Early Astronomy 22.1

Astronomy • The scientific study of the universe; it includes

the observation and interpretation of celestial bodies and phenomena.

Geocentric

• Describes the concept of an Earth—centered universe.

Heliocentric

• Describes the view that the sun is at the center of the solar system.

Retrograde Motion • The apparent westward motion of the planets

with respect to the stars.

Ellipse

• An oval.

Astronomical Unit (AU)

• Average distance from Earth to the sun; 1.5x10⁸ Kilometers.

Key Concept

• In the geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter– orbit Earth.

Key Concept

• In the heliocentric model, Earth and the other planets orbit the sun.

Key Concept

• Copernicus concluded that Earth is a planet. He proposed a model of the solar system with the sun at the center.

Key Concept

• Brahe’s observations, especially of Mars, were far more precise than any made previously.

Key Concept

• Kepler discovered three laws of planetary motion.

Key Concept

• Galileo’s most important contributions were his descriptions of the behavior of moving objects.

Key Concept

• Although others had theorized the existence of such a force, Newton was the first to formulate and test the law of universal gravitation.