Learning From Celestial Beauty

VENUS slides between us and the Sun today, an exceedingly rare spectacle that scientists hope will expand their understanding of our solar system and refine their inquiries about distant planets.

A transit of Venus has been recorded just six times before; one will not recur until 2117. Each time, astronomers understand it far better than we did the last time; when it is over, our curiosity only grows. Each time, too, it stirs our imaginations. The 1882 transit inspired Edmond-Louis Dupain to paint, on a ceiling at the Paris Observatory, a near-nude Venus, goddess of love, floating across the path of the sun god Helios in his chariot in the sky.

If you view the real sky this afternoon — using a safe solar filter if you look directly, or viewing it on a projected image — you will see a dark spot cross the Sun, its diameter one-thirtieth the size of the Sun’s, and so round it might seem a bullet had pierced our star. Four NASA spacecraft will be observing, too — including two that will measure a drop of about one-tenth of 1 percent in the solar energy that reaches the Earth.

By measuring variations in this reduction of light during the transit, we hope to get clues about what happens when any planet, including so-called exoplanets far out in the universe, transits its star. NASA’s Kepler spacecraft has found more than 2,000 suspected exoplanets; perhaps 90 percent really are exoplanets, but nobody knows which 90 percent. Insights from this transit may help us find an answer.

Such ambitions could not have been imagined by Johannes Kepler, who observed that planetary orbits were elliptical, not circular (as even Copernicus had thought), and predicted the 1631 transit. Nobody recorded seeing it, but in 1639, 21-year-old Jeremiah Horrocks anticipated another transit, and it became the first ever observed. Horrocks also realized that transits occurred in eight-year pairs, with more than a century between one pair and the next.

More than 100 expeditions, from many countries, were sent to observe the next pair, in 1761 and 1769, in hopes that timing a transit would help to determine the solar system’s size. The method had been devised by Edmund Halley, of comet fame.

The British Admiralty sent a lieutenant, James Cook, to Tahiti, bringing scientists to watch the 1769 transit; he then explored the South Seas (and became a captain). The French abbé Jean-Baptiste Chappe d’Auteroche made his way across Siberia in 1761 for the first transit, then perished in Baja California in 1769, after witnessing the second. He had fallen victim to an epidemic, thought to be typhus, in his quest for the best observation point. It was written of him that he survived to see a lunar eclipse weeks later and died a happy man, having accomplished his aims.

Less gratified was the French astronomer Guillaume Le Gentil. After the British prevented him from landing in Pondicherry, India, to observe the transit of 1761, he stayed in Asia until the next transit, since it was only eight years away. But last-minute clouds spoiled his observations in 1769, and when he sailed home, he found he had been declared dead and his finances had been plundered. Only after a struggle did he reassemble his life.

For the next transits, in 1874 and 1882, even newfangled dry-plate photography could not overcome the so-called black-drop effect, an optical phenomenon in which Venus, for about a minute after it enters the solar disk, appears as a mysterious, dark, teardrop-like shape linked to the sky outside. A century before, the resulting uncertainty in timing the transit had limited the accuracy of Halley’s plan for measuring the solar system. And it would take spacecraft observations of a transit of Mercury in 1999 before the black-drop effect mystery was solved. (Glenn Schneider of the University of Arizona and I deduced that an effect of the sun’s gaseous nature helped explain the phenomenon — as a result of darkening near the sun’s edge and the limits of optical imaging.)