# The Solar System Chapter 24 Section 1.

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The Solar System Chapter 24 Section 1

Earth Centered Models (Geocentric)
Various models of the solar system with Earth at the center. Most commonly, the moon, the sun, the five planets (Mercury, Venus, Mars, Jupiter, and Saturn), and the stars were fixed in separate spheres that rotated around the earth. Didn’t explain retrograde motion.

Earth Centered Models (Geocentric)
Claudius Ptolemy Greek astronomer who lived 500 years after Aristotle (384 to 322 B.C.) Proposed that all planets move in a series of small circles called epicycles as they revolve around the Earth. The motion of the planet in its epicycle would make it appear to move backwards.

Sun Centered Model (Heliocentric)
Published by Nicolaus Copernicus (a Polish astronomer) in 1543. Stated that the moon revolved around the earth, but the earth and all the planets revolved around the sun. Each planet revolved at a different speed and a different distance from the sun. Also stated that the daily movement of the planets and the stars was due to the rotation of the earth.

Support for Sun Centered Model
Galileo Galiliei Italian scientist in the 1600’s. Observed the motions of the planets with the newly invented telescope. Collected evidence such as the phased of Venus which supported the heliocentric model of the solar system.

Mathematical Support of the Sun Centered Model
Johannes Kepler ( ) Developed three laws to describe the motion of planets: Law of Ellipses Law of Equal Areas Law of Periods

Law of Ellipses All planets orbit the sun in an elliptical path.

Law of Equal Areas Describes the speeds at which planets travel around the sun. Planets always travel through equal areas in equal periods of time.

Law of Periods Describes the relationship between the average distance of a planet from the sun and the orbital period of the planet. The orbit period is the time required for the planet to make one revolution around the sun.

Modern View of the Solar System
Eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune All planets along with many smaller objects orbit the sun. The sun contains 99.86% of all mass contained within the solar system.

Formation of the Solar System
Nebular Theory Proposed in 1796 by the French mathematician Marquis Pierre Simon de Laplace. Stated that the sun and planets condensed out of the same spinning nebula of gas, ice and dust at approximately the same time (4.6 billion years ago).

Formation of the Solar System
Formation of a slowly spinning cloud of material. Contraction of the cloud begins (possibly caused by something similar to the explosion of a nearby star). As the cloud contracts, more material is pulled into the center of the cloud causing it to rotate quicker. Higher density objects exert larger gravitational forces. Eventually the cloud forms a flattened disk with a dense area in the center.

Formation of the Solar System
Temperatures in the cloud increase until the core of the cloud is 10 million degrees Celsius and nuclear fusion begins (this is the birth of the sun). Nuclear fusion occurs when lighter mass atoms are fused into heavier mass elements (such as hydrogen into helium). A slight mass is converted into energy in this process.

Formation of the Solar System
Matter that is not pulled into the center of the cloud begin to form clumps (planetisimals), with each clump getting larger as it collides with other clumps (forming protoplanets and eventually planets). Close to the sun, temperatures are so high that lighter elements such as hydrogen, helium, methane, and ammonia were vaporized, causing the inner planets to be terrestrial and the outer planets to be gaseous.