S Letcher, Jr.s
"Self Contained Celestial Navigation with H.O.208" (1977). It differs
somewhat from Michael's suggestion, in that it relates a Moon altitude to a
time-sight of another object, not necessarily nearby. In that respect it's
more useful, in that it isn't relying on a rare astronomical event, a
near-conjunction. But Letcher doesn't dwell on the inaccuacies of his
procedure, though he does accept it is "slightly infrior to the traditional
lunar distance procedure."

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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