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From: George Huxtable (no email)
Date: Sun May 15 2005 - 16:22:58 EDT
I disagree somewhat with Gordon Talge, who wrote-
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>PS: The British, and rightly so, complained bitterly about us
>"up starts" ripping them off, with copyright violations. We copied
>without regard, by reprinting tons of British
>books and documents. It was kind of a sore in their side.
>
>The British Navy, in spite of America's victories in Revolutionary War
>and in the War of 1812, was still the best in the world and we knew it.
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Although what Gordon says may have been true in many respects, it did NOT
apply to the provision of almanac information, from 1855 onwards, which was
the topic we were discussing.
This is what William Chauvenet had to say about the matter, writing, from
an American viewpoint, in 1868, in his pamphlet "astronomy for US Navy
Officers (Navy scientfic papers No.1)". This paper has been circulated for
copying by many listmembers.
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"The ephemeris of the moon has until within a few years been inaccurate. It
is well known that for a number of years prior to the first publication of
the American Ephemeris and Nautical Almanac in 1855, the ephemeris of the
moon in the British nautical almanac (of which the American amnanacs at
that time were only reprints), and in other European publications of the
same kind, was greatly in error. For example, the solar eclipse of July 28,
1851, proved that the moon's right ascension as given in the British
almanac was on that date in excess 28" (of arc) and consequently the
distance of the moon's limb from any star in her path on that date, could
it have been used at that time, would have been given in the almanac with
an error of about the same amount. The error in the ephemeris, however, was
not constant, but varied during each lunation, so that the error of the
lunar distances was usually less than 20", and sometimes even under 5", but
it never disappeared entirely. Hence, even if the navigator had taken a
perfect observation and had computed it by a perfect method, he was sill
liable to the unavoidable error of the ephemeris, at that time unknown and
by most navigators not even suspected, as they were in the habit of relying
on the almanac as an absolutely perfect publication."
Later, Chauvenet adds-
"The improvement in the lunar ephemeris, first introduced in the American
Almanac by the use of Pierce's Tables of the Moon, and subsequently in the
British Almanac and other European Ephemerides, by the use of Hansen's
tables, reduces the mean error of the lunar distance to about 5"."
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It seems that the British almanac had become unduly complacent, and had
just kept on churning out lunar distance predictions that were no better
than those that had been supplied to Cook by Maskelyne, 90 years earlier,
in spite of the advances in astronomy that had taken place in the meantime.
Whatever had happened in the way of copying prior to 1855, from that date
the American almanac led the way in providing accurate predictions, and by
1868 the other almanacs had been forced to follow suit. Chauvenet was
rightly proud that in the field of astronomical prediction, the New World
was then leading the Old, perhaps for the first time in an important
scientific matter.
As for other matters raised by Gordon-
>It would seem to me that by 1855, having the correct time via
>accurate chronometers was not really much of an issue, if you
>could afford a good one. I think that doing Lunar Distances at sea
>was not that common.
Unless they had a full kit of three chronometers (and few ships did) then
there was no check a prudent mariner could make, to ensure that his
tick-tock, with its pivots and gears and escapement, had not been affected
by a bit of grit in the wrong place. Not without making the occasional
lunar-distance observation. Chauvenet is emphatic, in 1868, about the
importance of this matter for US Navy vessels.
>
>They may have been done on land or in port for getting time in
>isolated places, but I think the Noon Sun Sight ruled, and maybe
>still does. It is the easiest to do, and requires the least
>calculations.
The Noon Sun Sight gives latitude accurately, but not longitude, even if a
chronometer provides precise Greenwich Time. Some sort of Time-Sight, at a
time OTHER than noon, is required before precise longitude can be deduced.
We have discussed that matter at some length on this list in the past.
>I don't think that 30 seconds difference would be that big of
>a deal between the British and American Almanacs, LD's seem to
>me to be very hard to do right and accurate, especially on board
>a ship.
Well, we can agree with Gordon that LD's are very hard to do right and
accurate, especially from a sailing vessel at sea, with violent motion of
the deck and most of the sky obscured by sails. But however well or badly
they are done, errors in the almanac always increase the overall possible
error, making an uncertain observation even less certain. A 30-second arc
error is enough to add an uncertainty of an extra 15 miles to the
longitude, in equatorial waters. Well worth eliminating, when that's
possible. And the American Almanac showed, from 1855, that it WAS possible!
Astronomy, it seems to me, was the earliest scientific endeavour in which
the new nation of America showed that it could compete on equal terms with
the rest of the world. And Chauvenet was an important figure behind making
that happen.
George.
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contact George Huxtable by email at , 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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