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UNIT NINE: Matter and Motion in the Universe. Chapter 26 The Solar System Chapter 27 Stars Chapter 28 Exploring the Universe. Chapter Twenty-Six: The Solar System. 26.1 Motion and the Solar System 26.2 Motion and Astronomical Cycles 26.3 Objects in the Solar System.
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UNIT NINE: Matter and Motionin the Universe • Chapter 26 The Solar System • Chapter 27 Stars • Chapter 28 Exploring the Universe
Chapter Twenty-Six: The Solar System • 26.1 Motion and the Solar System • 26.2 Motion and Astronomical Cycles • 26.3 Objects in the Solar System
Section 26.1 Learning Goals • Explain the significance of gravity in maintaining the solar system. • Distinguish between Sun-centered and Earth-centered models of the solar system. • Explain the current model of the solar system.
Investigation 26A Phases of the Moon • Key Question: What causes the lunar cycle?
26.1 Motion and the solar system • Ancient astronomers used a landmark, such as a building or tree, to mark the point where the Sun rose or set each day. • The position of the sunset and sunrise changes over time.
26.1 Observing patterns in the sky • The Moon appears to change its shape and the time and position at which it rises and sets.
26.1 Observing patterns in the sky • The rising and setting positions of the stars do not appear to change along the horizon over short periods of time. • However, the timethat stars rise or set each night gradually changes during a year.
26.1 Observing patterns in the sky • A constellation is a group of stars that, when seen from Earth, form a pattern.
26.1 The Earth-centered model • 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.
26.1 The Earth-centered model • In 140 AD, the Greek astronomer Ptolemy developed a model that explained the apparent path of the planets. • He hypothesized that each planet moved on a circle, which, in turn, moved on a larger circle around Earth.
26.1 The sun-centered model • While the Ptolemaic model could predict the positions of the planets, Nicholas Copernicus found that its predictions became less and less accurate over the centuries. • In Copernicus’ model, the Sunwas at the center of the solar system and the planets orbited in circles around the Sun.
26.1 The sun-centered model • The phases of Venus, discovered by Galileo in the 1600s, were part of the evidence that eventually overturned Ptolemy’s model. • Using a telescope he built himself, Galileo made two discoveries.
26.1 The sun-centered model • First, he argued that the phases of Venus could not be explained if Earth were at the center of the planets. • Second, Galileo saw that there were four moons orbiting Jupiter.
26.1 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.
26.1 Gravitational force • Gravitational force is the force of attraction between all objects. • All objects that have mass attract each other.
26.1 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.
26.1 Orbits • In 1600, German mathematician Johannes Kepler determined that the orbits of the planets were not perfect circles but slightly elliptical.
26.1 Orbits • Isaac Newton explained that an orbit results from the balance between inertia (the forward motion of an object in space), and gravitational. • Without the pull of gravity, a planet would travel off into space in a straight line.
26.1 Current model of 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.
26.1 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.