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Astronomy Chapter Seventeen: The Solar System • 17.1 About the Solar System • 17.2 The Planets • 17.3 Other Solar System Objects Investigation 17A Non-Renewable Resources • How fast are we using non-renewable resources? 17.1 About the solar system • Ancient observers noticed that five bright objects seemed to wander among the stars at night. • They called these objects planets, from the Greek word meaning “wandering star,” and named them Mercury, Venus, Mars, Jupiter, and Saturn. 17.1 About the solar system • Galileo made two discoveries. • Galileo argued that the phases of Venus could not be explained if Earth were at the center of the planets. 17.1 About the solar system • Second, Galileo saw that there were four moons orbiting Jupiter. • Galileo’s discoveries helped prove that Earth and the other planets orbit the sun, disproving the early theory that earth was the center of the universe. 17.1 What is the solar system? • Today, we define the solar system as the sun and all objects that are gravitationally bound to the sun. • The solar system is roughly divided into the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune) • The dwarf planet Pluto is the oldest known member of a smaller group of frozen worlds orbiting beyond Neptune. 17.1 Gravitational force • Newton’s law of universal gravitation explains how the strength of the force depends on the mass of the objects and the distance between them. 17.1 Orbits • • An orbit is a regular, repeating path that an object in space follows around another object. An object in orbit is called a satellite. 17.1 Orbits • • An orbit results from the balance between inertia and gravitational force. Without the pull of gravity, a planet would travel off into space in a straight line. 17.1 Motion of the planets • • The orbits of the planets are slightly elliptical but almost circular. The Sun is at a point called the focus that is offset from the center of the orbit. 17.1 Motion of the planets • • In addition to orbiting the Sun, the planets also rotate. An axis is the imaginary line that passes through the center of a planet from pole to pole. 17.1 Comparing size and distance • The Sun is by far the largest object in the solar system. • One astronomical unit (AU) is equal to 150 million km, or the distance from Earth to the Sun.