Galileo Galilei

Galileo Galilei (1564–1642) was an early scientist from Italy who made careful studies of motion and the sky. He is known for using one of the first telescopes to look at planets and moons. A telescope is a tool that makes faraway objects appear larger and clearer. His observations changed how people understood the Solar System, which is the group of planets and moons that travel around the Sun.

Galileo was born in the city of Pisa. As a young man, he studied mathematics, which is the study of numbers, shapes, and patterns. He later taught at the University of Padua. During this time he began to test ideas about falling objects. He showed that objects fall at the same rate when there is no air resistance. Air resistance is the force that slows objects as they move through air. These tests helped describe the basic motion of objects.

In 1609 Galileo heard about a new device from the Netherlands that used glass lenses to make images appear closer. He built his own versions and improved them. When he pointed his telescope at the Moon, he saw mountains and valleys, which showed that the Moon had a rough surface. He also saw many stars that could not be seen with the eye alone. These early observations helped show that space contained far more stars than people once thought.

In January 1610 Galileo pointed his telescope at Jupiter. He saw four small objects moving around the planet. These were the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. Today they are called the Galilean moons. A moon is a natural satellite, which means it is a body that orbits, or goes around, a planet. His discovery showed that not all bodies in the sky move around Earth.

Galileo also studied the phases of Venus. A phase is the shape of the lit part of a planet or Moon as seen from Earth. He saw that Venus showed a full set of phases, similar to the Moon. This supported the idea that Venus moved around the Sun. At that time, some people believed that Earth was the center of all motion in the sky. Galileo's observations provided evidence that planets moved around the Sun instead.

Late in his life, Galileo continued to study motion and astronomy. Astronomy is the study of objects in space. He wrote books that explained his observations and described how he used experiments to test ideas. Although he faced conflict with leaders of his time for his support of a Sun-centered model, Galileo kept making careful observations and sharing his results. His work became an important part of early scientific study.