Galileo and the Heliocentric model

Observers in ancient Greece noticed that although the stars seemed to move, they stayed in the same position relative to one another. These patterns of stars, called constellations, kept the same shapes from night to night and from year to year. The Greeks thought that Earth was inside a rotating dome called a celestial sphere. Since the word geo is the Greek word for Earth, an Earth- centered explanation is known as a geocentric system.

In a geocentric system, Earth is at the center of the revolving planets and stars. About A.D. 140, the Greek astronomer Ptolemy further developed the geocentric model. Like the earlier Greeks, Ptolemy thought Earth was at the center of a system of planets and stars. In Ptolemy’s model, however, the planets moved on small circles that moved on bigger circles. Copernicus was able to work out the arrangement of the known planets and how they move around the sun.

A Greek scientist developed the heliocentric system. In a heliocentric system, Earth and the other planets revolve around the sun. In the early 1500s, the Polish astronomer Nicolas Copernicus developed a new model for the motions of the planets. His sun-centered system is also called heliocentric. Helios is Greek for “sun.” Copernicus was about to work out the arrangement of the known planets and how they move around the sun. Later, Galileo used the newly invented telescope to make discoveries that supported the heliocentric model. Copernicus thought that the planets’ orbits were circles. He based his conclusions on observations made by the ancient Greeks.

When Galileo pointed his telescope at Jupiter, the largest planet in our solar system, he made a startling discovery. The planet had four "stars" surrounding it. Within days, Galileo figured out that these "stars" were actually moons in orbit of Jupiter. His discovery challenged common beliefs of his time about the bodies of our solar system. Continuing Galileo's legacy, modern telescopes and space probes observe the wonders of Jupiter's many moons. Galileo turned his gaze toward Venus, the brightest celestial object in the sky - other than the Sun and the Moon. With his observations of the phases of Venus, Galileo was able to figure out that the planet orbits the Sun, not the Earth as was the common belief in his time.

In the late 1500s, Tycho Brahe made more accurate observations of the planets’ orbits. Johannes Kepler analyzed Brahe’s data. Kepler found that the orbit of each planet is an ellipse. An ellipse is an oval shape, which may be elongated or nearly circular. Kepler used the new scientific evidence gathered by Brahe to disprove the long-held belief that the planets moved in perfect circles. Since Galileo’s time, our knowledge of the solar system has increased dramatically. Today we know that the solar system consists of the sun, eight planets and their moons, and several kinds of smaller objects that revolve around the sun.