NavList:

Frank,

Thanks for digging up this interesting letter. Apart from its
entertainment value, it may contain a hint to an answer to a question
that has long been puzzling me:

Jefferson sent Lewis and Clark on their way with the instruction to take
"[...] careful observations of latitude & longitude, at all remarkable
points on the river [...]" and insisted that they "are to be taken with
great pains & accuracy, to be entered distinctly & intelligibly for
others as well as yourself, to comprehend all the elements necessary,
with the aid of the usual tables, to fix the latitude and longitude of
the places at which they were taken, and are to be rendered to the
war-office, for the purpose of having the calculations made concurrently
by proper persons within the U.S." (Jefferson to Lewis, 1803)

There seems to have been no explicit instruction to Lewis as to the
method of establishing longitude. Did Jefferson leave this important
"detail" to Ellicott, the instructor? It's unlikely, when we consider
the minute details specified elsewhere in the instruction. How come it
was understood that chronometer plus lunars was the method to be relied
on? This seems rather strange at a time when the rest of the world had
long been using the Jupiter satellites for cartography and exploration
with demonstrable success. Why was this expedition not equipped with
suitable telescopes? My preferred answer until now was that Jefferson
knew that the trick to a successful exploit of satellite eclipses was
the availability of control observations. Since the U.S. did not have an
observatory of their own, Jefferson would have had to depend on
cooperation in Europe. Maybe he felt that the moon ephemeris in the N.A.
was sufficiently reliable to stand on its own, at least more so than
that of the Galilean satellites.

But now that I read this letter, I see a second explanation. Could the
answer be found in the possibility that Jefferson did not understand the
eclipse method properly? He slips when he argues that it would be
necessary to keep accurate time during the entire voyage. In fact, one
normally requires a watch no more accurate than for a lunar distance,
because the eclipse is supposed to provide GMT.

The text suggests that the letter was drafted in a rush with more
pressing affairs to attend to. This could explain a temporary slip of
the mind. There is a good chance that Groves' idea was theoretically
sound (although totally impractical) and Jefferson missed the point. He
admited to reading the proposal only "hastily". One would have to see
Groves' letter to decide this. Longitude is the difference between GAT
and LAT. When one observes a Jupiter satellite eclipse and Jupiter's
altitude simultaneously one gets both at the same instant. So, if
Groves' suggested what I suspect that he did, one needs no time-keeper
at all, not even a watch.

Herbert Prinz



   FrankReedCT@aol.com wrote:

>Yesterday, I found a delightful letter (via  google books) written by Thomas
>Jefferson in 1808 telling one 'Captain Groves'  that his plan to find
>"longitude at sea by an observation of Jupiter and his  satellites, brought to the
>horizon by a double reflection" is a big waste of  time and he should apply
>himself to the "comfort of [his] family". TJ is very  polite about it all. He notes
>that he understands that the guy has written a  play to raise funds for his
>research (that's a script I would love to see!).  Jefferson points out that the
>longitude problem is solved... "fine time-keepers  have been invented" and
>more so by lunar distances; "every captain of a ship now  understands the method
>of taking these lunar observations, and of calculating  his longitude by
>them". Because of his personal interest in longitude, President  Jefferson must
>have seemed like the American equivalent of the British Board of  Longitude in
>that first decade of the 19th century.
> 
>

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