Not so much a theory of the universe as a simple picture of the planet we call
home, the flat-earth model proposed that Earth’s surface was level. Although
everyday experience makes this seem a reasonable assumption, direct observation
of nature shows the real world isn’t that simple. For instance, when a sailing
ship heads into port, the first part that becomes visible is the crow’s-nest,
followed by the sails, and then the bow of the ship. If the Earth were flat, the
entire ship would come into view at once as soon as it came close enough to
shore. The Greek philosopher Aristotle provided two more reasons why the Earth
was round. First, he noted that Earth’s shadow always took a circular bite out
of the moon during a lunar eclipse, which would only be possible with a
spherical Earth. (If the Earth were a disk, its shadow would appear as an
elongated ellipse at least during part of the eclipse.) Second, Aristotle knew
that people who journeyed north saw the North Star ascend higher in the sky,
while those heading south saw the North Star sink. On a flat Earth, the
positions of the stars wouldn’t vary with a person’s location. Despite these
arguments, which won over most of the world’s educated citizens, belief in a
flat Earth persisted among many others. Not until explorers first
circumnavigated the globe in the 16th century did those beliefs begin to die
out. Ptolemy, the last of the great Greek astronomers of antiquity, developed an
effective system for mapping the universe. Basing much of his theory on the work
of his predecessor, Hipparchus, Ptolemy designed a geocentric, or

Earth-centered, model that held sway for 1400 years. That Ptolemy could place

Earth at the center of the universe and still predict the planets’ positions
adequately was a testament to his ability as a mathematician. That he could do
so while maintaining the Greek belief that the heavens were perfect—and thus
that each planet moved along a circular orbit at a constant speed—is nothing
short of remarkable. Copernicus made a great leap forward by realizing that the
motions of the planets could be explained by placing the Sun at the center of
the universe instead of Earth. In his view, Earth was simply one of many planets
orbiting the Sun, and the daily motion of the stars and planets were just a
reflection of Earth spinning on its axis. Although the Greek astronomer

Aristarchus developed the same hypothesis more than 1500 years earlier,

Copernicus was the first person to argue its merits in modern times. Despite the
basic truth of his model, Copernicus did not prove that Earth moved around the

Sun. That was left for later astronomers. The first direct evidence came from

Newton’s laws of motion, which say that when objects orbit one another, the
lighter object moves more than the heavier one. Because the Sun has about

330,000 times more mass than Earth, our planet must be doing almost all the
moving. A direct observation of Earth’s motion came in 1838 when the German
astronomer Friedrich Bessel measured the tiny displacement, or parallax, of a
nearby star relative to the more distant stars. This minuscule displacement
reflects our planet’s changing vantage point as we orbit the Sun during the
year. How did the universe really begin? Most astronomers would say that the
debate is now over: The universe started with a giant explosion, called the Big

Bang. The big-bang theory got its start with the observations by Edwin Hubble
that showed the universe to be expanding. If you imagine the history of the
universe as a long-running movie, what happens when you show the movie in
reverse? All the galaxies would move closer and closer together, until
eventually they all get crushed together into one massive yet tiny sphere. It
was just this sort of thinking that led to the concept of the Big Bang. The Big

Bang marks the instant at which the universe began, when space and time came
into existence and all the matter in the cosmos started to expand. Amazingly,
theorists have deduced the history of the universe dating back to just 1043
second (10 million trillion trillion trillionths of a second) after the Big

Bang. Before this time all four fundamental forces—gravity, electromagnetism,
and the strong and weak nuclear forces—were unified, but physicists have yet
to develop a workable theory that can describe these conditions. During the
first second or so of the universe, protons, neutrons, and electrons—the
building blocks of atoms—formed when photons collided and converted their
energy into mass, and the four forces split into their