It was a beach party the likes of which no one had seen for more than a century. On the eastern edge of Nantucket Island on June 8, more than 100 sky watchers waited in the gray predawn light. Clouds hid the first rays as the sun rose over the ocean, but then the clouds parted, revealing a slowly moving beauty mark, the silhouette of Venus passing the face of the sun for the first time in 122 years. Venus’ transit over the sun’s lower half had begun some 4 hours earlier, but that was before sunrise on Nantucket, one of the first places in the United States to view the passage. With still another 2.5 hours of transit to go, those gathered on the beach peered sunward with special glasses, binoculars, and telescopes.
The telescopes, some mounted on the sand, showed Venus at its most riveting: a bullet hole in an orange orb.
Among the sky watchers was veteran astronomer Dorrit Hoffleit, former director of the island’s Maria Mitchell Observatory (see “Island Astronomy,” below). Cataracts now cloud Hoffleit’s vision, and she can no longer directly observe her lifelong passion, a class of stars whose brightness varies over time. But the transit loomed bright enough for Hoffleit to enjoy. At age 97, she was probably the oldest astronomer to view the event, yet even she was born 25 years too late to have viewed the planet’s last such passage, in 1882.
Fuzzy data
Just as fishermen once set sail from Nantucket Island on perilous whaling expeditions, astronomers in centuries past embarked on hazardous journeys around the globe to chase the transit of Venus (SN: 4/17/04, p. 247: Shades of Venus). Observations of a transit of Venus in the 18th century provided the first evidence for an atmosphere of a planet other than Earth
Moreover, by comparing the times that Venus entered and exited the sun’s disk at far-flung locations, scientists could calculate the planet’s distance to the sun and thereby determine one of the era’s most sought-after cosmic details: the size of the solar system. But that strategy ran into an unexpected problem that scientists during this year’s transit were still investigating.
During the first transit explorations, in 1761 and 1769, astronomers found their determinations of the timing confounded by a perplexing phenomenon. As it enters the sun’s disk, and again when it exits, Venus’ silhouette becomes fuzzier and elongates from a sharply defined circle to a teardrop shape. This black-drop effect makes it difficult to know exactly when the transit begins and ends, so determinations of the sun’s distance by this method were less precise than the 18th century astronomers had expected.
The exact origin of the black-drop effect has remained a matter of debate, but an article published in the April Icarus argued that the fuzziness didn’t result from sunlight shining through Venus’ bloated atmosphere. Glenn Schneider of the University of Arizona in Tucson, Jay Pasachoff of Williams College in Williamstown, Mass., and their colleagues reported that they had found a similar effect while observing the 1999 transit of Mercury across the face of the sun. Since Mercury has no atmosphere, they reasoned, the phenomenon must arise from other sources.
The effect is most likely a combination of optical distortions by Earth’s atmosphere, the inherent limitation of telescope mirrors to discern detail, and the appearance of the sun, which looks brighter at its center than it does at its rim.
To further investigate the black-drop effect, Pasachoff and his colleagues stationed themselves for the Venus transit at an observatory in Thessaloniki, Greece, where they viewed the event in its entirety. Schneider and Pasachoff are also comparing ground-based images with those taken by the Transition Region and Coronal Explorer (TRACE) satellite, an Earth-orbiting observatory aimed at the sun, which took spectacular images of the transit. Because that observatory flies above Earth’s atmosphere, they’re looking to learn what part of the black-drop effect stems from the other factors.
“To be able to cast [new] light on a scientific problem that was among the most important in the world 240 years ago is really a high point of my work,” Pasachoff says.
Night vision
While Schneider and Pasachoff were in Greece to see the transit with their own eyes, Schneider’s colleague Paul Smith, also of the University of Arizona, viewed the transit indirectly at night using a 2.3-meter telescope at Steward Observatory on Kitt Peak. He recorded the Venusian transit in sunlight reflecting off the moon. Less detailed than the view from Nantucket or Thessaloniki, Smith’s images closely resemble the recent observations of distant, extrasolar planets passing in front of their parent stars.
Astronomer David Charbonneau of the California Institute of Technology in Pasadena, whose team used such observations to make the first detection of the atmosphere of a planet outside the solar system, combined a vacation with transit viewing. He carried a small telescope, filters, and a digital camera along on a visit to Italy’s northwest coast. The transit has a “particularly close connection to my work,” says Charbonneau.
Back on Nantucket, amateur astronomer Anthony Russo of Wallingford, Conn., kept shifting his telescope to keep Venus’ silhouette in view. Emily and Dominic, his two youngsters, frolicked on the beach, all but oblivious to the historic event he was witnessing.
For the last half hour, the clouds over Nantucket teased the sky watchers but parted one last time as Venus dropped off the rim.
Then Vladimir Strelnitski, the current director of the Maria Mitchell Observatory, paraphrased the final words of the opera “I Pagliacci.” He proclaimed, “Finita la commedia.” Indeed, the show was over—at least until June 6, 2012, when Venus makes its second and final passage of the century.
Island Astronomy
Maria Mitchell discovers a comet: Nantucket’s heavenly legacy
Strolling Nantucket’s narrow cobblestone streets lined with gray-shingled houses, brick mansions, and stately homes with rooftop lookouts, it’s not hard to imagine the time when this island was the whaling capital of the world. Men risked their lives—and earned fortunes—harpooning the sperm whale.
Harder to fathom is that Nantucket has a place in the history of astronomy. Born in 1818 to a large Quaker family, Maria Mitchell learned to use the tools of navigation—the sextant and the telescope—from her father. At age 12, Mitchell was already assisting her father in both nautical navigation and sky watching, regularly sweeping the heavens in search of comets.
On Oct. 1, 1847, Mitchell slipped out from a party at her family’s house, ran up to the roof where her telescope was mounted, and spied a fuzzy blob just above the North Star. She returned to the parlor and announced that she thought she’d found a comet.
She had.
The Nantucket comet, as it came to be called, was the first to be discovered by a U.S. citizen and the first by a woman. Accolades followed, including a gold medal awarded to her by the King of Denmark and a $100 prize from the Smithsonian Institution in Washington, D.C. In 1848, Mitchell became the first woman elected to the American Academy of Arts and Sciences. Seventeen years later, when the newly opened Vassar College hired her, she became the first female astronomy professor in the United States. Mitchell led her students on solar eclipse expeditions, raised funds for the Vassar Observatory, and witnessed the Great Comet of 1881.
The small telescope that Mitchell used to discover the Nantucket comet is now mounted in her childhood home on Vestal Street, across from the headquarters of the Maria Mitchell Association, the group her descendants founded in 1908 to continue Mitchell’s lifelong passion for the natural sciences and science education. Each summer, the association invites about 10 undergraduates to conduct experiments in astronomy, biology, and earth science on the island. On June 8, this year’s students were part of the beach party that witnessed the transit of Venus.
