ONA WARM June evening in 1609, Galileo Galilei, a 45-year-old Italian mathematics teacher and father of three, listened as a friend described the latest invention to hit Europe: a long tube containing two glass lenses, called a spyglass.

“It makes faraway things appear close! We could use one of those here in Italy,” the friend sai “Perhaps you could try to build one, if you’re interested.”

Was he ever! Galile loved trying to figure o how the world and the things in it worked. That very night he leaped into the project with gusto. In a short time, he not only figured out how to construct a spyglass, he improved upon the existing model as well.

When he later presented his device to the rulers of Venice, they marveled at how far it could see. The spyglass would be quite useful for spotting distant enemies, making it hard for anyone to launch a sneak attack!

Galileo wasn’t satisfied, however, and he kept fiddling with his “telescope,” as his fellow mathematician friend Giovanni Demisiani had called it (the word means “far-looker.”) He tried new adjustments and worked at grinding better lenses. Within a few months, he had a telescope that was three times more powerful than his first.

By now, autumn had arrived, and evenings darkened early. One night as the moon rose, Galileo pointed his telescope toward the sky. If it could see far on Earth, why not into the heavens as well? Who knew what the moon might look like close up?

What Galileo saw astonished him. The moon was not perfectly smooth, as it appeared to the naked eye. It was a bumpy moon. Its edges had “ridges of darkness” and “pips of lights,” and it was covered with what looked like craters, mountains, and valleys. Entranced by these unexpected details, Galileo drew picture after picture of what he saw through the telescope.

A craggy moon wasn’t all that he discovered. Galileo could now see that the Milky Way, seemingly a bright cloud across the sky, was in fact made of millions of stars.

He also noticed that while fixed stars of constellations looked like twinkling lights, the “wandering stars,” or planets, seemed to be solid spheres—like the moon.

All through that December, Galileo peered into the night sky at what had never been seen before. Sometimes his hands shook with the cold. The chilled lens of his telescope fogged up whenever he put his eye near the glass, and he had to keep wiping it clear.

On January 7, 1610, Galileo focused his gaze on Jupiter. He noticed three bright stars beside it and drew a sketch of them. The next night he looked again. The three stars had scooted to new positions! How odd, he thought. The following night, Galileo saw only two stars. What was going on? A few nights later he saw four. Galileo kept watching and recording his observations until he concluded that these dots were neither stars nor planets— they were little moons, circling Jupiter!

Galileo wrote down his observations and thoughts in a book called The Starry Messenger. The book immediately sold out. People were very excited—and troubled—by his discoveries.

Little moons circling not Earth, but Jupiter? Our own moon a bumpy one? Sightings like these could change how people thought about the universe. Some skeptics chose not to look through the new telescopes; others refused to believe what they saw. They insisted that Galileo put specks in his telescopes to trick people.

Feverishly, Galileo built more telescopes, hoping to make the truth visible to everyone. But what kind of truth was this? Could you really trust a telescope? And what did it all mean?

These early telescopes weren’t perfect, but what they showed was true enough: the heavens were not “flawless,” nor did they revolve around the earth, as many believed. Instead, it looked like the many planets, including Earth, revolved around the sun.

Those who lived in the 1600s were not quite ready for this scientific breakthrough. At the time, many people passionately believed Earth should be at the center of the universe. Galileo was ordered to stop writing about his observations and was imprisoned in his house. He was put on trial, and his books were banned for two hundred years.

However, people had already begun to see what they had not been able to see before, and they started to ask questions. While some refused to let go of their old ideas, new and marvelous mysteries and discoveries awaited those who did.

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