Mav Mark 11/3/11 When electricity leaves a power plant is it AC or DC, and is it at high voltage or low voltage?

Observing the Solar System Notes

Earth at the Center Most early Greek astronomers believed that Earth was the center of the universe. They also noticed that a few “stars” in the sky seemed to follow paths separate from the others. The Greeks called them planets from their word for “wanderers.” The Romans would later label these planets Mercury, Venus, Mars, Jupiter, and Saturn after their own gods.

Earth at the Center A model of the universe in which Earth is at the center is called a geocentric system. In a geocentric system, all planets and stars revolve around Earth.

Earth at the Center The Greek astronomer Ptolemy developed a complex geocentric model of the universe around the year 140 CE. Ptolemy’s model seemed to explain motions in the sky. Because the planets occasionally appear to go backwards in the sky, Ptolemy had to come up with some kind of solution for this motion, so he claimed that the planets, in their orbits, would make small circles called epicycles. Ptolemy’s theories fit so well all of the observations of the planets and stars, that his model of the universe would last for nearly 1,500 years, until nearly the end of the Italian Renaissance.

Sun at the Center A system in which the sun is at the center is called a heliocentric system. Earth and other planets revolve around the sun in a heliocentric system. In 1543, a Polish astronomer named Nicolaus Copernicus developed a good heliocentric model of the universe, but still contained a few flaws. Despite the fact that he was mostly correct, his theories would still be rejected for over 50 years.

Sun at the Center In the 1600s, the Italian scientist Galileo Galilei made discovers that supported the heliocentric model. For example, using his telescope, Galileo discovered the four largest moons of Jupiter (known now as the Galilean Moons), proving that not everything orbited Earth. His observations of a full set of phases for Venus also supported a heliocentric view of the universe. Galileo too was ridiculed for his theories. He was even put under house arrest by Pope Urban the VIII for “vehement suspicion of heresy,” where he would eventually die.

Sun at the Center The most accurate measurements of the stars and planets was made by a Danish astronomer named Tycho Brahe. Unfortunately, Brahe was a believer of Ptolemy’s geocentric vision of the universe. At the same time, though, he had to concede that some observations fit a heliocentric view. In a way to keep both versions, he created a new model of the universe. His model came to be known as a geo- heliocentric model, or the Tychonic model.

Sun at the Center Brahe’s greatest contribution, beside his very precise observations, was actually his apprentice, Johannes Kepler. Brahe knew that Kepler was smarter, so he hid some research from Kepler, and presented him with minor tasks, such as figuring out Mars’ orbit, and only gave Kepler part of his research. This would actually allow Kepler to make the greatest discovery of his life.

Sun at the Center After years of study, and Brahe’s eventual death, Kepler had finally reached a very controversial conclusion. The orbit of Mars was not a perfect circle. Kepler discovered that not only Mars, but all of the planets travel in a path in the shape of a flattened circle, or an ellipse.

Sun at the Center Kepler used his own calculations, as well as Brahe’s observations to create his Three Laws of Planetary Motion: 1. The orbit of every planet is an ellipse with the Sun at one of the two foci.orbitplanetellipsefoci 2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.lineareas 3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.squareorbital periodproportionalcube semi-major axis

Kepler’s Three Laws of Planetary Motion lers_Laws_fullscreen.mov

Modern Discoveries Early astronomers only knew of six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Uranus was eventually discovered by William Herschel in 1781, but he originally though it to be a comet. "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it". Due to it’s nearly circular orbit, it was later named a planet, Uranus, after the Greek sky god Ouranos.

Modern Discoveries Neptune was discovered in 1846, but was actually discovered by mathematical prediction rather than by direct observation. Oddities in the orbit of Uranus led to the prediction of a large gravitationally massive object outside of Uranus’ orbit.

Modern Discoveries While Herschel was looking in the sky with his telescope for Uranus, he noticed fuzzy, white patches that he could not identify. Despite the fact that he didn’t know what they were, he noted their positions anyway. In 1923, with the invention of photography, an astronomer named Edwin Hubble studied these “fuzzy patches.” Edwin concluded that they were entire galaxies of stars, each containing their own hundreds of billions of stars. Before Hubble, we really thought that we were the only galaxy out there.

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