Astronomers around the world are gearing up to watch a momentous event – the last transit of Venus in this century on June 5/6, 2012. During this event, a tiny dot, the planet Venus, will slowly sail in front of the sun. To learn more about the transit, including whether you will be able to see it, click here. Before this century, the last transit of Venus was in the year 1882. What a different world it must have been! We can’t imagine what it was like to have been there, but – thanks to this video from Tony Misch and William Sheehan – we can almost glimpse the 1882 Venus transit through the eyes of 19th century astronomers.
Transit of Venus are rare. They occur in pairs occurring in a cycle that repeats every 243 years, with pairs of transits separated by about 121 years and 105 years. When one transit occurs, another one happens eight years later. Then there are no Venus transits for more than a century. After the 1882 transit, the next one to occur was in 2004. Then there is the June 5/6, 2012 transit. The next one won’t be until December 11, 2117.
EarthSky asked Misch and Sheehan about the 1882 Venus transit, and about how and why they decided to “reanimate” the event on video.
What did astronomers expect to learn from the 1882 transit of Venus?
The simple answer is that they hoped to come home with a refined value for the solar parallax, which is a tool astronomers use to measure relatively nearby distances in space. Solar parallax is a fundamental quantity that can be derived by comparing transit data from widely separated obervations, and from which the Earth-sun distance – called the astronomical unit – can be calculated.
An accurate value for the solar parallax is the key to measuring the entire solar system, which in turn bootstraps all sorts of other fundamentals.
A century prior to the 1882 transit, in the 18th century, transits of Venus were as important as they were rare, being the best hope of getting a good measurement for the scale of the solar system. At the time, observing the Venus transits of 1761 and 1769 was probably the biggest coordinated scientific program ever undertaken. The observations, however, proved far more difficult than anticipated.
The next Venus transit was in 1874. Big advances in technology — notably the advent of photography, but also better telescopes, measuring devices, and time-keeping, along with much greater ease of travel—made for high hopes, but again the results came in below expectation.
Moreover, by this time several other techniques for determining the solar parallax, not dependent on rare transits of Venus or requiring observations made at remote points on the globe, had produced values at least as good as the best that were likely to be obtained from further transit observations. So one might say that the 1882 transit was a somewhat anticlimactic last hurrah, driven as much by the inertia of earlier efforts as by scientific need. Many astronomers traveled to many sites and made many observations, but the real urgency was gone.
What steps were involved in the creation of your video?
In 2002, while doing research for a book on the historical transits of Venus, Bill’s attention was caught by a letter in Lick Observatory’s archive referring to a series of transit photographs made on Mt. Hamilton by Amherst astronomer David P. Todd in 1882, at the time Lick Observatory was being built on the summit. Todd, who had reduced the 1874 data for the U.S. Transit Commission, expected to be tapped to lead one of its eight expeditions in 1882, but was passed over. He was thus available and eager to accept the invitation of the Lick Trustees — who felt that their observatory had also been left out in the cold — to come to Mt. Hamilton and observe the transit with a fine new 40-foot photoheliograph.
Todd’s skill and careful preparation were rewarded with beautiful weather on transit day. Venus was already on the face of the sun when it rose at Mt. Hamilton. Over the next four and half hours, Todd made exposures at two-minute intervals on wet collodian plates (trickier to handle than the new-fangled dry plates favored by the official expeditions, but chosen by Todd for their finer resolution). The original negatives were stored on Mt. Hamilton, but with interest in the actual result having cooled, they were eventually forgotten.
Following up on Bill’s tip, we struck gold on a high-up, corner shelf in the observatory’s plate vault, where all 147 negatives, most in good condition, had lain patiently for 120 years. The find turned out to be — to the best of our knowledge — the most complete photographic record of a transit of Venus in existence (this was, of course, prior to the 2004 transit). Digitizing the plates for archival purposes was the obvious next step, and the idea of making them into a stop-motion animation flowed naturally from that.
WIth a good scanner, Photoshop, and Final Cut, we were able to do in a few days what would have been impossible for Todd, even had the idea occurred to him — and it’s not out of the question that it did enter his fertile imagination. The Frenchman Jules Janssen had, after all, developed his “revolver” — a device that in some respects anticipated the motion picture camera — to photograph the 1874 transit. At about the same time Eadweard Muybridge, Étienne-Jules Marey and others were conducting early experiments with stop-motion photography. In any case, the transit movie is certainly among the earliest events to be photgraphically reanimated.
It’s an interesting if sad footnote that Todd is now more remembered as cuckold than astronomer, for it was probably during his absence from Amherst to photograph the transit that his beautiful and talented wife, Mabel Loomis Todd, an author in her own right and champion of Emily Dickinson, began a lifelong love affair with the poet’s brother Austin.
Are you planning to observe the June 5, 2012 transit?
Bill will be taking part in the activities planned at Mt. Wilson Observatory, and Tony will be at Mt. Hamilton where, among other things, he’ll be showing the movie and telling the story behind it.