Galileo and
the Moons of Jupiter

By Tony Helies

	In 1989, NASA named a new spacecraft after Galileo and sent it toward Jupiter. Spacecraft Galileo (shown at left) is still sending back information about Jupiter (in the background) and its moons. Shown to the left of the spacecraft is one of those moons, called Io, which has many active volcanoes.

Four hundred years ago, most people believed that Earth was the center of the universe. They thought that everything in the heavens, including the planets and the Sun, orbited our world.

An astronomer named Nicolaus Copernicus had written a book with a startling new theory: All planets, including Earth, orbit the Sun. Was Copernicus right or wrong?

In 1609, the Italian scientist and mathematician Galileo Galilei heard about a new device invented in Holland. Galileo thought it could help answer this question. The new instrument let the eye see distant objects as if they were nearby. It was the first telescope.

The early telescopes were made with two pieces of curved glass, called lenses, set into a metal tube. Telescope lenses are like the lenses in eyeglasses. Both types of lenses work on the same principle: Curved glass bends light. Telescope lenses are designed so this bending magnifies objects, making them look closer.

Galileo was excited by the idea of using this new invention to look at the heavens. But these early telescopes were not powerful enough for astronomy. So Galileo set about building a better one. He worked for six months, experimenting with different designs. His hard work paid off. The telescope he built made objects look thirty times closer than they did with the eye alone.

On January 7, 1610, Galileo pointed his telescope toward one of the brightest objects in the night sky, the planet Jupiter. On either side of Jupiter, he saw little points of light arranged in a straight line, two to the left and one to the right:

Galileo believed these were fixed stars, and expected they would remain in place as Jupiter moved across the sky.

The next night, he was surprised to find that the three stars were all to the right of Jupiter:

Galileo was puzzled. He had expected to find the stars farther to the left of Jupiter. What had caused them to move to the right? He waited impatiently to see where these stars would be on the third night, but cloudy skies blocked his view.
The next day, clear skies returned. Galileo was astonished by what he saw that night. Two of the stars had moved back to Jupiter’s left and one had disappeared:

Galileo suspected that the third object was behind Jupiter.

Over the next two nights, the three stars continued to move from one side of Jupiter to the other. Galileo now realized that the three points of light he was observing were orbiting Jupiter. They were moons, not stars.

Galileo had proved that not all heavenly bodies orbit Earth. The Earth was not at the center of everything after all. This was the first evidence supporting Copernicus’s theory. It helped create our modern view of the universe.

On January 13, Galileo saw a fourth moon, which had been hidden behind Jupiter. These four moons—Io (EYE-oh), Europa (your-OH-pah), Ganymede (GAN-ih-meed), and Callisto (kuh-LIH-stoh)—are today known as the Galilean moons, in his honor.