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From: George Huxtable (no email)
Date: Thu Sep 16 2004 - 16:20:24 EDT
We were recently discussing William Chauvenet's "A Manual of Spherical and
Practical Astronomy".
I've just discovered an interesting 20-page pamphlet by Chauvenet,
"Astronomy, comprising suggestions to U.S. naval officers, bearing on
points connected with nautical astronomy, astronomical geography, and
general astronomy", dated 1868.
This was No. 1 of a series of "Navy scientific papers", issued by the
Bureau of Navigation, Navy Department. That's a series I have not come
across before, and don't know how many papers it extended to.
Nor do I recall ever seeing any reference to Chauvenet's pamphlet
elsewhere. It seems to have vanished from the record.
It offers lots of useful and practical advice to navigators. I don't know
where, or how, or even if, Chauvenet acquired his maritime experience. At
that time he was Chancellor of Washington University, Saint Louis,
Missouri; about as far from the sea as it's possible to get. He seems to
have been an academic astronomer and mathematician: does anyone know
better, about any maritime experience?
There's useful stuff about meridian and ex-meridian latitude observations,
about Sumner lines, about the behaviour of chronometers.
He is a strong advocate of using lunars for checking chronometers, even as
late as 1868, and of his own "approximate" method for clearing lunars,
which he published in the American Ephemeris for 1855 (also published
separately as Chauvenet: "New method of Lunar Distances"), and which he
claims to be good to 2 arc-seconds, using tables to 4 figures. Presumably,
this is the same (rather long-winded) method given in his Spherical and
Practical Astronomy, 5th ed. 1863.
He has some interesting words to offer about the state of predictions of
the Moon's position.
"The ephemeris of the moon has until within a few years been inaccurate. It
is well known that for a number of years previous 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 almanacs at
that time were only reprints), and in other European almacs of the same
kind, was greatly in error. Foe example, the so;ar eclipse of July 28,
1851, proved that the moon's right ascension as given in the British alanac
was at that date in excess of 28" (in 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 of 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 less than 5", but it
never disappeared entirely."
To me, this was something of an eye-opener. I was aware that in Cook's time
and Vancouver's time, the moon's predicted position could be
nearly-an-arcminute out in celestial longitude, which could put
Earth-longitudes derived from lunars out by 30 arc-minutes or sometimes
more. But I had thought that by the mid 1850s lunar predictions would have
improved to the point at which errors in the predictions were negligible
compared with the errors in the measurement. Not so, however, according to
Chauvenet.
Clearly, he takes some justified pride in the fact that the still-newish
nation has started to be able to tell the old world a thing or two, going
on to tell us that "The improvement in the lunar ephemeris, first
introduced in the American Almanac, by the use of Peirce's Tables of the
Moon, and subsequently in the British and other European Ephemerides, by
the use of Hansen's tables, reduces the mean error of the lunar distance to
about 5"."
It's all interesting stuff, and, coming from Chauvenet's pen, highly
authoritative. I wonder if anyone knows of other titles in that series of
"Navy scientific papers"?
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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