STORY HAS BEEN FORMATTED FOR EASY PRINTING
transit of the sun since 1882
In 1761, Harvard
professor John Winthrop left
The astronomer and his
team lugged their equipment up a hill to their chosen observing site, and
camped out. The skies were clear as dawn broke on June 6, and the months of
preparation paid off.
At 4:18 a.m. the men
became the only North Americans to witness the “transit of Venus,” the
rarer-than-twice-a-century event when the second planet passes directly
between the Earth and the sun, and appears as a tiny black dot in front of
the bright solar disk.
“We had the high
satisfaction of seeing that most agreeable Sight, VENUS ON THE SUN, and of
shewing it in our telescopes to the Gentlemen of the place, who had assembled
very early on the hill to behold so curious a spectacle,’’
The transit of Venus has
happened only three times since then – in 1769, 1874, and 1882 – but next
Tuesday morning, for the ﬁ rst time in 122 years, Venus will
once again glide silently in front of the sun.
Such transits occur in
pairs eight years apart, so there will be another transit in 2012, but then
more than another century will pass before the planets align perfectly again.
Back in the 18th century astronomers used the transit as a scientific
opportunity to calculate the distance to the sun, and, from that, the
distance to the planets. Today, the transit is of more interest to amateur
astronomers than to professionals; many hobbyists are planning to travel
thousands of miles for the best view.
The transit -- which will
be visible through a telescope or binoculars (see sidebar on how to view the
sun safely) in the Boston area from sunrise at 5:09 a.m. until 7:26 a.m. --
will tell astronomers little that's new about either Venus or the sun. There
is still scientific interest, though, in the so-called "black drop
effect," a complex phenomenon in which the second planet appears in the
shape of a raindrop as it first passes in front of the sun, and later as it
exits the solar disk.
Jay Pasachoff, an
astronomer at Williams College, believes he's got the black drop figured out:
It seems to be an interplay between the "point-spread function" --
a natural blurring of any image seen through an aperture such as a telescope
-- and solar "limb darkening," the rapid decrease in the sun's
brightness at the edge of its disk. He and a group of students will observe
the transit from northern Greece -- where the entire duration of the event
will be visible -- in the hope of studying and quantifying the black drop
Like an eclipse or the
passage of a bright comet, the transit is expected to trigger a renewed
public interest in all things astronomical.
"We'll have the
attention of much of the world on June 8," Pasachoff said recently.
"So this is a great time for public education, and making people
appreciate the value of science in general and astronomy in particular."
The first transit of
Venus known to have been seen by human eyes occurred in 1639, and only two
people reported glimpsing it -- a young Englishman named Jeremiah Horrocks
and his friend William Crabtree. By the time of the next transit in 1761, the
alignment had been predicted well in advance and dozens of expeditions,
including Winthrop's, set out to locations around the globe to record the
Winthrop, one of the
brightest scientific minds of his time, sailed to Newfoundland because the
transit wasn't visible from New England. "In his day he was one of the
better mathematicians and scientists in North America," said Sara
Schechner, curator of the Collection of Historical Scientific Instruments at
Harvard, which still houses a number of Winthrop's instruments.
Besides equipment and
manpower, Winthrop also needed "letters of safe passage" --
documents that would guarantee safe travel across French-controlled waters.
The fact that the governor was willing to request these from the enemy -- and
that they complied -- shows just how important all parties viewed the
endeavor to be. England and France may have been trading cannon fire up and
down the Atlantic coast of North America, but the pursuit of science was seen
as being above such petty squabbles.
The next transit came in
1769, and again drew astronomers to the far reaches of the globe. Viewing the
transit was James Cook's motive for his epic voyage to the South Pacific.
Winthrop would observe this one, too -- this time from the comfort of Harvard
In the decades that
followed, astronomers would combine the data from the two 18th-century
transits to work out the Earth-sun distance with a remarkable degree of
It boils down to a
problem of triangulation. As seen from different locations on the Earth's
surface, Venus takes slightly different paths across the face of the sun. If
you compare those paths -- whose lengths can be calculated based on precise
timing of the transit -- and if you know the distance separating the various
observing sites, then, with a bit of geometry, you can work out both the
distance to Venus and the distance to the sun. Using this method, astronomers
eventually arrived at a figure within a few percentage points of the modern
figure: 149.6 million kilometers to the sun.
The last time Venus
passed directly between the Earth and the sun, Chester Alan Arthur was in the
White House and the Brooklyn Bridge was under construction.
After the 1882 event,
astronomer William Harkness mused on the long wait ahead: "What will be
the state of science when the next transit season arrives God only
knows," he told his audience. He could hardly imagine that far-off day
when "the twenty-first century of our era has dawned upon the earth, and
the June flowers are blooming in 2004."
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