Models of the Solar System

Earliest Astronomers (Before 400 BC) ► Early civilizations (e.g., Maya, Babylonians) observed the heavens for religious and political reasons, but mostly for farming. ► They made very accurate observations and predictions, but had no rational explanations for what they saw.

Aristotle ( B.C.) ► Aristotle’s model was influenced by his ideas of motion: things on Earth can change and tend to fall towards the center, while things in the heavens are eternal and never change their motion.

► Aristotle ’ s model was geocentric; the earth is in the middle of the universe, and the sun, moon, planets and stars revolve around it. ► Everything in the heavens is perfect and unchanging. ► The celestial bodies are attached to transparent crystal spheres, one inside the other, which rotate around the earth.

Aristarchus of Samos (ca. 275 B.C.) ► Aristarchus believed in a heliocentric model: the sun is in the middle of the solar system, and the planets, including earth, revolve round it. Only the moon revolves around the earth, and is closer than the sun, which is immensely far away. The stars are even farther away than the sun.

► Unfortunately, almost no one believed Aristarchus; they went for Aristotle ’ s model instead. ► Aristarchus ’ s model had two major weaknesses: 1. The earth did not appear to move 2. Parallax of the stars could not be observed ► The geocentric model was taught as truth for the next 1700 years.

Ptolemy (ca. 125 A.D.) ► Ptolemy ’ s model was geocentric like Aristotle ’ s, but had some improvements that fixed some things about Aristotle ’ s model that did not agree with actual observations: 1. The planets seemed to slow down and speed up as they passed through different parts of the zodiac (variable periods) 2. Some planets, notably Mars, appeared to actually go backwards at times (retrograde motion)

► Ptolemy solved these problems by adding deferents and epicycles to the orbits of some of the planets to help account for the odd motions that did not agree with the geocentric theory.

► During the Middle Ages and early Renaissance in Europe, the Church eagerly accepted Ptolemy ’ s model, because it matched their science, but more importantly because it placed humans in the center of an orderly universe created by a rational, compassionate God.

Nicolaus Copernicus ( ) ► Copernicus was a priest who was given the job of making a map of the motion of the sky to help out with a new calendar. ► He rediscovered the heliocentric model around 1515, when he decided it just made more sense logically, and matched his observations better. ► He was uneasy about his discovery, though, and did not publish his book until just before his death. ► Copernicus’s model does not explain the orbits or the motions of the planets, it just describes them; it is a purely geometric model.

Galileo ( ) ► Galileo was the first astronomer with a telescope, and a strong supporter of Copernicus ’ s model. ► He made several important observations that supported the heliocentric model: 1. The moon has mountains and valleys 2. Venus and Mercury have phases just like the moon, which can only be explained if they orbit the sun and not the earth 3. He discovered four moons of Jupiter 4. He discovered the Milky Way, which suggested that the universe is much larger than people thought.

► Galileo’s loud (and arrogant) support for the heliocentric model were not welcomed by some in the Church; in 1633, he was accused of heresy, his works were condemned, and he was placed under house arrest for the rest of his life. His life was only spared because he agreed to keep quiet.

Johannes Kepler ( ) ► Kepler improved the heliocentric model by developing laws of planetary motion: 1. Law of elipses: the orbits of the planets are elliptical, not perfectly round 2. Law of equal areas: planets sweep out equal areas in equal times. 3. Harmonic Law: The square of a planet ’ s orbital period is equal to the cube of its average distance from the sun. (translation: planets close to the sun go faster than ones that are far away because gravity is weaker far away)