After 121 years, Venus is ready for transit

From the Baltimore Sun

After 121 years, Venus is ready for transit

By Frank D. Roylance

Sun Staff

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.

Not quite.

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 Massachusetts.

While there is little scientific interest anymore, he said, "this kind of event can be inspirational for students."

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 witness it.

(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 miles.

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 glass.

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 telescopes.

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 their disks.

For more information, visit www.transitofvenus.com

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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 a.m.

* 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 Marriottsville Road.

* Eastern Shore: with the Delmarva Stargazers, from the parking lot of Woodland Beach, east of Smyrna, Del., on Route 6. Sunrise.