May 25, 2020

Tracking Tales of Transits in Lewes, Delaware

Seen in Lewes, Delaware… photo by author.

I had to stop.

As we always love to say, astronomy and history is where you find it, even in a graveyard in Delaware beside the road.

The Delaware coast is an unlikely birthplace for modern American science. But back in the mid-18th century, it was the site of cutting edge astronomy. At the time, measuring the distance from the Earth to the Sun was the gold standard, a key to unlocking the scale and size of the solar system using Kepler’s laws. A transit of Venus across the the face of the Sun represented just such an opportunity to make a parallax measurement at the precise moments of ingress and egress as the black inky disk of the planet slips across the face of the Sun. This was one of the first truly international efforts in science, as several observations had to be made from multiple geographically separate locations.

The transit of Venus on June 3rd, 1769 represented just such an opportunity. Now, Venus transits occur in pairs eight years apart, which are in turn spaced out by alternating spans of 121.5 and 105.5 years. The American Philosophical Society chaired by Benjamin Franklin soon realized this event could put U.S. astronomy on the map.

And it was thus that Owen Biddle and Joel Bailey were dispatched to set up The Transit of Venus Observatory in Lewes, Delaware. Three separate locations along the U.S. east coast were actually selected as a redundant measure against clouds and inclement weather. Why Lewes? Well, the site needed to be surveyed and its location measured to a high degree of accuracy for the observation to be valid; the nearby Transpeninsular Line along the mouth of Delaware Bay had already been surveyed. There was also another motive in selecting Lewes, as the measurement for the observatory would also establish the precise latitude and longitude of the Cape Henlopen lighthouse at the inlet to the bay.

The credentials of the team were impressive: Bailey had worked with Charles Mason and Jeremiah Dixon, surveying the Mason-Dixon Line dividing the states of Maryland and Pennsylvania. Owen Biddle was an accomplished clockmaker, a handy skill when needing to reckon precise time for the upcoming observation.

The team was equipped with three telescopes, including a 3-inch Gregorian reflector and a pair of smaller Dollond refractor supplied by optics firms in London. Solar observing during the transit was done using smoked glass filters, a dangerous pursuit common in the day. The site surveying measurements had to be made quickly, as the team arrived in Lewes on Friday, May 26th, only eight days before the transit was to occur, and they were hampered by weather for several nights needed to make crucial measurements. In those days, time and location were fixed to solar noon, when the Sun transited the meridian running from north to south; nighttime measurements were also done in a similar manner, measuring the precise moment when key stars crossed the meridian. Biddle and Bailey also used transits and eclipses of Jupiter’s moons to get a precise fix on local time, a tough observation to make with a small telescope!

The day of the transit of Venus dawned clear, and the observation went off without a hitch. The team actually trained local schoolchildren to chant the passage of seconds, so the observers wouldn’t miss recording the crucial ingress of Venus onto the Sun’s disk before both set below the western horizon.

The value of the solar parallax from the Lewes observations was 8.862”, giving a distance of 95.37 million miles for the Astronomical Unit from Earth to Sun, a value that stood for about a century. Not bad, considering the modern value using radar observations stands at 92,955,807 miles. Had astronomers of the day used the Lewes observations combined with European observations exclusively, the measurement would have been very nearly spot on to within 0.068”(!) an impressive feat, then or today. Astronomer Simon Newcomb praised the observations of the Lewes team. Elsewhere, teams weren’t so fortunate: for example, astronomer David Rittenhouse fainted(!) at the start of the transit and was unable to make reliable observations.

Today, the sign commemorating the site of the Transit of Venus Observatory is at the cemetery gate along Savannah Road a mile outside of the heart of the old town, just across from Lloyd’s Market.

After visiting the site, be sure to check out the Dogfish Head Brewery just outside of town, and hoist a brew or three to the brave transit team of 1769. We would recommend the stout 120 Minute IPA. And hey, Dogfish is making spirits now, too!

If you take the way out by the Lewes airport to the brewery, don’t miss one of the only surviving UFO houses, a remnant from an era of American kitsch straight out of the Space Age-obsessed 1960s.

Finally, we made an interesting observation whilst out for the day on the Cape Lewes Ferry, of first Fata Morgana at sea, complete with reversed mirror images of buildings hanging over the horizon. We love it when we spot the bizarre on an ordinary outing.

Watch this space, for more tales of revolutionary astronomy to come.



  1. [...] and rabbit holes that most traditional biographies only hint at. Rittenhouse passed out during a transit of Venus. Tycho had his nose shot off during a [...]

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