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Re: Thomas Jefferson and Lunar Obs.
From: Frank Reed CT
Date: 2005 Mar 26, 02:13 EST
From: Frank Reed CT
Date: 2005 Mar 26, 02:13 EST
One more
"longitudinal" quotation from the Lib o' Congress web site...
From the
President of the United States, Thomas Jefferson, to William Dunbar in a
letter dated May 25, 1805:
"While Capt. 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 timekeeper, and I knew that a thousand accidents might happen to that in such a journey as this, & thus deprive us of the principal object of the expedition, to wit, the ascertaining the geography of that river, I sat 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 occured to me that as we can always have a meridian at land, that would furnish what the want of it at sea obliges us to supply by the timekeeper. Supposing Capt. Lewis then furnished with a meridian, & having the requisite tables & Nautical Almanac with him, 1. 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 know star) & measure her distance in that moment from his meridian. This distance would be the diference of longitude between Greenwich & the place of observation. Or 2dly. Observe the moon's passage over his meridian & her right ascension at that moment. See by the tables the time at Greenwich when she was on his meridian. Or 3dly. observe the moon's distance from his meridian at any moment, & her right ascension at that moment, & 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 & a known star are in the same vertical.
"While Capt. 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 timekeeper, and I knew that a thousand accidents might happen to that in such a journey as this, & thus deprive us of the principal object of the expedition, to wit, the ascertaining the geography of that river, I sat 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 occured to me that as we can always have a meridian at land, that would furnish what the want of it at sea obliges us to supply by the timekeeper. Supposing Capt. Lewis then furnished with a meridian, & having the requisite tables & Nautical Almanac with him, 1. 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 know star) & measure her distance in that moment from his meridian. This distance would be the diference of longitude between Greenwich & the place of observation. Or 2dly. Observe the moon's passage over his meridian & her right ascension at that moment. See by the tables the time at Greenwich when she was on his meridian. Or 3dly. observe the moon's distance from his meridian at any moment, & her right ascension at that moment, & 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 & a known star are in the same vertical.
I suggested
this to Mr. Briggs, who considered it as correct & practicable and proposed
communicating it to the Phil. society; but I observed that it was too obvious
not to have been thought of before, and suppose had not been adopted in practice
because of no use at sea where a meridian cannot be hand, and where alone the
nations of Europe had occasion for it. Before his confirmation of the idea
however, Capt. Lewis was gone. In conversation afterwards with Baron Humboldt,
he observed that the idea was correct, but not new & that I would find it in
the 3d vol. of Delalande. I recieved two days ago the 3d & 4th vols. of
Montuda's his of Mathematics, finished & edited by Delalande; and find in
fact that Morin Y Vanlangren in the 17th century proposed observations of the
moon on the meridian, but it does not appear whether they meant to dispense with
the timekeeper: but a meridian at sea being too impracticable, their idea was
not pursued. The purpose of troubling you with these details is to submit to
your consideration and decition whether any use can be made of them
advantageously in our future expeditions, & particularly that up the Red
river.
Your letter
on the current of the Missisipi, and paper on the same subject, corrected at
once my doubts on your theory of the current of that river. Constant emploiment
in a very different line, permits me to turn to philosophical subjects only when
some circumstances forces them on my attention."
LOL. Constant employment in a very different line... :-D
-FER
42.0N 87.7W,
or 41.4N
72.1W.
www.HistoricalAtlas.com/lunars
www.HistoricalAtlas.com/lunars