Centuries later, Phila. will be tracking Venus again

David Rittenhouse spent months fine-tuning his handmade instruments and setting up a small observatory on the grounds of his farm, 20 miles outside the bustling young city of Philadelphia. On a clear June day in 1769, he was ready to participate in a landmark moment in science's efforts to measure the heavens.

The rare event was called the transit of Venus, and it can be seen once again Tuesday evening - likely the last such chance for anyone now alive. For several hours, the path of Venus will take it directly across our view of the sun - looking something like a small blueberry against a fiery volleyball, for those with a telescope and proper eye protection (number 14 welder's goggles will work).

Starting Friday, the American Philosophical Society Museum is marking the occasion with 10 days of events and an exhibit of historic artifacts, including one of Rittenhouse's brass telescopes and his wooden clock. And the first few hours of this year's transit can be seen from the roof of the Franklin Institute, starting at 6:03 p.m. Tuesday. The next one won't be until 2117.

In 1769, the transit of Venus enabled scientists for the first time to calculate Earth's distance from the sun with a fair degree of accuracy.

This was possible because the planet appears to take a different path across the sun depending on the observer's point of view. With measurements from observers at multiple locations on Earth, scientists could use trigonometry to calculate the dimensions of a giant triangle in the sky, and thus, the distance to the sun.

Various teams spent months getting into position to capture the best data, including one led by England's Captain James Cook, who sailed to Tahiti. National pride was at stake, said Owen Gingerich, professor emeritus of astronomy and the history of science at the Harvard Smithsonian Center for Astrophysics.

"It was, in its own way, the equivalent to our space-age competition," said Gingerich, who has lent historic documents to the Philosophical Society museum's exhibit in Philadelphia.

Rittenhouse and colleagues were eager to make their name, as well, as the colonies were seen by the British as a cultural and scientific backwater, said Sue Ann Prince, director of the museum on South Fifth Street.

Rittenhouse's measurements were only somewhat helpful in gauging the distance to the sun, because he could not see the entire transit from his vantage point on the East Coast. What's more, he is said to have fainted for a few minutes during the early part of the transit, from excitement or illness, according to various accounts.

Yet, today, Rittenhouse is credited with being among the first - perhaps the first - to correctly record evidence of the atmosphere of Venus, said Williams College astronomy professor Jay M. Pasachoff. Others had claimed to see such evidence, but Pasachoff has analyzed their writings and found them wanting.

To a careful observer of a transit of Venus, the planet appears to be limned with an arc of light as it crosses the edge of the sun. Rittenhouse correctly attributed that phenomenon to the planet's dense atmosphere, Pasachoff said.

Others who took measurements in the colonies included a team stationed near Cape Henlopen, in what is now Delaware, and one behind the building now called Independence Hall.

Rittenhouse was renowned for his skill at making instruments, yet it was nevertheless a challenge to make the observations in 1769. Timekeeping was something of an uncertain affair before the advent of railroads and standardized clocks. And it was difficult to identify one's longitude with pinpoint accuracy.

Rittenhouse correctly predicted that the transit would begin shortly after 2 p.m. for observers at his farm, located in what is now East Norriton Township, Montgomery County, north of Norristown. Just to be sure, his team kept a lookout for more than an hour in advance.

Writing down his thoughts afterward, he waxed poetic.

"Imagination cannot form any thing more beautifully serene and quiet, than was the air during the whole time; nor did I ever see the Sun's limb more perfectly defined, or more free from any tremulous motion," Rittenhouse wrote.

The results from Rittenhouse's team were published by the Royal Society in London. Nevil Maskelyne, the astronomer royal, wrote that the findings "seem excellent and compleat, and do honor to the gentlemen who made them."

The results helped cement the scientific reputation of Rittenhouse, who succeeded Benjamin Franklin as president of the Philosophical Society, followed by Thomas Jefferson.

There are two main reasons that Venus eclipses our view of the sun so infrequently. One is that Venus' path around the sun is tilted by several degrees with respect to Earth's orbit. Also, Venus orbits the sun 13 times for every eight orbits by Earth.

"We have to have a couple of different cycles sync up for these things to happen," said Derrick Pitts, chief astronomer of the Franklin Institute.

The transits occur in pairs, separated by eight years, with more than a century elapsing between each pair. There was a transit of Venus in 2004, but none during the 20th century.

Transits are no longer needed to determine Earth's distance to the sun, as better values have been obtained with radar. The current accepted value is about 92,900,000 miles, whereas a paper published by the Royal Society after the 1769 transit pegged the answer at about 93,700,000 miles.

Still, transits interest today's astronomers, as they can guide the search for new planets far beyond the solar system. When such planets cross in front of their stars, they cause a faint dimming of the starlight, as Venus does with the sun.

By studying a transit by a known planet such as Venus, scientists can make sure the dimming of distant stars is due to a planet and not some other factor, said Pasachoff, the Williams College scientist.

The Philosophical Society museum exhibit includes information on such modern research, as well as a healthy dose of history and lectures.

And on the lighter side, the museum will host multiple performances of a comic piece called "The Astronomer Collapses" - an interpretation of Rittenhouse's fainting spell. That will take place in the garden across the street from the museum on Fifth Street.

If the scientist is remembered for fainting, better that than the fate of Frenchman Guillaume le Gentil, who went to India to get a good vantage point for the transit of 1761. Unable to measure that event after British forces prevented him from going ashore, he resolved to stay in Asia for eight more years to catch the event in 1769.

But this time, he was defeated by cloudy weather. And upon returning home, 11 years after setting out, he found he had been presumed dead and his estate carved up by relatives.

To learn more about Philadelphia's role in the transit of Venus and see a NASA simulation, visit www.philly.com/venusEndText