Thomas Jefferson Papers

William Lambert to Thomas Jefferson, 14 November 1811

From William Lambert

City of Washington, November 14th 1811.


The President of William and Mary college having lately sent me the result of your observations of the solar eclipse of Septr 17th at Monticello, I have calculated the longitude from Greenwich, using the first and last contacts, which will always give a near approximation to the truth, if the apparent times and latitude of the place have been correctly ascertained. I have taken great pains to find all the elements accurately by various rules, and have never, in any previous computation, discovered so great a variance in the time of true conjunction, found from the beginning and End of an eclipse or occultation. It will readily occur to you, that if there be an error in the apparent times, the Moon’s parallaxes, and consequently the true difference of longitude of Sun and Moon, will be proportionably affected. Permit me to remark, that a repetition of the process, using the longitude found, instead of the estimated longitude, would be advisable, in which case, the altitude and longitude of the nonagesimal, and the parallaxes in longitude and latitude would be obtained with greater certainty, and one, among the rules I have given, might be sufficient for each. If it be contemplated to find the longitude from the internal contacts, the difference of the Sun and Moon’s semidiameters instead of the sum, must be used for the purpose, and the difference of apparent longitude of Sun and Moon computed for those times, respectively; but I do not suppose it necessary, as the result already obtained, is believed to be near the truth.

From the data afforded by the same eclipse, the longitude

of the capitol in this city, is found to be
difference of longitude   1.37.19. 043.
I am, Sir, with great respect, Your most obedient servant,

William Lambert.

RC (DLC); addressed: “Thomas Jefferson, late President of the United States, Monticello, Virginia”; notation by TJ on verso: “Long. Monticello 78°–35′–11″”; endorsed by TJ as received 22 Dec. 1811 and so recorded in SJL.

Bishop James Madison was president of the College of William and Mary. The nonagesimal is the point of an ecliptic highest above the horizon (OED description begins James A. H. Murray, J. A. Simpson, E. S. C. Weiner, and others, eds., The Oxford English Dictionary, 2d ed., 1989, 20 vols. description ends ).

Index Entries

  • astronomy; and calculations of Monticello’s longitude search
  • astronomy; and solar observations search
  • Capitol, U.S.; longitude measurement search
  • Lambert, William; calculates Monticello’s longitude search
  • Lambert, William; letters from search
  • Madison, James, Bishop; and observations of solar eclipses search
  • Monticello (TJ’s estate); longitude of search
  • Monticello (TJ’s estate); solar eclipse observed by TJ at search
  • sun; annular eclipse of1811observed search
  • Washington (D.C.); longitude of search
  • William and Mary, College of; president of search