“This was the big unknown for astronomy,” said Owen Gingerich, an emeritus professor of astronomy and history of science at Harvard. Without that number, much else about the solar system was also uncertain: the size of the Sun, the distance between planets.

The next transit of Venus will occur next Tuesday, and will be visible, at least for a while before sunset, across the United States. In New York and along the East Coast, the Sun will be low in the sky, requiring observers to find locations not be obscured by trees or buildings. (A west-facing window up high in a skyscraper could be a good place to watch.) The usual precautions about not looking directly at the Sun apply. Special eclipse viewing glasses can be used, or the image of the Sun can be projected through a pinhole or binoculars onto a sheet of paper.

While no longer of great scientific import, as it was to Rittenhouse and Captain Cook, a Venus transit is still a rare and striking event, occurring in pairs, eight years apart, about once a century. The last transit occurred in 2004, and almost no one alive today will be around for the next one, 105 years from now, on Dec. 11, 2117. (That one will not be visible at all from most of the United States. New Yorkers, however, will have a prime viewing spot for the following transit, on Dec. 8, 2125.)

It was only in 1627 that anyone realized Venus transits occurred at all. That year, Johannes Kepler, the mathematician and astronomer, published data about the planetary orbits that predicted that Venus would pass directly between Earth and the Sun in 1631.

Kepler died in 1630, and it appears no one saw the 1631 transit. (It was not visible from Europe.) The first recorded observation of a transit was in 1638, which Kepler had not predicted. Jeremiah Horrocks, an English astronomer, realized Kepler had made an error in his calculations.