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I have switched to a more relevant threadname.

Frank Reed wrote-

"By the way, if you want to think about lunar distance observations
differently, each lunar generates a line of position, which you could plot
on a chart just like any other LOP. But unlike standard altitude LOPs, an
error of 0.1 minutes of arc in a lunar distance observation implies a 6 n.m.
error in the lunar LOP. One advantage is that you can shoot lunars when the
horizon is hidden. Cross two lunar LOPs and you get a rough position fix."

Earlier, I questioned that second sentence, suggesting that the 6 miles
given should have been 3. But questioning the rest of that paragraph, also,
may help our understanding.

It calls for some clear thinking about what we are discussing, a lunar
distance observation. Does that, in itself, generate a line of position? Not
on its own, no. A lunar distance itself is nothing more than a measurement
of the position of the Moon in its orbit. From that, you get Greenwich time,
nothing else. You can set your watch from it, that's all. It tells you
NOTHING about your position, just the same as reading Greenwich time from
your chronometer tells you nothing about where you are.

It is generally true that in order to correct a lunar it is normal to
measure the altitudes of the two bodies involved as well as the lunar
distance itself. But that's not necessary, and it's quite possible (but a
bit complicated) to correct a lunar by computing where the two bodies must
be in the sky, without making any altitude observations at all. All it calls
for is a reasonable estimate of latitude, which a mariner could usually
provide. Indeed, to "shoot lunars when the horizon is hidden", as Frank
suggests, that is the only way the job can be done. It's what Frank's
on-line computer will do for you.

If, however, the lunar corrections are based on a measurement of the two
altitudes, then high precision in those altitudes isn't necessary. That
makes it possible to measure star-Moon lunars at night, when there's only an
indistinct moonlit horizon, though still, care needs to be taken to avoid a
"false horizon".

As long as it's been possible to precisely measure both altitudes, under
favourable circumstances (sharp horizon), then each such altitude can be
used to generate a LOP, once GMT has been deduced. That's the standard
process of Sumner-line navigation, and they can then be crossed to give a
position. Alternatively, those precise altitudes, to get position, may be
measured at a more propitious moment, away from the lunar itself, the
chronometer having been set from the lunar so that Greenwich time is then
known.

So what I am saying here is that a lunar observation itself may provide no
position line at all, or the observations taken with it may provide two
position lines, and so a fix. Which is why I question Frank's statement that
"each lunar generates a line of position, which you could plot on a chart
just like any other LOP". It doesn't. If it did, which direction would that
LOP run?

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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