Sky & Telescope - December 2024 - 24

Solar System History
to " sneer a little at us. " At last, he set up his equipment in
Queenstown, on the shores of glacial Lake Wakatipu and
within sight of the mountains justly known as the Remarkables.
(The site is now occupied by the Millennium Hotel
with its Observatory restaurant.)
In the end, Peters' gallant efforts were vindicated. Most
of New Zealand lay under heavy cloud on transit day, but
on the high ground where he deployed his telescopes, he
enjoyed short intervals of sunshine. He succeeded in timing
the first and second contacts and exposing 59 photographic
plates - a yield that exceeded the combined results of the
rest of the New Zealand expeditions. One imagines he celebrated
with a good cigar.
A Matter of Timings
Although the photographic results drew the greatest attention,
visual observations also proved valuable. Perhaps due to
improved optics, the black-drop effect did not figure as conspicuously
this time as it did in 1761 and 1769. Nevertheless,
the images on the photographic plates - from which so much
was expected - proved to be no less subject to uncertainty
than timings captured by eye.
Even the Janssen revolver, for all its promise, proved to
TRANSIT DOWN UNDER This illustration from H. C. Russell's Observations
of the Transit of Venus, 9 December 1874; Made at Stations in
New South Wales shows the " black drop, " as recorded by F. Allerding in
Sydney, New South Wales, Australia, with a 3½-inch refractor stopped
down to an effective aperture of 2 inches.
be a bust. It produced diffuse, distorted images that were
no more reliable than the best visual reports. Evidently this
was in large part due to the blurring caused by Earth's everseething
atmosphere. At an 1881 international conference
in Paris to discuss results of the transit and to plan for the
next one in 1882, Airy admitted that " the ardor of observers
had been much cooled by the apparent general failure of the
photographic principle. "
Nonetheless, the slog of working out a new value of the
astronomical unit began. Charles Edward Burton, an assistant
at the Royal Greenwich Observatory, undertook the
task of reducing the British data, including the disappointing
photographic results. He applied himself to the painstaking
task until his eyesight and health began to fail. George
Lyon Tupman, who had organized the British expedition to
observe the transit from the Sandwich Islands, took over. He
described the heartbreak of the task:
When the negatives are placed under the microscope with an
amplification of only 5 or 6 diameters, the limbs of both planet
and Sun, even those which are pretty sharp to the unaided eye,
become extremely indistinct, and the act of bisecting the limb
with the wire or cross of the micrometer is mere guesswork.
Not surprisingly, the solar parallaxes Tupman derived
ATMOSPHERIC SIGHT The brilliant arc of sunlight refracted by
the atmosphere of Venus is recorded in these sketches by Captain A.
Onslow, who observed the 1874 transit from Goulburn, in the Southern
Tablelands of New South Wales, Australia, with a 33
/4
-inch, equatorially
mounted refractor. (This drawing also appeared in H. C. Russell's book.)
24 DECEMBER 2024 * SKY & TELESCOPE
were very poor. Indeed, only by discarding the photographic
values did the results show better mutual agreement than the
visual timings made in the 18th century! The technological
advances and new techniques of this bold new era ultimately
made little difference.
Meanwhile, it had become obvious that there were better
methods for determining a value for the astronomical unit.
PRIVATE COLLECTION OF WILLIAM SHEEHAN (2)

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
Sky & Telescope - December 2024 - 6
Sky & Telescope - December 2024 - 7
Sky & Telescope - December 2024 - 8
Sky & Telescope - December 2024 - 9
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Sky & Telescope - December 2024 - Cover4
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