Over the past couple of days, astronomers have been putting solar filters on their telescopes and taking test shots of the sun in anticipation of capturing today’s Transit of Venus.
But this isn’t the first time photographs have been the method of choice for preserving this rare cosmic event. After hand-drawn images dominated observations from the 1761 and 1769 transits, photography came on the scene and was up to the task of creating a stunning record of the 1874 and 1882 events.
In the 18th century, astronomers traveled to far flung locations around the globe to observe the transit of Venus. It was, in 1761 and 1769, the best chance astronomers had to measure the size of the solar system; by comparing transit times from different locations, they could use simple geometry to determine the distance from the Earth to the sun. But this precursor to modern international cooperatives didn’t return the precise results many had hoped. Instruments were imprecise, and many observers were amateurs.
When the next pair of transits rolled around in 1874 and 1882, things were different. Specifically, the technology available to astronomers was vastly improved. Telescopes were better, and measuring devices and time keeping methods were more precise. This, combined with easier means of traveling to optimal viewing locations, gave many high hopes for successful observations.
The other major technology to come on the scene for these 19th century transits was photography. It was the favored tool of American astronomers whose 200 photographs of the 1874 event hinted at the impressive results photography might yield during the 1882 transit. The latter was visible in the United States, enabling the U.S. Naval Observatory to capture nearly 1,400 photographs.
Occasionally images from the 19th century transits are published but most have been in various archives for the past century. Including those taken by Amherst astronomer David P. Todd in 1882. Todd was unexpectedly passed over to lead an expedition to observe the transit that year, especially after he reduced the 1874 data for the U.S. Transit Commission. It proved to be a fortuitous arrangement. When the Lick Observatory invited Todd to travel to Mt. Hamilton and observe the transit with a fine new 40-foot photoheliograph, he eagerly accepted. Todd’s was lucky to get clear skies the day of the transit, and over four and half hours took an exposure every two minutes on a wet collodion plate.
Todd’s negative were stored on Mt. Hamilton where they were eventually forgotten. That is, until a letter in the Lick Observatory’s archives caught astronomer William Sheehan’s attention. It referred to the series of transit photographs Todd made. Sheehan went looking for them and, with fellow astronomer Tony Misch, found all 147 of Todd’s original negatives in good condition.
Digitizing the negative was the next logical step, and putting them together in sequence to create a short movie of the 1882 transit soon followed.
It’s not often that we get such a pristine look into a rare historical event like a 19th century transit of Venus, but Sheehan and Misch have given us one. It’s an amazing look back in time. The video shows us the same event we’re all gearing up to see as astronomers saw it over a century ago. You can’t get a better literal historical perspective than this.
Source: Earth Sky