RC (PPAmP); at foot of text: “Thos. Jefferson President of the U.S. and of the A.P.S.”; endorsed for the American Philosophical Society. Recorded in SJL as received 6 Jan. 1802. Enclosure: “Astronomical observations made at the Borough of Lancaster in the State of Pennsylvania,” containing data of observations made 2 Nov. to 24 Dec. 1801 to find longitude by daily observations measuring distances between the sun and the moon; Ellicott reporting that “in several respects” measurement of “lunar distances” to find longitude seemed to be superior to the generally preferred method, which was based on observations of Jupiter’s moons (MS in PPAmP; in Ellicott’s hand, unsigned and undated; endorsed for the APS description begins American Philosophical Society description ends ).

TJ transmitted Ellicott’s letter and the enclosed observations to the APS description begins American Philosophical Society description ends , where the paper was read at a meeting on 15 Jan. 1802. The paper was referred to a committee consisting of Robert Patterson and Benjamin Latrobe, who at the next meeting, on 5 Feb., reported that it was worthy of publication. The observations were not, however, printed in the society’s Transactions. Instead, the paper appears to have been superseded by another set of observations Ellicott sent in December 1802, this time in a letter to Patterson. Observations in the new paper overlapped with some of the data Ellicott sent with the letter above, and both papers were aimed at testing the theory of using the moon to find longtitude. The new report was read at an APS description begins American Philosophical Society description ends meeting in January 1803, approved for publication, and included in the sixth volume of the Transactions (APS description begins American Philosophical Society description ends , Proceedings, 22, pt. 3 [1884], 320, 330, 333; APS description begins American Philosophical Society description ends , Transactions, 6 [1809], 61–9).

Partly because measurements of the moon’s position could be made more frequently, and with a less powerful telescope, than observations of the disappearance and reappearance of Jupiter’s moons, Ellicott, Patterson, and TJ concluded that the Lunar Theory was the method best suited for determining longitude in the field. In 1803, they instructed Meriwether Lewis and William Clark to collect data using the lunar method during the journey of the Corps of Discovery to the Pacific (Bedini, Statesman of Science description begins Silvio A. Bedini, Thomas Jefferson: Statesman of Science, New York, 1990 description ends , 342–9; Richard S. Preston, “The Accuracy of the Astronomical Observations of Lewis and Clark,” APS description begins American Philosophical Society description ends , Proceedings, 144 [2000], 168–91; Donald Jackson, Thomas Jefferson & the Stony Mountains: Exploring the West from Monticello [Urbana, Ill., 1981], 176).