Stories of Penn scientists: David Rittenhouse

In celebration of the 250th anniversary of his observations of the 1769 transit of Venus, a glimpse into the story of the man whose name became synonymous with astronomy and mathematics.

On June 3, 1769, months of work and planning all became worthwhile thanks to clear skies over East Norriton, Pennsylvania. 

sketch of rittenhouse seated at a table with a telescope in the background
Portrait of Rittenhouse by Charles Wilson Peale (ca. 1780) (Image: University of Pennsylvania Archives Image Collection)

In the mid-morning hours of that fateful day, David Rittenhouse waited nervously for the moment when Venus would briefly become visible as it passed across the sun. The transit was a rare opportunity to measure the parallax and calculate the distance between the sun and Earth, a chance that would not happen again for 105 years. 

But long before the day 250 years ago that established Rittenhouse as a world-renowned astronomer, the man who would become treasurer of Pennsylvania, president of the American Philosophical Society, and a trustee, professor, and vice-provost of the University of Pennsylvania plus the first director of the U.S. Mint, he was a boy growing up in rural Pennsylvania. 

Rittenhouse was born in 1732 near Germantown. Colonial farm life was arduous and offered no time for formal schooling, but Rittenhouse, armed with a curious mind and inspired by science, taught himself physics and math. After inheriting his uncle’s woodworking tools, along with a number of science books such as Isaac Newton’s “Principia,” Rittenhouse set out to make his livelihood away from farming. 

After setting up his own workshop and making his first clock at the age of 19, Rittenhouse rose to local prominence, with the steady demand for his grandfather clocks being enough to make a livelihood. He also became increasingly interested in astronomy and astronomical instruments, to which he devoted himself between his work as a clockmaker and later as a local surveyor.

Rittenhouse was increasingly known for his astronomical abilities, and William Smith, the first provost of the College of Philadelphia (now Penn), selected him to lead the effort coordinated by the American Philosophical Society to observe the 1769 transit of Venus. With £100 to buy new telescopes and equipment for the 22 observation stations in the area, Rittenhouse found himself involved in what was arguably one of the first international scientific collaborations. 

As the final preparations were completed, Rittenhouse and his colleagues were thankful to see clear skies after several days of cloudy weather that spring. As crowds of people from the countryside gathered to watch history unfold, Rittenhouse himself was laid flat on the ground near the observatory so he could watch the path of the transit through his newly-developed telescope as Venus moved along its path directly overhead. 

a diagram of venus moving across the sky on old paper
Rittenhouse’s diagram and notes from his observations of the 1769 transit of Venus.

Just 36 seconds later than Rittenhouse predicted, a “triangle of flame” appeared in the sky, and he and his assistants recorded the exact timings of its movement in order to calculate the parallax. With data collected from stations across the world, Rittenhouse calculated it as 8.805 seconds, equivalent to nearly 93 million miles between Earth and the sun. Modern methods, by comparison, calculate the parallax as 8.803 seconds.

Rittenhouse moved to Philadelphia in 1770, where he set up shop on the corner of Seventh and Mulberry (now Arch) streets and later became the city surveyor of Philadelphia. With his work in astronomy, involvement with the American Philosophical Society, and time spent as an instructor at the College of Philadelphia, Rittenhouse relished any opportunity he had to focus on science. But just as he was being named the first public astronomer for the province, the American Revolution began.

With his skills in surveying and manufacturing, Rittenhouse was appointed to the Committee of Safety as an engineer, where he was charged with assessing sites for forts and finding new ways to cast cannon and improve rifles. He also served on the Pennsylvania Assembly and the Pennsylvania Constitutional Convention of 1776 and was named the Pennsylvania treasurer in 1777. Even while fulfilling his public duties, Rittenhouse managed to stay active in science, writing commentaries about Isaac Newton and observing eclipses and the movements of Mercury in the spare time he could find. 

As the Revolution came to a close, Rittenhouse stepped away from his public roles to devote himself to astronomy through his role as a professor of astronomy at the College of Philadelphia and as the vice president of the American Philosophical Society. But his retirement from public service didn’t last long as he was nominated in 1792 by President George Washington to be the first director of the U.S. Mint. 

In a surprising parallel to the career path of Newton, who worked as the master of the Mint at London in his later years, Rittenhouse gave much of his time to the challenge of setting up a Mint in the newly-formed nation. He faced numerous criticisms but was able to get the Mint up and running, and retired after three arduous years there. He was then free to focus on science and remained dedicated to science until his death in 1796. 

text and tables in an old manuscript
Penn Libraries purchased a unique eighteenth century manuscript in November 2014 that was likely produced in Philadelphia in 1746/7, which has a small pencil annotation listing Rittenhouse as the author. Consisting of a single bifolium, it appears to be part of an almanac containing eclipse charts, predictions for weather, and astrological signs. (Image: Penn Libraries)

While Rittenhouse’s achievements as a scientist and as a public servant are numerous, it’s his orrery that stands to this day as a physical testament to his success. The second orrery he created, a clockwork model of the solar system that shows the motions of the planets known at the time, was inspired by smaller pieces commissioned for the fourth Earl of Orrery that toured British America in the mid-18th century. The first orrery went to Princeton, much to the dismay of William Smith, Penn’s first provost, but through his second endeavor Rittenhouse likely had the opportunity to make improvements on his original design.

The Penn Libraries’ David McKnight, director of the Kislak Center for Special Collections, Rare Books and Manuscripts, says that the work was commissioned by Smith, who taught natural philosophy and wanted to use the orrery as a tool for teaching students astronomy. The components of the orrery, including the hundreds of gears and cranks, as well as the beautifully-detailed carved wood panels, were all made in Philadelphia in the late 1760s. “It was designed to be an elegant instrument,” McKnight says of the orrery, which in 2013 was moved from the first floor of the Van Pelt library to the Kislak Center, where it dominates the view of anyone exiting the elevators onto the sixth floor. 

the orrery in a wooden cabinet at the far end of a room full of chairs
Pen’s orrery is currently on display at the Kislak center. The massive wooden cabinet, towering more than eight feet in height, and hundreds of metal cogs and gears were all made in Philadelphia, showcasing the excellent craftsmanship from the region during the pre-revolutionary time period. (Photo: Penn Libraries) 

Rittenhouse’s orrery is unique not just in its elegant grandeur but also in its technical achievements. Unlike others of the time, Rittenhouse’s successfully incorporated Newton’s newly-published theories about gravity and the elliptical rotation of the planets into the construction.  

It’s a piece of history that McKnight says symbolizes America’s pursuit of scientific knowledge that characterized the time period. “The notion that Provost Smith had the insight to have this instrument built to teach astronomy—you have the American Philosophical Society, which was founded as a learned society, Rittenhouse and his scientific endeavors, Franklin and electricity—Rittenhouse himself is part of the spirit of the period, a symbol of the American enlightenment,” says McKnight.

Despite humble beginnings and no formal education, the self-taught scientist who rose to international renown as an astronomer and mathematician was described by one of his recent biographers: “To men who valued astronomy and mathematical research, and recognized the indispensability of scientific method, Rittenhouse had no peer in America.”

 David McKnight is director of the Kislak Center for Special Collections, Rare Books and Manuscripts in the Penn Libraries at the University of Pennsylvania