For those of you that may have missed it, last week saw a special Toasters exposé on the transit of Venus, as presented by engineerd™. It was a special event that only occurs on 100+ year cycles (quoth the engineerd, “Pairs of transits, separated by 8 years, occur every 105.5 or 121.5 years. The pattern repeats every 243 years.”), and so since the excitement has passed for another 100-ish years, it seems appropriate to look back on the late 1800s to see the first time that this astronomical anomaly was photographed.
In 1874 the first in the series of 2 closely spaced transits occurred, with the second coming in 1882. The US Naval Observatory and Transit of Venus Commission set up 8 expeditions to travel to various points on the globe to photographic each event. For the ’74 transit, wet bromo-iodide plates were used, but by ’82 dry collodion emulsion plates were available. Only 11 plates survive from the American 1882 expeditions, and none of the plates have survived from the 1874 groups. The exact sites from which each of these plates was taken is unknown.
An instruction manual was created by the Naval Observatory for the crew manning each viewing outpost, excerpts of which can be found here. The manual was created based of the experiences of the 1874 teams, and explained what the observers should expect to see and how to ascertain the moment of contact, as well as guidelines for getting a superb observation.
“Day of the Transit.–It is essential that every observer intending to make a really accurate observation should have little else to attend to during at least an hour or two before the first contact he is to observe, and should be entirely free from visitors and inquirers. The points to be particularly attended are the firmness of the telescope, his ability to move it in such a way as to keep any required part of the sun’s limb steadily in the center of the field, and the accuracy of the focal adjustment. A mere estimate of an accurate focus about the time of observation should not be trusted to if it can be avoided, because the eye itself is liable to change its accommodation in this respect.”