The Solar System

528 PART 4^ |^ THE SOLAR SYSTEM

Over the half-century following the discovery of Uranus,
astronomers noted that Newton’s laws did not exactly predict the
observed position of the planet. Tiny variations in the orbital
motion of Uranus eventually led to the discovery of Neptune, a
controversial story you will read later in this chapter.

The Motion of Uranus

Uranus orbits nearly 20 AU from the sun and takes 84 years to
go around once (Celestial Profi le 9). Th e ancients thought
of Saturn as the slowest of the planets, but Saturn orbits in a bit
over 29 years. Uranus, being farther from the sun, moves even
slower than Saturn and has a longer orbital peri od.
Th e rotation of Uranus is peculiar. Earth rotates approxi-
mately upright in its orbit. Th at is, its axis of rotation is inclined
only 23.5° from the perpendicular to its orbit. Th e other planets
have similarly moderate axial inclinations. Uranus, in contrast,
rotates on an axis that is inclined 97.9° from the perpendicular
to its orbit. It rotates on its side (■ Figure 24-3).
Because of its odd axial tilt, seasons on Uranus are extreme.
Th e fi rst good photographs of Uranus were taken in 1986, when
the Voyager 2 spacecraft fl ew past. At that time, Uranus was in
the segment of its orbit in which its south pole faced the sun.
Consequently, its southern hemisphere was bathed in continuous

The So-Called Scientifi c Method

Why didn’t Galileo expect to discover
Jupiter’s moons? In 1928, Alexander Fleming
noticed that bacteria in a culture dish were
avoiding a spot of mold. He went on to
discover penicillin. In 1895, Conrad Roentgen
noticed a fl uorescent screen glowing in his
laboratory when he experimented with other
equipment. He discovered X-rays. In 1896,
Henri Becquerel stored a uranium mineral on
a photographic plate safely wrapped in black
paper. The plate was later found to be fogged,
and Becquerel discovered natural radioactivity.
Like many discoveries in science, these seem
to be accidental; but, as you have seen in this
chapter, “accidental” doesn’t quite describe
what happened.
The most important discoveries in science
are those that totally change the way people
think about nature, and it is very unlikely that
anyone would predict such discoveries. For the
most part, scientists work within a paradigm
(look back at How Do We Know? 4-1), a set
of models, hypotheses, theories, and expecta-
tions about nature, and it is very diffi cult to
imagine natural events that lie beyond that

paradigm. Ptolemy, for example, could not

have imagined galaxies because they were not

part of his geocentric paradigm. That means

that the most important discoveries in science

are almost always unexpected.

An unexpected discovery, however, is not

the same as an accidental discovery. Fleming

discovered penicillin in his culture dish not

because he was the fi rst to see it, but because

he had studied bacterial growth for many

years; so, when he saw what many others

must have seen before, he recognized it as

important. Roentgen realized that the glowing

screen in his lab was important, and Becquerel

didn’t discard that fogged photographic plate.

Long years of experience prepared them to

recognize the signifi cance of what they saw.

A historical study has shown that each time

astronomers build a telescope that greatly

surpasses existing telescopes in capability, their

most important discoveries are unexpected.

Herschel didn’t expect to discover Uranus with

his 7-foot telescope, and modern astronomers

didn’t expect to discover evidence of dark

energy with the Hubble Space Telescope.

The discovery of pulsars, spinning neutron stars,

was totally unexpected, as important scientifi c

discoveries often are. (NASA/McGill/V. Kaspi et al.)

X-ray image

Pulsar

Scientists pursuing basic research are rarely

able to explain the potential value of their

work, but that doesn’t make their discoveries

accidental. They earn their right to those lucky

accidents.

24-1

Scientifi c Discoveries

sunlight, and a creature living on Uranus (an unlikely possibility,

as you will discover later) would have seen the sun near the

planet’s south celestial pole. Th e sun was at southern solstice on

Uranus in 1986, and you can see this at lower left in Figure 24-3.

Over the next two decades, Uranus moved about a quarter of the

way around its orbit, and, with the sun shining down from above

the planet’s equator, a citizen of Uranus would see the sun rise

and set with the rotation of the planet. Th e sun reached equinox

on Uranus in December 2007, and you can see that geometry by

looking at the lower right in Figure 24-3. As Uranus continues

along its orbit, the sun approaches the planet’s north celestial

pole, and the southern hemisphere of the planet experiences a

lightless winter lasting 21 Earth years. In other words, the eclip-

tic on Uranus passes very near the planet’s celestial poles, and the

result is strong seasonal variation.

The Atmosphere of Uranus

Like Jupiter and Saturn, Uranus has no surface. Th e gases of its

atmosphere—mostly hydrogen, 15 percent helium, and a few

percent methane, ammonia, and water vapor—blend gradually

into a fl uid interior.

Seen through Earth-based telescopes, Uranus is a small, fea-

tureless greenish-blue disk. Th e green-blue color arises because