Sky & Telescope - December 2024 - 21

ducing uncertainties that contributed to the inaccuracy of
the results. But in 1874 astronomers not only had better
telescopes, but they could also rehearse by viewing artificial
transits. For example, French astronomers at the Paris Observatory
aimed telescopes at a series of lamps and screens positioned
in the window of a library in the nearby Luxembourg
Gardens. Similarly, American astronomers set up their own
transit simulation at a building near the U.S. Naval Observatory
in Washington, DC.
More crucial was that astronomy now had a new weapon
in its arsenal: photography. Visual observations were subjective,
but the photographic plate was regarded as objective
and not subject to the vagaries of what American astronomer
Ormsby M. Mitchel called the " personality of the eye. "
Thus, the most serious efforts to observe the transit of 1874
employed photography, which would allow astronomers to
leisurely measure the contact times after the fact.
Janssen and His Revolver Camera
The most innovative device deployed at the 1874 transit was
the revolver photographique, invented by the French astronomer
Pierre Jules César Janssen (1824-1907). Janssen's device
produced a series of images, which could be shown in succession
to make a primitive motion picture (preceding the
famous " horse in motion " sequence captured by English
photographer Eadweard Muybridge by four years). Janssen's
" revolver camera " consisted of two slotted disks - a large
spinning one with 12 slots and a smaller, fixed one with a
single slot that formed a window that admitted only a small
part of the solar image onto a photographic plate. The larger
disk was advanced by a gear mechanism based on the one
employed in the Colt revolver. The camera was designed to
capture the " fortuitous moment " in which the contacts
between Venus and the Sun's limb took place. Astronomers
hoped that this new camera could defeat the uncertainties
that plagued 18th-century observations.
Janssen was a wonderfully colorful figure. Lifelong suffering
from a lame leg due to an accident at age eight plagued
him, but he refused to allow his handicap to hold him back.
Janssen devoted himself to scientific research and became
an inveterate eclipse chaser. Among his many exploits, he
observed the so-called King of Siam's eclipse of August 18,
1868, at Guntur, India, where he discovered (independently
of Norman Lockyer of England) a spectroscopic method for
observing solar prominences without the need for an eclipse.
To observe the December 1870 total eclipse in Oran, Algeria,
which occurred during the Siege of Paris, Janssen escaped
from France in a balloon. However, despite his heroic efforts
to get to the centerline, he was clouded out.
Since transits are really miniature eclipses, Janssen was
excited about the forthcoming transit of Venus in 1874.
Without knowing specifically where he would be setting up
his instruments, Janssen and his team arrived in Japan after
weathering a typhoon off the coast of Hong Kong. After
stopping first at the port of Yokohama, they continued on to
HISTORY MADE HERE Top: During the author's visit to Japan in
2004, he and a group of Japanese enthusiasts climbed Mount Konpirayama
near Nagasaki to find this small pyramid marking the location
where Janssen's team observed the transit of Venus in 1874.
MADE FOR JAPAN Bottom: In this portrait of the Janssen party upon
their arrival at the Japanese port of Yokohama Bay, Janssen sits in the
front middle, while the tube of a telescope is visible in the background
along with the revolver photographique, on a table at far left.
Nagasaki, where they established themselves on the mountain
Konpira-yama - a magnificent setting with breathtaking
views of the city below. As a precaution against bad weather,
Janssen sent a detachment of observers to Kobe, more than
500 km away. As it turned out, the weather on transit day
was perfect at both sites. The following day, Janssen telegraphed
the president of the transit commission in Paris with
the message:
10 December. Transit observed at Nagasaki and Kobe, interior
contacts obtained. No ligaments [i.e., black-drop effects].
Photographed with revolver . . . Venus seen over Sun's corona
before transit.
American and British astronomers also mounted expedisk
yand tele scope .o r g * DECEMBER 2024 21
MARKER PYRAMID: WILLIAM SHEEHAN; JANSSEN AND CREW: WIKIMEDIA COMMONS / PUBLIC DOMAIN
http://www.skyandtelescope.com
Sky & Telescope - December 2024

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