Sky & Telescope - December 2024 - 20

Solar System History
The 19th Century Takes Up the Gauntlet
The next Venus transit was due on December 9, 1874. And so,
18th-century astronomers passed the torch to their 19thcentury
counterparts, who would be able to deploy an array
of improved techniques and technology.
The 105 years between transits saw monumental political
upheavals and the beginning of the Industrial Revolution,
with its steam engines, spinning jennies, and locomotives.
The adoption of precise standards of measurement, crucial
to economic and military success, was emphasized as the
distinguishing feature of science by the British physicist Lord
Kelvin, who proclaimed, " When you can measure what you
are speaking about, and express it in numbers, you know
something about it. "
Astronomers shared this obsession, and nothing beckoned
with greater urgency than precisely determining the astronomical
unit. The application of Halley's method (including
a refinement introduced by the French astronomer JosephNicolas
Delisle) was at the foundation of a vast enterprise to
observe the transit of Venus.
In the 18th century, land travel was slow and sea travel
unpredictable. In 1761, one transit observer, Jean-Baptiste
Chappe d'Auteroche, took five months to travel from Paris
to Tobolsk (Siberia) by land and arrived with little time to
spare. He didn't return to France until 1763. In 1769, he
needed eight months to make the journey from Paris to San
José del Cabo in Baja, California (now Mexico), where he
subsequently died in an outbreak of yellow fever. Only one
member of his expedition managed to return home with
their observations.
By 1874, however, it was possible to cross the Atlantic
in less than eight days, in contrast to the 77 that Chappe
required. The improvements in land travel due to the introduction
of railroads was, if anything, even greater. The
greater ease of movement meant that some 100 official
expeditions set off to observe the December 9, 1874, transit,
which was visible in its entirety across Japan, Southeast Asia,
Indonesia, Australia, and New Zealand.
Governments helped sponsor overseas expeditions, and
observatories and universities organized a few more. Of these,
Great Britain, Russia, the United States, and Germany backed
the largest number of expeditions, though Australian entities
also organized many small parties, since the continent lay in
the transit zone.
The 18th century also heralded the arrival of vastly
improved astronomical instrumentation and techniques.
Observers of the 18th-century transits had to make precise
visual timings of the planet's disk making contact with the
Sun's limb, using small telescopes often of imperfect optical
quality. At the critical moments, various optical effects
hampered precise timings. Most notorious was the aptly
named, black-drop effect, which occurs as the disk of Venus
and the edge of the Sun barely touch - the planet's silhouette
distends like a drop of water about to drip from a faucet.
The effect meant that even observers positioned side by side
often disagreed on the precise moment of contact - introTRANSIT
ZONE As a Cambridge-trained mathematician who devoted his life to the popularization of astronomy, Richard A. Proctor published the
standard works on the 19th-century transits of Venus in the English-speaking world. Proctor himself drew this aesthetically pleasing chart showing the
circumstances of the 1874 transit, based on his own calculations.
20 DECEMBER 2024 * SKY & TELESCOPE
ECLIPSE-MAPS.COM / PUBLIC DOMAIN
http://www.ECLIPSE-MAPS.COM

Sky & Telescope - December 2024

Table of Contents for the Digital Edition of Sky & Telescope - December 2024

Contents
Sky & Telescope - December 2024 - Cover1
Sky & Telescope - December 2024 - Cover2
Sky & Telescope - December 2024 - 1
Sky & Telescope - December 2024 - Contents
Sky & Telescope - December 2024 - 3
Sky & Telescope - December 2024 - 4
Sky & Telescope - December 2024 - 5
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