From the Baltimore Sun
After 121 years, Venus is ready for transit
By Frank D. Roylance
Originally published May 17, 2004
After a huge blizzard, Isabel's
record storm surge, and an invasion by 17-year cicadas, Marylanders might think
Nature would be finished serving up rarities for now.
On June 8, Maryland and most of the
inhabited world will have a chance to witness one of the rarest spectacles in
Nature - the Transit of Venus.
At sunrise that morning, the planet
Venus will be passing directly between the sun and the Earth, moving in
silhouette across the disk of the sun as seen from Earth and dimming its light
by one-tenth of 1 percent.
It is the most infrequent of solar
"eclipses." The last time it occurred was Dec. 6, 1882, and all who
saw it then are gone.
"I think it's a glorious event
to be able to see something that nobody on Earth has seen," said Jay
Pasachoff, an astronomer and eclipse expert at Williams College in
While there is little scientific
interest anymore, he said, "this kind of event can be inspirational for
During the 18th and 19th centuries,
the Transit of Venus was a galvanizing event for scientists. They knew that
careful observations might reveal both the sun's distance and the true size of
the solar system. So they launched expeditions to the far corners of the globe
to measure it.
The 1882 transit caused a stir in
Baltimore, too, according to an article published by The Sun the next day:
"From windows and housetops, in
groups and singly, the remarkable phenomenon was observed," the newspaper
said. "Schoolboys stood in the streets with smudged faces and looked
through their bits of smoked glass.
"The event was a matter of
general popular interest," the paper said, "and the inquiry of the
day was, 'Have you seen the transit?'"
When the 1882 transit ended, William
Harkness, of the U.S. Naval Observatory, lamented that "there will be no
other until the 21st Century of our era has dawned upon the Earth, and the June
flowers are blooming in 2004."
Those flowers have finally sprouted
121 1/2 years later. But will the dawn be clear on the 8th? If not, most of us
will get another chance to witness the transit when it recurs in eight years,
on June 5, 2012. But after that, it will be 105 years - Dec. 11, 2117 - before
the cycle comes around again.
The June 8 transit will arrive amid
stern warnings against looking directly at the sun, which can cause instant eye
damage and blindness.
Observers will have to use
protective devices such as No. 14 welder's glass (available at local welding
supply stores). They can also watch with experienced astronomers using solar
filters or projection techniques, or see the transit live on the Internet.
But with the right gear and clear
skies, Marylanders should be able to see Venus as a tiny black spot creeping
across the bright disk of the sun.
Without magnification, Venus will be
the same size as a dime at a distance of 67 yards, according to Herman Heyn,
Baltimore's original "Streetcorner Astronomer."
Like Halley's Comet and total
eclipses of the sun, the Transit of Venus is an event that amateur astronomers
anticipate all their lives. "It's another milestone in my astronomical
life, and like everything else it came up fast," Heyn said.
Observers in Europe and most of
Africa and Asia will be able to watch for more than six hours as Venus makes
its way across the lower quarter of the sun's disk.
By the time the sun rises on the
east coast of the Americas, (at 5:39 a.m. EDT in Baltimore), the transit will
be more than half over. But observers will still have more than two hours to
(In 2012, the entire transit will be
visible from North America.)
Only the moon, and the planets
Mercury and Venus can pass directly between the sun and observers on Earth. But
none of the three crosses the sun's face on every orbit. If they did, we would
see a solar eclipse every month, and a transit of Venus every 19 months.
Because the orbits of all three
objects are tilted with respect to Earth's orbit, they usually pass unnoticed,
just above or below the sun.
Transits by Venus occur in pairs, in
cycles that repeat every 121 1/2 years. The first transit in a cycle is
followed by a second eight years later. The subsequent cycle begins 105 1/2
years after that.
The German astronomer Johannes
Kepler, who discovered the laws of planetary motion, noticed in 1629 that Venus
would transit the sun just two years later.
Kepler died before the 1631 transit,
which wasn't visible from Europe anyway. But eight years later, in 1639,
English astronomers Jeremiah Horrocks and William Crabtree became the only
humans with the knowledge and equipment to see it.
In 1691, English astronomer Edmund
Halley figured out that precise timing and mapping of a transit by Venus from
widely separated points on Earth could (with a bit of surveyors' geometry
called triangulation) yield the distances from Earth to Venus and the sun.
Using Kepler's laws, astronomers could then deduce the distances to all the
planets, and the size of the solar system - a scientific bonanza.
Halley, then 35, knew he wouldn't
live to see the 1761 transit. But a later generation of European scientists
took up the challenge, sending observers around the globe.
Troubled by bad weather, imprecise
clocks and uncertain geography, the efforts were disappointing. But they'd get
another chance in 1769.
In 1768, Capt. James Cook sailed the
Royal Navy ship Endeavour for Tahiti, and when the day arrived on June 3, 1769,
conditions at what was named Point Venus were ideal. But the precise timing
critical to the experiment was foiled by a mysterious distortion of the image
of Venus, now known as "the black drop effect," for many years blamed
on the atmospheres of Venus or the Earth.
(A satellite study led by Pasachoff
last year disproved the theory, tying the problem to blurring common to all
telescopes and darkening at the edge of the sun's disk.)
Even so, when Cook sailed home three
years later, scientists combined his data with others and calculated an
Earth-sun distance to within a few percent of its true value - about 93 million
Bad weather and the black drop
problem again troubled observations of the 1874 Transit of Venus. But
scientists quickly began planning for the 1882 transit, establishing more
observation points than ever before.
One of them was on the grounds of
Johns Hopkins Hospital, then under construction in East Baltimore. On the
morning of the transit - Dec. 6, 1882 - physicists and astronomers led by
Hopkins physics professor Charles Sheldon Hastings set up their telescopes at
the hospital. Ordinary citizens made do with the dubious protection of smoked
Just before 9 a.m., Venus appeared
at the edge of the sun. "It entered upon the sun's southern border and
passed across his face in a westerly direction," the newspaper said.
"Some likened the appearance of
the planet to a black dot on the sun the size of a pea; others thought it
looked like a small piece cut out of the sun by a gun-wad cutter."
Between critical phases of the
observations, dozens of Hopkins faculty and their wives, clergy, judges and
students, "anxious to have a glimpse," took their turns at the
In 2004, astronomers are searching
for the transits of planets orbiting other stars, a key tool in their search
for planets - and perhaps life - beyond our solar system.
These "extrasolar" planets
can't be seen directly. But precision instruments have now measured the slight,
periodic dimming of four stars that reveal the transit of unseen planets across
For more information, visit
Where to see it
For safe viewing of the June 8
Transit of Venus:
* Webcasts: www.exploratorium.edu
beginning at 1 a.m. Also http://sunearth.gsfc.nasa.gov
* Maryland Science Center: from the Inner Harbor promenade, 6 a.m.
* Druid Hill Park, Baltimore: with Herman Heyn. west end of Druid Lake, at 5:30
* Towson: with "Starman"
Jerry Feldman, Parkville High School, 6:30 a.m.
* Howard County: with the Howard
Astronomical League. Sunrise at Alpha Ridge Park, off Route 99, west of
* Eastern Shore: with the Delmarva
Stargazers, from the parking lot of Woodland Beach, east of Smyrna, Del., on
Route 6. Sunrise.
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