Zenith Telescope

Zenith Telescope

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This, the most precise scientific instrument made in America in the 18th century, serves as tangible evidence that Americans could achieve the same level of technological sophistication and scientific accuracy as did their European counterparts.
The story of this telescope (or sector, as it was often called) began on May 8, 1785, when David Rittenhouse wrote to Pennsylvania Gov. John Dickinson about the projected survey of the boundary between Pennsylvania and New York: "I believe there is no Instrument fit for the purpose in this part of America excepting the 6 feet Sector belonging to Mr. Penn. But I have been for some time employed in making one which will be much more portable than that of Mr. Penn, and, I doubt not, equally accurate." The reference here is to the sector that had been made by John Bird in London, which Thomas Penn had purchased in 1763 for Mason and Dixon's survey of the boundary between Maryland and Pennsylvania. Rittenhouse was familiar with this instrument, having used it to establish the boundary between New York and New Jersey in 1769, and for another Pennsylvania survey in 1774.
Andrew Ellicott provided further information about the origins of the American telescope in a letter to Albert Gallatin dated August 7, 1803: "Mr. Rittenhouse, and myself" undertook to make this instrument for the Commonwealth of Pennsylvania, "but being called down to our Mills in Maryland myself the principal part of the work was done by Mr. Rittenhouse." Ellicott then noted that the objective lens was an achromat of 5.5 feet focal length that belonged to Thomas Pryor of Philadelphia. It was probably the only suitable lens available in America, and explains why the American instrument was slightly smaller than the English one.
The American telescope was inaugurated in the summer of 1786. As Ellicott explained in a letter to Robert Patterson: "We commenced our operations by running a guide line west, with a surveying compass from the point mentioned on the Delaware, 20½ miles, and there corrected by the following zenith distances taken at its western termination, with a most excellent sector, constructed, and executed, by Dr. Rittenhouse." At the end of the observing season, Ellicott noted that "from the excellency of the Sector, and the stability of the Triangle which I had set up on this occasion, the Latitude may be depended on, within 2" and perhaps less." Ellicott used the zenith sector again the following year, as he continued the New York-Pennsylvania line to the shore of Lake Erie, and he used it to survey the western boundary of New York in 1789.
Ellicott used the telescope again in 1796-1800 when he surveyed the southern boundary of the United States between the Atlantic Ocean and the Mississippi River. In his report on this work, Ellicott noted that the zenith sector was "principally executed by my late worthy and ingenious friend Mr. Rittenhouse, except some additions which I have made myself." This instrument, he said, was "similar to the one made by Mr. Graham for Dr. Bradley and Mr. Molyneux, with which the aberration of the stars, and nutation of the earth's axis were discovered and the quantities determined." And its plumb line was "suspended from a notch above the axis of the instrument, in the manner described by the Rev. Dr. Maskelyne the present Astronomer Royal at Greenwich, in the introduction to the first volume of his Astronomical Observations." Ellicott went on to say that the sector "is of all instruments the best calculated for measuring zenith distances which come within its arch."
Ellicott took the zenith telescope with him to West Point in 1813 when he became professor of mathematics at the U.S. Military Academy. In 1817, in conjunction with the survey of the boundary between the United States and Canada following the Treaty of Ghent, he used it to determine the latitude of St. Regis Island in the St. Lawrence River. A descendant, Andrew Ellicott Douglass, deposited it with the Smithsonian in 1898.
Ref: Andrew Ellicott, "Astronomical and Thermometrical Observations, made on the Boundary between the United States and His Catholic Majesty," Transactions of the American Philosophical Society 5 (1802): 204-205; and "Observations for Determining the Latitude and Longitude of the Town of Natchez," Transaction of the American Philosophical Society 4 (1799): 447-450.
Andrew Ellicott astronomical notebooks, Political History Collection, National Museum of American History.
Currently not on view
Object Name
zenith telescope
Rittenhouse, David
overall: 5 1/2 ft; 1.6764 m
overall: 96 in x 25 in x 23 in; 243.84 cm x 63.5 cm x 58.42 cm
ID Number
catalog number
accession number
Credit Line
Dr. A. E. Douglass
See more items in
Medicine and Science: Physical Sciences
Measuring & Mapping
Surveying and Geodesy
Data Source
National Museum of American History
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