History of Astronomy Historical People and Theories
Plato Believed he could solve everything through reasoning. Circles and Spheres are good because they are perfect (never ending) and pleasing to the eye. Earth was the center of everything.
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Aristotle (384-322 B.C.) Student of Plato 2 main concepts: Earth was the center of the Universe. Rest of the Universe was locked on concentric spheres around the Earth.
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What Aristotle Had Correct Correctly explained the phases of the moon Correctly explained the Sun is farther than the moon Knew the Earth was spherical because 1. Earth s shadow is curved during a lunar eclipse
Earth s shadow is curved during a lunar eclipse
Things Aristotle Did Wrong Believed the Earth was stationary and did not rotate because: When you throw something up it comes straight back down and He could not see the stars change position (parallax).
Aristarchus (250 B.C.) He came up with a relative distance between the moon and the Sun. Measuring the moon or the Sun angle, he could estimate how much further away the Sun is than the moon. Concluded the Sun was the center of the Universe.
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The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Eratosthenes (276-195 B.C.) Calculated the circumference of the earth fairly accurately.
Hipparchus Charted positions of the stars Introduced precession of the axis of Earth Invented the Magnitude system for the brightness of stars (1 st magnitude=brightest stars and 6 th magnitude=barely visible stars) Introduced the epicycle to explain retrograde motion of planets.
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Ptolemy (200 A.D.) Followed previous geocentric models of the Universe meaning the Earth is stationary at the center. Did not satisfy the motions of other planets. Changed his model to include epicycles and moved the Earth from the center which did explain motions better but took away uniform circular motion.
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Copernicus Sun centered solar system Circular motion with epicycles Argued: Sun orbiting the Earth is the same as the Earth orbiting the Sun Rotation of the Celestial Sphere can be explained by the Earth s rotation (No longer believed Earth to be stationary) Natural Explanation of Retrograde motion
Copernicus Continued Further away from Sun, the slower planets go. Sidereal Period =the length of time required for a planet to complete one orbit around the Sun. Superior Planet = A planet that has a larger orbit than the Earth. Inferior Planet = A planet that orbits inside Earth s orbit. The distance from the Earth to the Sun is defined as 1 Astronomical Unit (AU).
Copernicus Continued He had a lot of things right However he did not publish his findings until he was near death.
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Tycho Brahe He was an observer and charted the most accurate positions of stars and planets throughout their orbits. Measured the year to within 1 second. Earth was stationary, no stellar parallax. (eye/thumb test) Believed the Sun orbits the Earth and all other planets orbit the Sun. Supposed to have died at a party because of ruptured bladder.
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Johannes Kepler Associate of Tycho Brahe Made sense of his mentor s work after he died. Kepler s 3 Laws of Planetary Motion
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Law # 1: Planets orbits are ellipses with the Sun at 1 focus.
Kepler s 2 nd Law a planet moves so that an imaginary line connecting the planet to the Sun sweeps out equal areas in equal intervals of time.
Kepler s 3 rd Law The squares of the sidereal periods of the planets (P) are proportional to the cubes of the semimajor axes of their orbits (a) Draw on board If periods are measured in years and semimajor axes are measured in AU s then: P^2 = a^3 Kepler described how things work and Newton comes later and explains why Kepler is right.
Galileo Galilee Father of Scientific Method Builds a telescope and views craters on the moon, moons of Jupiter (Galilean Satellites), phases of Venus (proving Venus orbits the Sun), sunspots. Father of Calculus: invented it at 23 years old Invented Physics in 2 years Did not speak of his beliefs for years because it went against the biblical doctrine. When he finally did, was charged with heresy. Denied Copernicus theories to save his skin. House arrest for 9 years.
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Conclusions Heliocentric (Sun centered) model did prevail but not because of proof. Proof came 200 years later. H. Model was simpler to explain the motions of the planets with incredible accuracy.