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Delighted to hear that Doug is getting hooked on lunars.

I recommend a Sun-lunar to start off with. That way, you can't fail to
identify the two bodies! And you will have daylight to read the sextant and
note the answers. But for highest accuracy, a star or planet will be best.

Yes, to do the job properly, you will need the altitudes of Sun and Moon,
and that's what the artificial horizon is for.

My recommended sequence is-

Alt Sun
Alt Moon.
Lunar distances (3 or 4, say).
Alt Moon.
Alt Sun.

The idea is to get the averaged time of the lunars to be the same, or
nearly so, as the average time for Sun alt and the average time for Moon
alt.

With a sextant and an art. horizon, you won't be able to measure altitudes
above 60 degrees, and lower altitudes (say 30 degrees) are easier, if you
can find a time when both bodies are low enough in the sky. That may be
impossible: check with the almanac.

For the altitudes, you will need some sort of low-sided trough or tray
containing Mercury (ideal but hard to acquire), old engine oil (black and
shiny), black treacle (molasses). At a pinch, you can use water or black
tea, but these are easily ruffled by wind. Raise the trough on a firm stand
or table so you can get close to the reflecting surface. Or get down on
your knees. To minimise wind-ruffling, work if you can in the lee of a
building, or set up windshields around the liquid. You can use a "cloche"
arrangement, but this brings its own problems unless the glass quality is
really good.

The altitudes aren't needed to high precision but the lunar distances need
to be taken with the utmost possible accuracy you can achieve. Check the
index error carefully before and after. Be sure the sextant shades don't
distort. From on-land, you may be able to use a x10 telescope, if you have
one, rather then the usual x3, but it will be hard, then, to get both
bodies into view. For your first lunar, the x3 will probably best. For an
initial sighting, you could remove the telescope altogether. You will
probably need to spend some time leaning well back, so provide yourself
with appropriate cushions and backrests to lean against.

It's much easier if there's a helper to note altitudes and take the times.
In that case, the observer doesn't have to take his eye away from the
sextant between repeated shots, and  may be able to take shots at about
1-minute intervals. It's best to have a digital watch that's been checked
against GMT, so that the accuracy of the lunar measurement of time can be
checked. The record-taking needs to be efficient.

Because the relative movement between the two bodies changes about 30x
slower than an altitude does, you have to adapt your procedure accordingly
when measuring the lunar distance. You can't simply set a gap and wait for
the moment that the gap closes: that would be too slow. You have to
actually drive the micrometer knob until the edges kiss (always ending in
the same direction, of course, but you will know that one already).

Refraction corrections will certainly be different from those at sea level.
The refraction tables in the almanac go down only to 970 millibars: not
enough for your height, at a guess. Find the local pressure as a fraction
of standard pressure, then multiply standard sea-level refraction by that
factor. For the measurements in the art. horizon, and also for the lunar
distances, you don't need dip.

To check how well you are doing, you will need a good geographical
position, from GPS or from good mapping.

What I am looking forward to, when Doug goes back to sea, is for him to
lock away his chronometer and GPS, and claim the record for the first
passage, for more than 100 years, made by an ocean freighter using only
celestial navigation.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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