CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 527
over two years. In the process, of course, he found many double
stars that were binary stars that are actually close together in space
and orbiting each other (see Chapter 9).
On the night of March 13, 1781, Herschel set up the 7-ft-
long telescope in his back garden and continued his work. He
later wrote, “In examining the small stars in the neighborhood of
H Geminorum, I perceived one that appeared visibly larger than
the rest.” As seen from Earth, Uranus is never larger in angular
diameter than 3.7 arc seconds, so Herschel’s detection of the disk
illustrates the quality of his telescope and his eye. At fi rst he sus-
pected that the object was a comet, but other astronomers quickly
realized that it was a planet orbiting the sun beyond Saturn.
Th e discovery of Uranus made Herschel world famous. Since
antiquity, astronomers had known of fi ve planets—Mercury,
Venus, Mars, Jupiter, and Saturn—and they had supposed that the
list was complete. Herschel’s discovery extended the classical uni-
verse by adding a new planet. Th e English public accepted
Herschel as their astronomer-hero, and, having named the new
planet Georgium Sidus (George’s Star) after King George III,
Herschel received a royal pension. Th e former music teacher was
welcomed into court society, where he eventually met and married
a wealthy widow and took his place as one of the great English
astronomers. His new fi nancial position allowed him to build large
telescopes on his estate and, with his sister Caroline, also a talented
astronomer, he attempted to map the extent of the universe. You
read about their “star gauging” research in Chapter 15.
Continental astronomers were less than thrilled that an
Englishman had made such a great discovery, and even some
professional English astronomers thought Herschel a mere ama-
teur. Th ey called his discovery a lucky accident. Herschel defended
himself by making three points. First, he had built some of the
fi nest-quality telescopes then in existence. Second, he had been
conducting a systematic research project and would have found
Uranus eventually because he was inspecting all of the brighter
stars visible with his telescope. And third, he had great experience
seeing fi ne detail with his telescopes. As a musician, he knew the
value of practice and applied it to the business of astronomical
observing. In fact, records show that other astronomers had seen
Uranus at least 17 times before Herschel, but each time they
failed to notice that it was not a star. Th ey plotted Uranus on
their charts as if it were just another faint star.
Th is illustrates one of the ways in which scientifi c discoveries
are made. Often, discoveries seem accidental, but on closer
examination you fi nd that the scientist has earned the right to the
discovery through many years of study and preparation (How
Do We Know? 24-1). To quote a common saying, “Luck is
what happens to people who work hard.”
Continental astronomers, especially the French, insisted that
the new planet not be named after an English king. Th ey, along
with many other non-English astronomers, stubbornly called the
planet Herschel. Years later, German astronomer Johann Bode sug-
gested the name Uranus, one of the most senior of the Greek gods.of Newton’s refl ecting telescope (see Chapter 6). William Herschel
and his brother Alexander began building telescopes, and William
went on to study astronomy in his spare time.
Herschel’s telescopes were similar to Newton’s in that they
had metal mirrors, but Herschel’s were much larger. Newton’s
telescope had a mirror about 1 in. in diameter, but Herschel
developed ways of making much larger mirrors, and he soon had
telescopes as long as 20 ft. One of his favorite telescopes was 7 ft
long and had a mirror 6.2 in. in diameter. Using this telescope, he
began the research project that led to the discovery of Uranus.
Herschel did not set out to search for a planet; he was trying
to detect stellar parallax produced by Earth’s motion around
the sun (see Figure 9-3). No one had yet detected this eff ect,
although by the 1700s all astronomers accepted the idea that
Earth moved. Galileo had pointed out that parallax might be
detected if a nearby star and a very distant star lay so nearly along
the same line of sight that they looked like a very close double star
through a telescope. In such a case, Earth’s orbital motion would
produce a parallactic shift in the position of the nearby star with
respect to the more distant star (■ Figure 24-2). Herschel began
to examine all stars brighter than eighth magnitude to search for
double stars that might show parallax. Th at project alone took
■ Figure 24-2
A double star consisting of a nearby star and a more distant star could be
used to detect stellar parallax. Seen from point b in Earth’s orbit, the two
stars appear closer together than from point a. The effect is actually much
too small to detect by too small to detect by the unaided eye.
As seen from aAs seen from bOrbit of EarthbaNearer star close to
line of sight seen
from point b.