38 PART 1^ |^ EXPLORING THE SKY
use their imaginations to visualize how nature works, but with a
key diff erence; they test their ideas against reality (How Do
We Know? 3-1). You can take comfort that today’s astrono-
mers explain solar eclipses without imagining celestial monsters.
A solar eclipse occurs when the moon moves between Earth
and the sun. If the moon covers the disk of the sun completely,
you see a spectacular total solar eclipse (■Figure 3-6). If, from
your location, the moon covers only part of the sun, you see a
less dramatic partial solar eclipse. During a solar eclipse, people
in one place on Earth may see a total eclipse while people only a
few hundred kilometers away see a partial eclipse.
Th e geometry of a solar eclipse is quite diff erent from that of
a lunar eclipse. You can begin by considering how big the sun
and moon look in the sky.
The Angular Diameter of the Sun
and Moon
Solar eclipses are spectacular because Earth’s moon happens to have
nearly the same angular diameter as the sun, so it can cover the sun’s
disk almost exactly. You learned about angular diameter in Chapter
2; now you need to think carefully about how the size and distance
of an object like the moon determine its angular diameter.
Visual wavelength images■ Figure 3-6
Solar eclipses are dramatic. In June 2001, an automatic camera in southern
Africa snapped pictures every 5 minutes as the afternoon sun sank lower in
the sky. From upper right to lower left, you can see the moon crossing the
disk of the sun. A longer exposure was needed to record the total phase of
the eclipse. (©2001 F. Espenak)
The So-Called Scientifi c Method
How do scientists produce theories to
test? Good scientists are invariably creative
people with strong imaginations who can look
at raw data about some invisible aspect of
nature such as an atom and construct mental
pictures as diverse as a plum pudding or a
solar system. These scientists share the same
human impulse to understand nature that
drove ancient cultures to imagine eclipses as
serpents devouring the sun.
As the 20th century began, physicists were
busy trying to imagine what an atom was like.
No one can see an atom, but English physicist
J. J. Thomson used what he knew from his
experiments and his powerful imagination to
create an image of what an atom might be
like. He suggested that an atom was a ball of
positively charged material with negatively
charged electrons distributed throughout like
plums in a plum pudding.
The key difference between using a plum
pudding to represent the atom and a hungry
serpent to represent an eclipse is that the
plum pudding model was based on experi-
mental data and could be tested against new
evidence. As it turned out, Thomson’s student,
Ernest Rutherford, performed ingenious new
experiments and showed that atoms can’t be
made like plum puddings. Rather, he imagined
an atom as a tiny positively charged nucleus
surrounded by negatively charged electrons
much like a tiny solar system with planets
circling the sun. Later experiments confi rmed
that Rutherford’s description of atoms is closer
to reality, and it has become a universally
recognized symbol for atomic energy.
Ancient cultures pictured the sun being
devoured by a serpent. Thomson, Rutherford,
and scientists like them used their scientifi c
imaginations to visualize natural processes andA model image of the atom as electrons orbiting a
small nucleus has become the symbol for atomic
energy.
then test and refi ne their ideas with new experi-
ments and observations. The critical difference is
that scientifi c imagination is continually tested
against reality and is revised when necessary.