The Caldwell Objects

30 feet. The mirror weighed 469 pounds and had
to be lifted by a crane. It cracked while cooling.
Herschel recast the mirror, which now weighed
538 pounds, but this time the metal fell through
the bottom of the furnace, damaging the floor
and causing the Herschels to run for cover. At the
end of this episode, Caroline later wrote, her
"poor brother fell, exhausted with heat and
exertion, on a heap of brickbats."
A NEW WORLD
Between these mirror-making escapades,
Herschel made his greatest discovery, that of the
planet Uranus. He found it on Tuesday, March
13, 1781, from the backyard of his new residence
at Bath. The discovery was no accident. That
night Herschel was using a telescope (with a focal
length of 7 feet, an aperture of 6.2 inches, and a
magnification of 227x) to systematically study,
for the purpose of registering double stars, all
stars brighter than 8th magnitude, when he
recorded a new object in his journal:
In the quartile near ζ Tauri the lowest of two is a
curious either nebulous star or perhaps a comet. A
small star follows the comet at ⅔ of the fields
distance.
After verifying that the "comet" had moved,
Herschel notified the Astronomer Royal, Nevil
Maskelyne, at Greenwich, who first suggested
that the object might be a new planet. He also
contacted his fellow astronomer and correspondent,
Rev. T. Hornsby at Oxford. Subsequent
observations proved that Herschel had indeed
become the first person in history to discover a
planet beyond those visible to the naked eye, and
this granted him immediate and lasting fame. In
November 1781 he was awarded the Royal
Society's Copley Medal for the discovery.
In response to a request by Maskelyne the
following year, Herschel named his new object
456
Georgium Sidus (the star of George) after his
king and mentor, King George III. "I cannot but
wish to take this opportunity of expressing my
sense of gratitude," Hershel wrote, "by giving the
name Georgium Sidus to a star, which with
respect to us first began to shine under his
auspicious reign." (The name Uranus, proposed
by Johann Elert Bode, was not universally
accepted until after Herschel's death). Herschel's
request did not go unnoticed by the king, who
suggested that Herschel concentrate on
astronomy full time. To this end, he granted
Herschel an annual stipend of £200 (not a large
sum), in exchange for which the astronomer was
to occasionally show members of the royal family
something of interest in the sky.
Herschel's career as a professional astronomer
commenced in August of 1782. To fulfill his
duties to the king, Herschel (with Caroline)
moved to a house in Datchet, about one and a
half miles from Windsor Castle, on the left bank
of the Thames. It had a convenient garden, in
which he placed a "small 20-foot reflector of 12
inches aperture." By 1783, Herschel had built a
"large 20 feet" (focal length) telescope with an
aperture of 18.7 inches, and he began observing
the heavens with it on October 23rd of that year.
All of his "sweeps" were made with this
instrument, as were most of his other important
observations.
Upon completing the large 20-foot telescope,
Herschel did not miss a single hour of "Star-Light
weather from which I used either to watch
myself or to keep somebody to watch. My leisure
hours of the day were spent in preparing and
improving telescopes." Herschel was to build
numerous telescopes for sale as well as for his
own use.
SWEEPING THE HEAVENS
Five days after he deployed his large 20-foot
reflector, Herschel began a systematic sweep
Deep-Sky Companions: The Caldwell Objects