Thomas Jefferson Papers

William Lambert to Thomas Jefferson, 22 November 1811

From William Lambert

City of Washington, November 22d 1811.


The observations relating to the solar eclipse of the 17th September last, were made in this city opposite Rhodes’s hotel, North 71.º West, 1.. m American measure, from the Capitol. The apparent times of the principal appearances, to the nearest second, are as follow:—

  h. m. s.
Beginning of the Eclipse, 0. 22. 9.
Annulus formed, 2. 2. 6.
do broken, 2. 6. 53.
End of the Eclipse, 3. 36. 53.
  º ′′  dec.
Latitude of the place of observation, (assumed)  38. 53. 25.
do reduced, ( 320 to 319) 38. 42. 54. 465.
  Constant log. to reduce the ☽’s equat. hor. parallax1   }  9.9994653.
for lat. and ratio 

An allowance of −1.″ 623. dec for irradiation of the Sun’s, and −2.′′ 977. dec for inflexion of the Moon’s light, has been made in the calculation.

From the foregoing data, and the elements connected, which have been computed to great exactness, the solar time of true conjunction—

h. m. Sec. dec.
 By the beginning of the Eclipse, is 1. 49. 17. 635.
  the End 1. 49. 17. 678.
Mean True Conjunction at Washington, 1. 49. 17. 656
 Ditto at Greenwich 6. 57. 14. 915
Longitude in time, of the place of observation, 5. 7. 57. 259.
º ′′
 Equal to 76. 59. 18. 885.
difference of longitude West of the Capitol,   1. 26. 978.
Longitude of the Capitol, by the external contacts,  76. 57. 51. 907.
 Say 76.º 57.′ 52.′′

If we connect this result with that obtained from the occultation of n Pleiadum (Alcyone) by the Moon, on the 20th of October 1804, a mean of the two will be, nearly 76.º 55.′ 45.′′, or 5. h. 7. m. 43. Sec. in time, being an approximation which will not, perhaps vary much from the truth, when it shall be tested by future observations.

Before this comes to hand, you will, probably, have received from bishop Madison, my calculation of the longitude of Monticello, from the external contacts of the same Eclipse.

I am, Sir, with great respect, Your most obedient servant,

William Lambert.

RC (DLC); at foot of text: “Thomas Jefferson, late President U.S.”; endorsed by TJ as received 22 Dec. 1811 and so recorded in SJL.

1Abbreviation for “equatorial horizontal parallax.”

Index Entries

  • astronomy; and calculations of Monticello’s longitude search
  • astronomy; and calculations of prime meridian search
  • Capitol, U.S.; longitude measurement search
  • Lambert, William; calculates longitude of U.S. Capitol search
  • Lambert, William; calculates Monticello’s longitude search
  • Lambert, William; letters from search
  • Madison, James, Bishop; and W. Lambert’s astronomical calculations search
  • Monticello (TJ’s estate); longitude of search
  • Rhodes’ Hotel (Washington) search
  • sun; and calculation of longitude search
  • Washington (D.C.); longitude of search
  • Washington (D.C.); Rhodes’ Hotel search