In 1610, when Galileo first pointed his newly built telescope toward the sky, no one had ever had the capability to see distant objects in any detail, and he had no idea what to expect. Focusing on the Moon first, he discovered that its surface was not smooth as people had previously thought, but covered with craters and mountains not unlike those on Earth.

Next, Galileo turned his attention to another bright object in the night sky — the planet Jupiter. Today, we know quite a bit about Jupiter. With a diameter about 11 times greater than Earth's, Jupiter is the largest planet around our Sun. It is so enormous that its mass is more than twice the combined mass of all the other planets in our solar system. Composed mostly of gaseous hydrogen and helium, Jupiter is a gas giant. It has a trademark banded look that results from its turbulent atmosphere, and its Great Red Spot is a giant storm that has been raging for centuries. Jupiter also has a faint ring system and, as of May 2005, 63 known moons surround it.

In a very short time, Galileo discovered Jupiter's four largest moons — Io, Europa, Ganymede, and Callisto, now called the Galilean moons. Each moon has unique characteristics that make it stand out in the solar system. For example, Io is the most volcanically active object in the solar system. Europa may have an ocean of liquid water beneath its icy surface and may be the best candidate for another habitable world. Callisto is one of the most heavily cratered bodies in the solar system, and Ganymede, which is even larger than the planet Mercury, is the largest moon.

The four Galilean moons were the first celestial bodies to be discovered with a telescope. Although Galileo at first thought they were fixed stars, his continued observations showed that their positions changed on a nightly basis. Galileo soon recognized that these new objects were satellites of Jupiter. The discovery that celestial objects could revolve around something other than Earth provided strong evidence in support of the heliocentric theory.

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