Early Models of the Universe How we explained those big shiny lights in the sky
The Greek philosopher Aristotle (384 322 BCE) believed that the Earth was the center of our universe, and everything rotated around Earth in perfect circles.
The illusion is almost perfect, and fools almost everyone for thousands of years
The planets were attached to transparent shells, and the outermost shell of fixed stars was unchanging. This is known as the Geocentric model This was the accepted view of our universe for almost 2000 years.
Aristarchus (200 BCE) proposed that the Sun was at the center of the universe, and that the Earth spins around the Sun on its axis, but his ideas are not easily accepted. If the Earth spins on an axis, why don t objects fly off? If the Earth is in motion around the Sun, why doesn t it leave behind clouds, birds and other sky objects, like the moon?
Eratosthenes of Cyene (276 194 B.C.E.) was a mathematician and astronomer in ancient Egypt. He was the first person to measure the diameter of the Earth
The Greek astronomer Ptolemy (87 150 CE) improved on Aristotle s model and solved the big problem of retrograde motion by assigning 3 of the planets EPICYCLES: an additional level of circular motion.
Ptolemy s Epicycles
Aristotle s Universe
Ptolemy s model made our universe a very complicated place to be, and wasn t able to perfectly account for retrograde motion.
Almost 1500 years later, a Polish astronomer, Nicholas Copernicus (1564 1601) proposed a simpler model; the Sun centered or Heliocentric system. His model made more sense, the Sun centered model solved many problems in relation to motion, but he lacked proof. He was so afraid of being punished by the church, he didn t allow his work to be published until after his death.
Galileo Galilei (1564 1642) uses a Dutch made telescope for a detailed study of the heavens. Church officials refuse to look through this instrument.
Galileo sees Sunspots, which goes against the Churches idea of an unchanging heaven. Worse yet, the sunspots move; the Sun rotates. (Why shouldn t the Earth?) Moons of Jupiter, more new things in an unchanging universe. Jupiter seems to rotate without leaving its moons behind; can Earth do the same? Venus has phases like our moon. This can only be explained if the Sun was the center of our universe, not the Earth.
Galileo also saw other things that went against the church s perfect idea of the universe; Planets were discs, not points of light like the stars. Saturn had ears Moon was not a smooth sphere, but had mountains and craters.
Galileo s challenge of the Church s authority lands him in house arrest for the last years of his life, and he is forced to recant his ideas of a Sun centered universe. His dying words.. But it does move
The Danish astronomer Tycho Brahe (1546 1601) is a believer in the Earth centered model, but made some important observations. Kept detailed records of Mars and its path through the sky for over 10 years Recorded a supernova in 1572 (visible for 1 ½ years) representing a change in an unchanging sky In 1577 observes a comet s path, which was not a circular path.
Brahe s German assistant, Johannes Kepler (1571 1630) believes in the Sun centered model, and reasons that planets move in flattened circles, or ELLIPSES.
Kepler developed 3 Laws of Planetary motion to explain our solar system more clearly: 1. Planets move in ellipses 2. Planets move more quickly when they are closer to the Sun. They move more slowly when they are further away. 3. The time it takes a planet to revolve/orbit the Sun is directly related to its distance from the Sun.
Isaac Newton (1642 1727) is considered to be the most influential scientist who ever lived. He developed three laws of motion which explain, once and for all, the Sun centered version of our universe.
He also invented the reflecting telescope a telescope the uses a curved mirror to focus the light to a point. It is now called the Newtonian design.
Newton s Laws are used to predict new planets, first Uranus, and then Neptune.
Textbook Practice Pg. 373 1. What does the term geocentric mean? (and what does heliocentric mean) 2. What are astrolabes used for? 3. Compare and contrast Aristotle s and Copernicus s model of the universe. 4. What piece of evidence led Galileo to believe the universe was heliocentric?
Explain star trails using both the ancient and modern model of the universe.