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From: Frank Reed (no email)
Date: Sun Dec 07 2003 - 10:39:57 EST
Jim wrote:
"Frank's fascinating introduction lead me to Mystic Seaport's website, on
which I found this gem: the 1804 American impression of the English Nautical
Almanac, scanned page by page in its entirety."
I also have a bunch of scans of my own of various early N.A.'s (the British
versions). When I return from Chicago (which seems far into the future... it's
still snowing), I'll post them on my web site.
And:
"The "Explanations" chapter is especially good fun."
Yes, and one of the best bits of practical advice for shooting Sun-Moon
lunars is right there, direct from Maskelyne's pen. If you spend any time shooting
lunars, you will learn this trick anyway: preset the sextant to the correct
lunar distance within a half a degree or so. Then align the instrument so that
the "horns" of the Moon are perpendicular to the sextant frame. The other
object should pop right into view, and you save a lot of time flailing around the
sky. The trick with the horns works exactly for the Sun, almost exactly for the
planets, and very well for most of the lunars stars. Maskelyne exaggerates by
omission: he makes it sound as if the trick will always work, but three of
the "lunars stars" are well off the ecliptic (and unfortunately all in the same
large section of the sky), so they do not align perpendicular to the Moon's
horns.
Presetting the sextant might seem like cheating, but it would only be so if
you've completely lost the Greenwich date. And in that case, you would first
observe the Moon's position among the stars and "by eyeball" you would have your
longitude to within about 25 degrees (that's a longitude with NO
instrumentation, but you do need a Nautical Almanac and probably a better than average
star atlas so it's not really useful). In practice, the ship's DR longitude (or
"longitude by account" as they used to call it) is translated into Greenwich
time to get the entry for presetting the sextant. By the way, losing the date
and recovering it by rough lunar is an idea I first encountered in Letcher's
excellent book. I read it shortly after it came out in the late 70s and it's kept
me interested in lunars ever since.
Here's a bit of trivia. In the early Nautical Almanacs, you will find an
entry for the "longitude" of the Sun and other celestial objects. This is the
ecliptic longitude and it could be used in calculations of aberration or in
re-deriving lunar distances for times other than the usual 3 hour intervals in the
almanac. Notice that the headings for the other columns are obvious enough. The
declination column is headed "D.M.S." or degrees, minutes, and seconds. But
the heading for ecliptic longitude is "S.D.M.S." Can you figure out what the
first S represents? It's not too tough, but it may surprise you when you figure
it out. It suprised me.
Frank E. Reed
75% Mystic, Connecticut
25% Chicago, Illinois
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