The transit of Venus, a rare astronomical event, is the passage of the planet Venus across the face of the sun. In the 18th century it was a very important scientific event because it gave astronomers a way to gauge the size of the solar system. Specifically, when careful measurements were made of the transit, astronomers could use the data to estimate the average distance between the sun and the earth as it orbited the sun. This distance came to be known as the astronomical unit (AU). Only six transits have been observed by persons who knew what they were seeing. The first was in 1639 and the next two occurred in 1761 and 1769. For the 1769 transit, the European astronomers sought help from the colonists because it was wise to plan many observations in case clouds hid the transit (a transit, like an eclipse, is not viewable all over the Earth). Thus it was that the American Philosophical Society, with Benjamin Franklin as president, supported three transit “observatories” one of them in Lewes, Delaware. The data from Lewes along with matching information from elsewhere in the world was assembled and published in the Transactions of the Royal Society, under the guidance of Nevil Maskelyne, the Astronomer Royal in England.
Subsequent transits occurred in 1874 and 1882. And after a long wait, another transit was observed on June 8, 2004. The next transit will be on June 5, 2012. It will be visible in Lewes, weather permitting, in the evening. No one who observes that transit will see another. The next will occur on December 11, 2117.
Join The Lewes Historical Society at the end of Cedar Street in Lewes on June 5, 2012 beginning around 6:00pm to view the transit.