Transit History
The first recorded transit in history was observed in 1639 by Britons Jeremiah Horrocks and William Crabtree. Horrocks found that astronomers of the day varied widely in their opinions of if or when a transit would take place, so he calculated the time on his own, and was accurate to within a few minutes.
Later, Sir Edmund Halley (for whom Halley's Comet is named) predicted that astronomers could use the next Venus transits, in 1761 and 1769, to determine the distance from Earth to the Sun with a high degree of accuracy. As a result, astronomers mounted several dozen expeditions to view these events. Some of the more interesting include the expeditions of Captain Cook, Guillaume Le Gentil, Jean Chappe, and Jeremiah Dixon and Charles Mason, who later surved the famous Mason-Dixon line in the United States.
Despite the best efforts of these teams, the uncertainty in the Earth-Sun distance was still several million miles. So new expeditions were launched in 1874 and 1882, including many by the United States Naval Observatory. Each expedition was equipped with the same set of instruments, including cameras for photographing the transit on glass plates. Some of these artifacts and plates are on display at the National Museum of American History. Despite some political infighting within the Naval Observatory, it eventually produced an Earth-Sun distance that was less than 100,000 miles off the true value.
Guillaume Le Gentil: Gloomy Skies
If it weren't for bad luck,
Guillaume Le Gentil wouldn't have had any luck at all. The 18th-century
French astronomer spent more than a decade of his life in the pursuit of two
rare events: transits of the planet Venus across the face of the Sun. He survived
a war, a hurricane, severe illness, and various other trials and tribulations.
Yet he missed out on both transits -- and faced even more depressing
news when he returned home.
Transits of Venus happen in pairs that are
eight years apart. But pairs are separated by more than a century.
So more than 60 expeditions were dispatched around the globe to watch the transit
of 1761. Astronomers hoped that observations of the transit would help them
measure the distance from Earth to the Sun.
Le Gentil headed for Pondichery, on the eastern
coast of India. But the Seven Years War between England and France was in full
swing. Pondichery fell under British control, and Le Gentil had to watch the
transit from the heaving deck of a ship, where it was impossible to make accurate
measurements.
The next transit was due in 1769, tso Le Gentil
decided to wait for it. He settled in Mauritius for a while, and studied the
local life, culture, and geology. He soon decided that the Philippines would
be a good spot to watch the next transit, so he headed for Manila. But the
Spanish governor was suspicious of foreigners, so Le Gentil moved again --
this time to Pondichery, which was back under French control.
With the help of the jovial local governor,
he set up an elaborate observatory, and was prepared for the transit of June
3, 1769. The day started off bright and clear -- but clouded up just hours
before the transit. The clouds loitered until shortly after the transit
ended. Le Gentil was so depressed that he couldn't even write his report
to the French Academy of Sciences.
Worn out after a decade away from home, Le
Gentil headed toward France. It took months to secure passage; during the wait,
he developed severe dysentery and almost died. He finally got a ship, but it
had to turn around when it ran into a hurricane. A second ship also encountered
severe weather, but finally made it to Europe.
Yet Le Gentil's bad news wasn't
over. His family had heard a rumor that he was dead, so it had divvied up much
of his estate. The French Academy had given away his position, too. Le Gentil
was almost bankrupt.
His luck eventually turned around, though.
He was awarded a new position with the Academy, and he married a wealthy woman.
But Le Gentil missed out on the science of the transits of Venus -- events
that wouldn't be repeated for more than a century.
Jean Chappe: Deadly Transit
Baja California is a popular spot for tourists these days. But in 1769, it
was a place of deprivation and death for a group of astronomers who were there
to watch the transit of Venus.
The French expedition was led by Jean Chappe, who had led a team to Tobolsk,
Siberia, to watch the transit of 1761. The team survived a treacherous river
crossing and a lengthy traipse through difficult boggy conditions and arrived
at its viewing site just six days before the transit. Just before the transit,
Chappe was almost murdered by the locals, who thought he had caused a unusually
severe spring floods by interfering with the Sun. A cordon of Cossack guards
saved the party, which managed to obtain good observations of the transit.
Chappe tried again in 1769, leading a team of three other scientists and two
assistants from Spain. They arrived at Vera Cruz, Mexico, in May, 1768. It
took them months to cross Mexico, across forbidding countryside and through
intense heat that spoiled most of their food.
They arrived at San Jose del Cabo two weeks before the transit. Natives warned
them of an epidemic, but the Europeans didn’t have time to pick another
viewing spot, so they stayed. Chappe, who was trained as a doctor, helped care
for the ill villagers, but he soon developed the fever himself and died within
days. The disease claimed all but three of Chappe’s party, and two of
those died during the trek across Mexico. Only a man named Pauly survived,
and his health was ruined by the ordeal.
But he returned to France with Chappe’s notebooks in 1770, and presented
the observations, which were some of the best of the 1769 transit, to the French
Academy of Science.
Mason and Dixon: Lining Up the Transit
Charles Mason and Jeremiah Dixon are best known for a purely American accomplishment.
They surveyed the disputed boundary between the colonies of Maryland and Pennsylvania,
establishing a border that is still used today. But the Mason-Dixon line came
to symbolize the split between northern and southern states that led to the
Civil War.
Yet Mason and Dixon were neither American nor political. They were Britons
-- Mason an astronomer who had worked at the Royal Observatory at Greenwich,
Dixon a surveyor -- who had worked together two years before beginning the
American survey project by observing the transit of 1761.
The Royal Society dispatched Mason and Dixon to Sumatra. But the Seven Years’ War
was in full fury, and a French frigate attacked their ship, HMS Seahorse, just
hours after it sailed from England on January 8. The battle killed 11 members
of the British party and injured more than 30, leaving the survivors no choice
but to return to Portsmouth.
Mason and Dixon tried to back out of the transit business, but threats of
lawsuits and ruin by the Royal Society convinced them to head to sea once more.
But the weeks of repairs made it impossible for them to reach Sumatra in time,
so they stopped at the Dutch colony of Cape Town, South Africa. The local governor
provided an observing site and logistics support, and they set up their equipment
in May of 1761. Despite some hit-and-miss clouds on the day of the transit,
Mason and Dixon obtained several accurate measurements of Venus’ position
on the solar disk, providing some of the most useful data from the 1761 event.
They remained in South Africa for several months, making observations of stars
and planets, before returning to England in the fall.
The Royal Society dispatched them to view the transit of 1769 as well -- but
this time on separate expeditions: Mason observed from Ireland, Dixon from Norway.
-- Damond Benningfield
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