American astronomer and inventor who was an early observer of the atmosphere of Venus.

A clockmaker by trade, Rittenhouse built mathematical instruments and, it is believed, the first telescope in the United States. He also introduced the use of natural spider webbing to form the reticle (system of cross hairs) in telescope transits and other position-measuring instruments. Rittenhouse was highly esteemed as a surveyor; he supervised the establishment of the boundaries between Pennsylvania and Maryland, New Jersey, New York, and the Northwest Territory, and parts of those between New York and New Jersey and between New York and Massachusetts.

Rittenhouse was elected to the American Philosophical Society in 1768, and in 1769 he observed the transit of Venus across the face of the Sun. During this transit he observed that Venus has an atmosphere. His findings were similar to those of the Russian scientist Mikhail Vasilyevich Lomonosov, who had identified Venus' atmosphere during a transit in 1761. Though both had written about their observations, neither report was published or publicized for more than a century.

Rittenhouse was the treasurer of the state of Pennsylvania from 1777 to 1789. In 1792 Pres. George Washington appointed him the first director of the U.S. Mint in Philadelphia, a position he held for three years. He was elected a fellow of the Royal Society of London in 1795 and served as president of the American Philosophical Society from 1791 until his death.