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Chapter 22 Origin of Modern Astronomy
Summary
22.1 Early Astronomy
• Astronomy is the science that studies the universe. It deals with the
properties of objects in space and the laws governing the universe.
In the geocentric model, the moon, sun, and known planets—Mercury,
Venus, Mars, and Jupiter—go around Earth.
• The Greeks believed in a geocentric universe, in which Earth was a
sphere that stayed motionless at the center.
• The path of an object around another object in space is called an orbit.
In the heliocentric model, Earth and other planets orbit the sun.
• Aristarchus (312–230 B.C.) was the first Greek to propose a suncentered, or heliocentric, universe.
• Retrograde motion is the apparent westward motion of the planets
with respect to the stars.
Copernicus concluded that the Earth is a planet. He proposed a model
of the solar system with the sun at the center.
Tycho Brahe’s observations, especially of Mars, were far more precise
than any made previously.
Galileo’s most important contributions were his descriptions of the
behavior of moving objects.
Although others had theorized the existence of a force that keeps the
planets from going in a straight line, Newton was the first to formulate
and test the law of universal gravitation.
• The law of universal gravitation states that every body in the universe
attracts every other body with gravitational force. The greater the
mass of the object, the greater the gravitational force.
22.2 The Earth-Moon-Sun System
The two main motions of Earth are rotation and revolution.
• Rotation is the turning, or spinning, of a body on its axis.
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Kepler discovered three laws of planetary motion.
• The oval-shaped path created by the orbit of the planets around the
sun is called an ellipse.
• The astronomical unit (AU) is the average distance between Earth
and the sun. It is about 150 million kilometers.