You’ve probably seen this picture before. It’s a photographic plate of the transit of Venus, taken by one of the eight 1882 American expeditions and it’s depicted in many books on the transit. Of the hundreds of dry collodion emulsion plates exposed by the American expeditions in 1882, only eleven survive today. And even of these eleven plates, no one can tell anymore from which station they originated. Or can they?
A couple of years ago I purchased an antique book from 1883, Transito de Venus por el Sol written by Luis Zegers. It gives a vivid description of the observations of the transit of Venus made from Chile by the French, Belgian, American and Chilean astronomers. The book also has an interesting frontispiece: an albumen photograph to scale of the transit of Venus, taken by the two photographers of the American party at Santiago.
The eleven photographs all have an inscription in the middle: ‘Fl 82’. The photograph from Chile has a similar writing: ‘Ch 82’. These letterings stem from the glass reticule plate which was placed in front of the sensitive plate and which had a grid etched on it, as well as some identification of the station it belonged to. That way, a grid to measure the pictures afterwards as well as an identification of the station became visible on each photograph that was taken.
The eight American expeditions were stationed at the US Naval Observatory (Washington), Cedar Keys (Florida), San Antonio (Texas), Cerro Roblero (New Mexico), Wellington (South Africa), Santa Cruz (Patagonia), Santiago (Chile) and Auckland (New Zealand). It’s great to have this photograph from Chile, because it makes clear that the two letters in de middle of the plate most probably refer to the station, leaving little room for doubt. If ‘Ch’ stands for Chile, than ‘Fl’ likely stands for Florida.
Comparing both pictures, it also is apparent that they were taken from two different hemispheres. On the picture taken from Santiago, Venus is on the lower half of the solar disk. Because on the southern hemisphere the Sun is seen ‘upside down’, the other picture should, in contrast, originate from the northern hemisphere.
This sounds very plausible, but still, additional research should show for certain. Each plate has a number, which corresponds to an entry in the photographer’s log, stating the time the picture was taken. It still needs to be checked if the positions of Venus on the Sun’s disk, as computed from the times listed in the log, correspond to the actual positions as measured on the photographic plates. The Cedar Keys’ log, however, is securely kept at the National Archives in Washington…