We are all familiar with the impact of Enlightenment thinking on the politics of the American Revolutions.  Overtures or Locke and Rousseau are found in debates leading to the Declaration of Independence and the Federalist Papers debating the Constitution.  Thomas Pane forcefully renounces the ideas of hereditary rule (Common Sense) and Patrick Henry literally screams for the ideals of the Enlightenment from his Virginia Statehouse lectern.  America was founded on a platform of refutation of Establishment and support of traditional political power.  At the same time, and for many of the same reasons, there is a global revolution in scientific thought during the 18th Century.  Long held beliefs about how the world works, its origins, and the nature of natural philosophy were being challenged.

We live today in a society that largely accepts modern science but this acceptance of the power of individual investigators to understand how the world works.  This was absolutely NOT the case in 1750!  “Perfect Science” and the authority of the Roman Catholic church dominated how learned people thought about the world.  Stones fell to the earth because, as Plato argued, that is where they belong.  Animals never evolve, they are, as it was written in Genesis, made by God.  It’s not really that people believed these things, but the act of challenging these beliefs was upsetting to the established political order of the world.  People believed what they were told by the accepted authorities (especially the Church) and all experience that conflicted with these accepted beliefs were viewed with extreme suspicion – witness the belief in witchcraft as an explanation for calamity.

Enlightenment science, starting in the late 17th Century, encouraged a focus on direct observation, inquiry and experimentation.  Things no longer happen because they should happen or because of divine intervention but they happen because of discoverable and understandable natural laws (that may have been authored but not enacted by God).  Furthermore, since these observations and experimentations are well within the ability and capability of all men, we have the rise of Men of Letters or Gentlemen Scientist.

David Rittenhouse, after acquiring significant fortune as a surveyor and international acclaim for the ornery he built for the University of Pennsylvania, turned his attention to just such direct observation of the heavens.

A ‘transit’ occurs when a smaller body passes in front of a larger one. A ‘Transit of Venus’ happens when Venus is seen in silhouette against the bright face of the Sun.  Although Venus, the Earth and the Sun roughly line up every 584 days, the alignment is not usually precise. Venus’ orbital motion usually takes it ‘above’ or ‘below’ the Sun as seen from Earth (for the same reason, the Moon does not block out sunlight – causing a solar eclipse – every month). On some rare occasions, the alignment is close enough that Venus blocks out some of the light from the Sun, causing a Transit of Venus.

In the 18th Century, transits of Venus presented valuable opportunities to tackle a fundamental problem of the time – finding an accurate value for the distance between Earth and the Sun, called the ‘Astronomical Unit’ (AU). The English astronomer Edmund Halley postulated that observations of Transits of Venus could be used to determine the distance between the Earth and the Sun by observing and timing a transit from widely spaced latitudes. Small differences in Venus’s track across the Sun (resulting from parallax) could be used to construct triangles with which to estimate the distance from the Earth to the Sun and by extension (Kepler established the relative distances of each planet to the Sun by determining their orbital periods) the distances from the Sun to all known planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George[i]). Unfortunately, Halley who died in 1742, was not able to conduct the needed observations but in 1761 and 1769 were observed by teams of scientist from the Royal Society, including David Rittenhouse, from many places around the world. James Cook’s observations in Tahiti and Devid Rittenhouse’s in Norristown Pennsylvania in 1769 were the most useful of these observations and established the Sun–Earth distance at 93,726,900 English miles[ii].

Historic Rittenhouse Town is not only the first paper mill in America; it is the birthplace of one of the men who first used modern empirical astronomy to establish the size of our solar system.  As Historic Rittenhouse Town celebrates the life and accomplishments of William Rittenhouse, it is only fitting we also celebrate his grandson, the first astronomer to observe the Transit of Venus in 1769[iii], the first director of the US Mint, first American to sight Uranus in 1781, and the first to employ grating diffractors to separate the wavelengths of light (then seen as only colors) in 1785[iv].

During the June 27, “Firstival” Event at Rittenhouse Town, I propose adding a booth for
David Rittenhouse to showcase 18^th Century scientific accomplishments.

[i] Sir William Herschel discovered Uranus in 1781 and originally suggested that the new planet be named Georgium Sidus, or George’s Star in honor of King George III.   This went against the naming convention that had developed in the Western world, in which planets were named after Roman deities. Several alternative names were tossed around, including Herschel (after its discoverer) and Neptune. German astronomer Johann Elert Bode, whose observations helped establish the new object as a planet, suggested another possibility: Uranus.

[ii] The radar-based value used today for the astronomical unit is 92,955,000 miles (149,597,000 km). This is only a difference of eight-tenths of one percent from the value established by Cook and Rittenhouse in 1771.

[iii] Cook’s observations were will be 5 hours and Greenwich’s observations 19 hours later.

[iv] These gratings would later be used to create interferometers and spectroscopes used to probe the heavens and determine the atomic content of stars.