The spherical sundial was a wooden sphere 10.5 inches in diameter, made at Monticello sometime between August 1809 and September 1816. Thomas Jefferson’s sundial was something of a novelty in its time. There is no evidence that spherical dials were in use in North America before Jefferson produced his version, although various types based on the same principle existed in Europe.
Horizontal lines were drawn for the Tropic of Cancer, the Equator, and the Tropic of Capricorn. Vertical lines extending from tropic to tropic represent the hours of the day, indicated by Roman numerals below the Tropic of Capricorn. Progressively shorter vertical lines represent half-hour, quarter-hour, and five-minute intervals. The sphere was mounted on a tapered neck with the Equator tilted on an angle based on the location’s latitude, 38° 1' North. The noon line was aligned with the true north-south axis. Solar time can be determined by moving the meridian, a bar of thin sheet iron that pivots on the north and south poles. Solar time was indicated when the bar cast the least shadow on the sphere.
The base or capital featured ears of corn and as such represents a new architectural order, of American origin, in the classical lexicon. Jefferson explained in an 1816 letter that the idea of creating a sundial was prompted by a gift from the architect Benjamin Latrobe.1 In 1809, Latrobe had given Jefferson a model of the capital that the architect had designed for the vestibule of the Senate wing of the United States Capitol suggesting it could be used as a base to a sundial if raised on a “frustum” (pedestal) about four feet high.2
In an 1817 letter to Latrobe, Jefferson remarked: “my dial captivates every body foreign as well as home-bred, as a handsome object & accurate measurer of time.”3
The whereabouts of Jefferson's original spherical sundial are unknown. So, the Thomas Jefferson Foundation created a reproduction sundial. The design for the re-creation was based on drawings by Jefferson and a detailed description in Jefferson's 1816 letter to Latrobe.4
The reproduction has some variances to the original. The dial was installed near where the original stood, based on a description by a visitor to Monticello in 1832.5 To discourage handling and to preserve the mechanism, the meridian has been set in place to indicate noon. In this version, it was decided to show all the hour lines but only the half-hour, quarter-hour, and five-minute intervals for nine hours each side of noon. Jefferson’s sundial was made of locust but mahogany was used because it is durable, stable, and readily available. Jefferson did not indicate the architectural order for the pedestal, but the fact that the dado (the surface between the cornice and base moldings) is square suggests that it is Doric. If it is indeed Doric then the dimensions given for the height of the base, the dado, and the cornice follow the proportions devised by the seventeenth-century French architect Claude Perrault. This form was adopted for this installation. The capital of cast-stone is modeled on the original corn capitals in the old vestibule of the Senate wing in the United States Capitol.
1. Jefferson to Latrobe, August 27, 1816, in PTJ:RS, 10:350. Transcription available at Founders Online. See also Latrobe to Jefferson, November 5, 1816, in PTJ:RS, 10:510. Transcription available at Founders Online.
2. Latrobe to Jefferson, August 28, 1809, in PTJ:RS, 1:473-75. Transcription available at Founders Online.