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Early Astronomy Astronomy is the science studying outer space and the universe. The “Golden Age” of astronomy was centered in Greece.
Early Astronomy The Greeks took measurements of distant objects such as the Sun and the moon. Famous Greek philosopher, Aristotle (384-322 B.C.) Aristotle, concluded Earth is round because it always cast a curved shadow on the moon. Another Greek astronomer, Hipparchus, determined the location of almost 850 stars. Hipparchus (190-120 B.C.)
Geocentric Model The Greeks believed in a geocentric universe, in which the known planets and the Sun revolved around the Earth. The path of an object as it goes around another object in space is called an orbit. Beyond the planets was an invisible sphere on which the stars traveled daily around Earth celestial sphere The Greeks attempted to explain the movements of all celestial bodies in space by using this geocentric model.
Ptolemaic System The Greek astronomer, Ptolemy, presented a geocentric model of the universe called the Ptolemaic System, with fixed stars in the background. Ptolemy (90-168 A.D.) Although Ptolemy’s theory was wrong in that the planets do not orbit Earth, it was able to account for the planets’ apparent motions, which he called epicycles. We concluded the planets’ individual rotation periods from this.
Heliocentric Model The first Greek astronomer to propose a heliocentric universe, in which Earth and the other planets orbit the Sun, was Aristarchus. Aristarchus (312-230 B.C.) Though much evidence was provided to support a heliocentric universe, the Earth-centered (geocentric) view dominated Western thought for 2000 years.
The Birth of Modern Astronomy After Ptolemy, very few advances were made in astronomy. The first great astronomer to emerge after the Middle Ages was a man from Poland named Nicolaus Copernicus. He believed the Earth was a planet, just like the other 5 known at the time, and Copernicus (1473-1543) supported the heliocentric model of the universe.
The Birth of Modern Astronomy After the death of Copernicus came a Danish astronomer, Tycho Brahe. Brahe became interested in astronomy from viewing the astonishing effects of a solar eclipse. Had an observatory built where he designed instruments in order to view and measure locations of celestial bodies. Still believed in a geocentric universe based on his observations of stars. Studied Mars extensively. Gained recognition for being extremely precise on his observations. Brahe (1546-1601)
The Birth of Modern Astronomy Before Brahe died he hired an assistant, Johannes Kepler, who carried on and inherited all of Brahe’s works. Didn’t agree Brahe’s view of a geocentric universe. Kepler (1571-1630) Discovered 3 Laws of Planetary Motion: 1. The path each planet takes around the Sun is oval- shaped and known as an ellipse.
The Birth of Modern Astronomy 2. Each planet revolves so an imaginary line connecting it to the Sun sweeps over equal area in equal time intervals. A planet travels faster when it is closer to the Sun and slower when it is farther from the Sun.
The Birth of Modern Astronomy 3. The square of a planet’s orbital period (the time it takes to orbit the Sun) is proportional to the cube of its average distance to the Sun:The orbital period of revolution (P) ismeasured in Earth years. The planet’sdistance (a) to the sun is expressed inastronomical units (AU’s), which isthe average distance between theEarth and the Sun, approximately 150million km. or 93 million miles.*a is also known as the semi-major axis
The Birth of Modern Astronomy Galileo Galilei was a great Italian scientist during the Renaissance. His most important contributions were his descriptions of the behavior of moving objects. Everything prior to Galileo was studied and examined without a telescope. He constructed his own telescope and used it to study the sky, making many important discoveries supporting Galileo (1564-1642) Copernicus’s view of the universe.