And the Distance Is . . .

The transits had come and gone, but the observations of 1761 and 1769 were subject to scrutiny and calculation far into the 19th century. In 1824 Johann Franz Encke, the director of Berlin Observatory, reviewed all the transit measurements and determined a parallax of 8.5776 arcseconds, making one a.u. equal to 153,340,000 km (95,280,000 miles). This was a great improvement over previous determinations but still well off today's value of 149,597,870 km.

And what of the traveling astronomers who made these measurements? They returned home, reported their results, often wrote a book about their travels and travails, and, in some cases, went on to make further astronomical discoveries. As for Le Gentil, it took him another two years, filled with numerous misadventures, to return to France, whereupon he found that his heirs had declared him dead and were in the process of dividing his property! Events finally took a better turn when he married a wealthy heiress, had a daughter on whom he doted, and lived in seeming contentment at the Paris Observatory.

Preparing for 1874

Encke threw down the gauntlet to astronomers worldwide to surpass the accuracy of this number at the next transits of Venus in 1874 and 1882. But first they would need to learn how to reduce the measurement errors caused by the black drop and other effects.

In order to practice anticipating effects like the black drop, and to get a better handle on the timing variations introduced by different visual observers in advance of the 1874 transit, astronomers at several institutions built artificial transit machines. In France, for example, Charles Wolf set up a series of lamps and screens in the window of a library at the Luxembourg Garden and had observers make timings with a telescope at nearby Paris Observatory. American researchers set up another transit simulator on a building near the US Naval Observatory in Washington, DC.

A New Tool

But suppose the observer could, in a sense, be eliminated altogether? As the 1874 transit of Venus was approaching, photography was coming into its own as an instrument of scientific research ("Picturing the Heavens," April 2004 issue, page 36.) Photography seemed to afford an objective and impartial record, not subject to the vagaries introduced by the "personality of the eye." And most conveniently, a photograph could be studied and measured long after the event was over.

In the buildup to the 1874 transit, photography was widely regarded as affording the best chance of obtaining an absolute and unvarnished record of the various fleeting contact phenomena and thereby obtaining the best value of the astronomical unit. Many nations, including France, Britain, the United States, Russia, and Germany, dispatched astronomers and their cameras to measure and photograph the transit from around the globe.

Amateurs usually employed eyepiece projection onto a white screen for solar viewing — the safest method then available. The only practical full-aperture solar filter obtainable in 1874 was smoked glass, but few wanted to smoke a valuable optical flat, much less their telescope's objective lens. So for direct viewing, observers typically placed an unsilvered prism in the telescope's optical path to deflect most of the Sun's heat and light away from the eyepiece. But enough heat and light got through to necessitate the use of tinted-glass filters to make viewing safe and comfortable, and even then there were considerable risks. In Australia one observer had the unnerving experience of having his dark glasses split, "causing him to lose the Ingress entirely."

As to photographic observations, most involved recording Venus's silhouette against the Sun so that its position could be carefully measured later. This meant taking special care that the wet-process plates then in use were carefully transported across great distances without bending or warping. Many plates were exposed, though most seem to have been discarded after they had served their purpose, and surprisingly few have survived.