The Transit of 1874: Disappointment

December 9, 1874, came and went. For some reason the black drop did not figure as conspicuously as it did during the 18th-century transits — probably because of the higher quality of the optics in use a century later — but timings by visual observers still differed exasperatingly among members of the same team, and the images recorded on photographic plates proved to be no less subject to vagaries than those captured by the eye.

Under the circumstances, it is hardly surprising that publication of a refined value of the astronomical unit was not immediately forthcoming after the 1874 transit. Moreover, as was already becoming obvious even before the 1882 event, there were better methods for determining the Earth-Sun distance. One was to measure the position of Mars relative to the background stars during one of the red planet's close approaches to Earth. With Mars, David Gill (the Astronomer Royal in Cape Town, South Africa) deduced a value of the astronomical unit only 0.2 percent off the modern value, better than any achieved from timings of a transit of Venus.

The Transit of 1882: Derision

The transit of December 6, 1882, thus came to occupy an unusual position in the history of astronomy. In part as a result of inertia, and in deference to the important role that transits of Venus had historically played in the quest for the grail of the astronomical unit, a number of government-sponsored expeditions were funded. That did not play well with the press. For example, the New York Times lambasted the US Naval Observatory for sending several expeditions to remote points of the globe when the transit could be observed from the continental United States. Indeed, the results of the 1882 transit were not really scientifically important. What was unique about the event was that it was the first of its kind within reach of the public.

The New York Times reported:

This is the first time within the memory of man that the unlearned common people have been permitted to observe a transit, and it is the first revelation of the fact that a transit can be seen through smoked glass.

People watched as they will again on June 8, 2004 — not in the hope of adding some tidbit of understanding to science, nor of refining the value of the astronomical unit, but out of sheer delight at the novelty of witnessing a rare happening in the heavens. Thus it will not be so much in the spirit of the great adventurers of the 18th century and the 1874 transit expeditions that we will watch the coming transit, but in a spirit of kinship with the "common people" who watched in 1882 realizing that this fleeting event would never again be seen in their lifetimes.

They relished it for its rarity, perhaps none more so than William Harkness of the US Naval Observatory, who reflected on the eve of the 1882 transit, "What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day."

The astronomy of that day — our day — features gigantic telescopes on high mountaintops, twin rovers wheeling across the Martian surface, a probe about to enter orbit around Saturn, and an armada of space telescopes peering into the heavens from orbit and observing cosmic radiation across the entire electromagnetic spectrum. We will have our transit of Venus on June 8th, then another on June 6, 2012. After that, there will be no more until 2117. What will be the state of science then God only knows.

This abridged article appeared in its entirety the February and May 2004 issues of Sky & Telescope. Archive subscribers may download the articles without restriction; nonsubscribers may buy them for US$2.95 per article. The regular price of a one-year archive subscription is $19.95; Sky & Telescope magazine subscribers save 50% and pay just $9.95. See our archive subscription page to sign up.

The Transit of Venus: Tales from the 18th Century
The Transit of Venus: Tales from the 19th Century