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Mercury, May/June 2004 Table of Contents

Courtesy of USNO Library

by Cameron Addis

Nearly two and a half centuries ago, American scientists had an opportunity to do two things simultaneously—help determine the distance to the Sun and show themselves as scientifically adept as their European colleagues.

By the 18th century the proportions of the Solar System were known, but its size remained a mystery. When the interior planet Venus crossed between Earth and Sun in 1769, astronomers hoped to use it as a reference point to figure the size of the Astronomical Unit, the average distance between the Sun and Earth, so that the overall size of the system would fall into place. Viewing and timing Venus’s crossing of the Sun's face from separate spots on Earth would reveal its parallax, the angle established by its apparently shifting position relative to the Sun, which could then be used to triangulate distance.

Today the distance to the Sun is determined by radar and lasers, and the transits of 8 June 2004 and 6 June 2012 will pass with little fanfare. Few colonial Americans knew about the transits either when they happened in 1761 and 1769. In Thomas Pynchon’s Mason & Dixon, a fictionalized character tells Charles Mason, "Why, most of us here in Virginia wouldn't know a Parallax from a Pinwheel if it came on up and said how-d'ye do."

But while Pynchon's fictionalized Virginian may have expressed a common sentiment, deciphering the Solar System mattered deeply to Enlightenment America’s amateur scientists and theologians. They cared so much that they were willing to put their lives on hold, haul equipment long distances, and invest countless hours constructing intricate instruments. Why did they expend such tedious effort on such an esoteric enterprise? One reason was simple curiosity.

The parallax was an important piece of the scientific jigsaw puzzle, and these stargazers saw the transit as an opportunity to view God's handiwork. Cultural prestige was also at stake. The colonials wanted to impress Europeans by proving they were up to snuff on cutting-edge science. Their efforts show how science and politics interacted prior to the Revolution, and they are a testament to ingenuity, determination, and fallibility.

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