Humans have always looked into the night sky and wondered about the mysteries hidden in its depths. Developing our modern understanding of the universe and its origins has taken thousands of years, with many wrong turns and detours along the way.
Until the 1500s, most natural philosophers (the early scientists) thought our planet was the center of the universe. If Earth were moving through space, they reasoned, we would be thrown off or blown away by violent winds. In their theory, stars and planets circled Earth, held in a series of invisible crystal spheres. It certainly looked like the natural philosophers were right. After all, as we gaze up at the sky, don't we seem to be standing still? The Moon, the Sun, and the stars travel across the sky in great circles above us.
This geocentric (Earth-centered) theory survived for almost two thousand years. It was supported in the writings of the Greek philosopher Aristotle around 350 BCE. About five hundred years later, in the second century A.D., the Egyptian astronomer Ptolemy wrote a detailed explanation of why Earth must be at the center of the universe.
Not all early astronomers agreed with this theory, however. Aristarchus of Samos, a Greek astronomer who lived from about 310-230 BCE thought Earth revolved around the Sun. But for two thousand years, Aristotle's geocentric theory was accepted by most scientists. The powerful Catholic Church, which relied on his teachings, also supported the geocentric idea because some biblical passages suggested that the Sun moved while Earth remained in one place. Few people were willing to dispute the church's favored theory. Anyone who challenged it was in danger of being imprisoned or even executed.
We have since learned that our planet is just a tiny speck circling an ordinary star, somewhere in a vast ocean of stars. Our understanding of the universe began to change around 1500, with the work of Polish astronomer Nicolaus Copernicus. Copernicus believed Earth and the other planets traveled in circular orbits around the Sun. His theory more accurately described the paths of the planets and stars in the sky than the geocentric theory did. But Copernicus's theory also removed Earth from its special place at the center of the universe.
Copernicus knew his idea was contrary to church teachings. Revealing it could be punished by death. He didn't publish his revolutionary idea until the very last days of his life. Copernicus's idea has been an essential part of cosmology ever since. The Copernican principle says Earth does not occupy a special, central time or place in the universe.
In 1609 Austrian astronomer Johannes Kepler improved upon Copernicus's idea. Kepler used mathematics to analyze the orbits of the known planets and found that they all follow the same rules of motion. The planets, he discovered, travel around the Sun in elliptical (oval) orbits, not circles. Each moves more slowly as its orbit takes it farther from the Sun, then speeds up again as it moves closer. Kepler also found that a planet's speed is proportional to its distance from the Sun. The farther a planet is from the Sun, the more slowly it moves. In short, the motions of the heavenly bodies follow regular mathematical rules.
In 1610 the Italian scientist Galileo Galilei provided additional support for Copernicus's theory. The telescope had recently been invented, and Galileo made his own improvements on the instrument. With his new telescope, Galileo found four moons orbiting Jupiter. These moons were further proof that not every object in the sky circled Earth. Galileo's discovery made it easier to believe that planets could revolve around the Sun, much as the moons revolved around Jupiter.
Just as important, Galileo first explained the idea of inertia. Inertia is the property of all matter to remain in motion (or at rest) unless a force acts to change its motion. One such force is friction—the force that slows the motion of two surfaces that touch each other. Galileo realized that in empty space, without friction, an object like a star or a planet could keep moving forever. Through Galileo's work, astronomers came to understand that everything in the universe is in constant motion.
The key to understanding the motion of the stars and planets became available in 1687, when English physicist Isaac Newton published his laws of motion and the law of universal gravitation. Newton's laws predicted the motion of objects on Earth. Even more important, the same laws calculated the motion of the planets. According to Newton's laws, an apple falling to Earth and the Moon falling in its orbit around Earth both follow the same rules. Newton realized that the Moon constantly falls toward Earth, but the Moon also moves forward fast enough to keep falling past Earth. That's how it maintains a constant orbit instead of colliding with Earth. The same laws of physics apply to the Sun, Earth, and the other planets. This may seem obvious to us. But in the 1600s, Newton's laws were revolutionary. They allowed scientists to explain events happening far from our own world.
Astronomers continued to gather more information about the stars. Larger, more powerful telescopes let them see farther and farther into space. One of the objects that had puzzled early astronomers was the Milky Way. What was this faint streak of light painted across the night sky? Galileo was the first astronomer to see individual stars in the Milky Way. In 1750 English astronomer Thomas Wright proposed that the entire Milky Way was actually a broad band of many stars.
Astronomers were also finding small, fuzzy patches of light scattered across the sky. They called these objects nebulae, from the Latin word for "cloud." In 1755 the German philosopher Emmanuel Kant suggested nebulae might be "island universes," or galaxies, made up of many individual stars. The word galaxy comes from the Greek word for "milk." The name we use for these large, spinning clouds of stars comes from the ancient name for our own galaxy.
British astronomer William Herschel studied nebulae through his telescope. He made a careful catalog of more than two thousand nebulae he found across the sky. Herschel confirmed Kant's hypothesis about the composition of these objects in 1785. His powerful telescope showed individual stars in several nebulae. In the late 1700s, Herschel also studied the movement of binary stars—pairs of stars that orbit one another. He discovered that their orbits follow Newton's laws. This proved that Newton's laws were truly universal—they could be applied to objects anywhere in the universe.
By the mid-1800s, astronomers were using photographic film to study the heavens. They kept their camera lenses open for long periods of time. This gathered much more faint light than you could with a simple snapshot. These long exposures displayed many thousands of stars invisible to the naked eye.
--The Big Bang, by Paul Fleischer