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The Caldwell Objects

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30 feet. <strong>The</strong> mirror weighed 469 pounds and had<br />

to be lifted by a crane. It cracked while cooling.<br />

Herschel recast the mirror, which now weighed<br />

538 pounds, but this time the metal fell through<br />

the bottom of the furnace, damaging the floor<br />

and causing the Herschels to run for cover. At the<br />

end of this episode, Caroline later wrote, her<br />

"poor brother fell, exhausted with heat and<br />

exertion, on a heap of brickbats."<br />

A NEW WORLD<br />

Between these mirror-making escapades,<br />

Herschel made his greatest discovery, that of the<br />

planet Uranus. He found it on Tuesday, March<br />

13, 1781, from the backyard of his new residence<br />

at Bath. <strong>The</strong> discovery was no accident. That<br />

night Herschel was using a telescope (with a focal<br />

length of 7 feet, an aperture of 6.2 inches, and a<br />

magnification of 227x) to systematically study,<br />

for the purpose of registering double stars, all<br />

stars brighter than 8th magnitude, when he<br />

recorded a new object in his journal:<br />

In the quartile near ζ Tauri the lowest of two is a<br />

curious either nebulous star or perhaps a comet. A<br />

small star follows the comet at ⅔ of the fields<br />

distance.<br />

After verifying that the "comet" had moved,<br />

Herschel notified the Astronomer Royal, Nevil<br />

Maskelyne, at Greenwich, who first suggested<br />

that the object might be a new planet. He also<br />

contacted his fellow astronomer and correspondent,<br />

Rev. T. Hornsby at Oxford. Subsequent<br />

observations proved that Herschel had indeed<br />

become the first person in history to discover a<br />

planet beyond those visible to the naked eye, and<br />

this granted him immediate and lasting fame. In<br />

November 1781 he was awarded the Royal<br />

Society's Copley Medal for the discovery.<br />

In response to a request by Maskelyne the<br />

following year, Herschel named his new object<br />

456<br />

Georgium Sidus (the star of George) after his<br />

king and mentor, King George III. "I cannot but<br />

wish to take this opportunity of expressing my<br />

sense of gratitude," Hershel wrote, "by giving the<br />

name Georgium Sidus to a star, which with<br />

respect to us first began to shine under his<br />

auspicious reign." (<strong>The</strong> name Uranus, proposed<br />

by Johann Elert Bode, was not universally<br />

accepted until after Herschel's death). Herschel's<br />

request did not go unnoticed by the king, who<br />

suggested that Herschel concentrate on<br />

astronomy full time. To this end, he granted<br />

Herschel an annual stipend of £200 (not a large<br />

sum), in exchange for which the astronomer was<br />

to occasionally show members of the royal family<br />

something of interest in the sky.<br />

Herschel's career as a professional astronomer<br />

commenced in August of 1782. To fulfill his<br />

duties to the king, Herschel (with Caroline)<br />

moved to a house in Datchet, about one and a<br />

half miles from Windsor Castle, on the left bank<br />

of the Thames. It had a convenient garden, in<br />

which he placed a "small 20-foot reflector of 12<br />

inches aperture." By 1783, Herschel had built a<br />

"large 20 feet" (focal length) telescope with an<br />

aperture of 18.7 inches, and he began observing<br />

the heavens with it on October 23rd of that year.<br />

All of his "sweeps" were made with this<br />

instrument, as were most of his other important<br />

observations.<br />

Upon completing the large 20-foot telescope,<br />

Herschel did not miss a single hour of "Star-Light<br />

weather from which I used either to watch<br />

myself or to keep somebody to watch. My leisure<br />

hours of the day were spent in preparing and<br />

improving telescopes." Herschel was to build<br />

numerous telescopes for sale as well as for his<br />

own use.<br />

SWEEPING THE HEAVENS<br />

Five days after he deployed his large 20-foot<br />

reflector, Herschel began a systematic sweep<br />

Deep-Sky Companions: <strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong>

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