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In a 1716 issue of the Philosophical Transactions of the Royal Society, Edmund Halley explained how it was possible to use transits of Venus or Mercury to determine the astronomical unit by measuring the apparent solar parallax between different points on the surface of the Earth.  Unfortunately, the next transits would occur in 1761 and 1769. Halley died in 1742, almost twenty years before the transit.

The Royal Society, with a grant from the Crown, took up the challenge of collecting observations from disparate locations across the British Empire, organizing and sponsoring expeditions around the world.  Charles Mason and Jeremiah Dixon observed from Cape Town.  Captain James Cook took the HMS Endeavour to Tahiti and established observatories, other observers went to Calcutta, Hawaii, Kerguelen, and Gibraltar.  David Rittenhouse led teams who would observe from 22 locations across Pennsylvania and Delaware.  Other scientific societies in Russia, France, and Norway joined in this historic endeavor.

Rittenhouse began his preparations in 1768, the same year that he became a member of the American Philosophical Society.  The American Philosophical Society persuaded the legislature to grant £100 towards the purchase of new telescopes, and members volunteered to staff half of the 22 telescope stations when the event arrived.

The transit of Venus occurred on June 3, 1769. Rittenhouse’s great excitement at observing the infrequently occurring transit of Venus resulted in his fainting during the observation. Lying on his back beneath the telescope, trained at the afternoon sun, he regained consciousness after a few minutes and continued his observations. His account of the transit, published in the American Philosophical Society’s Transactions, does not mention his fainting, though it is otherwise meticulous and well-documented.

The observers were ordered to record the transit in four phases of Venus’s journey across the sun. The first phase was when Venus began “touching” the outside rim of the sun. In the second phase, Venus was completely within the sun’s disc but was still “touching” the outer rim. In the third phase, Venus has crossed the sun, was still completely within the disc, but was “touching” the opposite rim. Finally in the fourth phase, Venus was completely off the sun but was still “touching” its outer rim.

Rittenhouse used the observations to calculate the distance from Earth to the Sun to be 93,726,900 English miles”. The radar-based value used today for the astronomical unit is 92,955,000 miles.  This is a difference of only eight-tenths of one percent.ons.

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