University of Virginia Library

4470. LATITUDE AND LONGITUDE, Lunar observations.—

While Captain Lewis's
mission was preparing, as it was understood
that his reliance for his longitudes must
be on the lunar observations taken, as at sea,
with the aid of a time-keeper, and I knew that
a thousand accidents might happen to that in
such a journey as his, and thus deprive us of
the principal object of the expedition, to wit,
the ascertaining the geography of that river,
I set myself to consider whether in making observations
at land, that furnishes no resource
which may dispense with the time-keeper, so
necessary at sea. It occurred to me that we
can always have a meridian at land that would
furnish what the want of it at sea obliges us to
supply by the time-keeper. Supposing Captain
Lewis then furnished with a meridian, and having
the requisite tables and nautical almanac

with him,—first, he might find the right ascension
of the moon, when on the meridian of
Greenwich, on any given day; then find by observation
when the moon should attain that
right ascension (by the aid of a known star),
and measure her distance in that moment from
his meridian. This distance would be the difference
of longitude between Greenwich and the
place of observation. Or secondly, observe the
moon's passage over his meridian, and her right
ascension at that moment. See by the tables at
Greenwich when she had that right ascension.
That gives her distance from the meridian of
Greenwich, when she was on his meridian. Or
thirdly, observe the moon's distance from his
meridian at any moment, and her right ascension
at that moment; and find from the tables
her distance from the meridian of Greenwich,
when she had that right ascension, which will
give the distance of the two meridians. This
last process will be simplified by taking, for
the moment of observation, that of an appulse
of the moon and a known star, or when the
moon and a known star are in the same vertical.
I suggested this to Mr. Briggs, who considered
it as correct and practicable, and proposed
communicating it to the Philosophical
Society; but I observed that it was too obvious
not to have been thought of before, and supposed
it had not been adopted in practice,
because of no use at sea, where a meridian
cannot be had, and where alone the nations of
Europe had occasion for it. Before his confirmation
of the idea, however, Captain Lewis
was gone. In conversation afterwards with
Baron von Humboldt, he observed that the idea
was correct, but not new; that I would find it
in the third volume of Delalandé. I received,
two days ago, the third and fourth volumes of
Montucla's History of Mathematics, finished and
edited by Delalandé; and find, in fact, that
Morin and Vanlangren, in the Seventeenth
Century, proposed observations of the moon on
the meridian, but it does not appear whether
they meant to dispense with the time-keeper.
But a meridian at sea being too impracticable,
their idea was not pursued.—
To Mr. Dunbar. Washington ed. iv, 578.
(W. 1805)