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The Dead Guys

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Timeline

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Ancient Astronomy

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Anaxagorus (500-428 B.C.)

• Greek Philosopher

• Claim to fame…Indefinite elements in all substances.

• Banished for saying “the sun is a red hot stone and the moon was like the Earth.”

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Pythagoreans (500 – 350 B.C.)

• Various students over many years.

• Original master was Pythagoras, a mathematician.

• Credited for observing the planet Venus.

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Aristotle (384 - 322 B.C.)

• He proposed three observational proofs that the Earth is a sphere:– Ships disappear slowly over the horizon.– The shape of the Earth's shadow at lunar

eclipse appears circular.– Different stars are visible at northern and

southern latitudes.

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Aristarchus (310-230 B.C.)

• First to propose a heliocentric universe.

• Proposed a much larger universe than what was originally thought.

• Studied various distances of the sun and other planets in relation to each other in space.

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Eratosthenes(276-192B.C.)

• Estimated how large the Earth was (250B.C.).

• The stadium, the size of a sports arena, was a unit of distance used by the Greeks.

• Eratosthenes estimated the circumference of the Earth to be 250,000 stadia. (40,320km)

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Calculating the Size of the Earth

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Hipparchus of Nicea (190-120B.C.)• Hipparchus proposed that the axis around which the heavens

seemed to rotate shifted gradually, though very slowly. • Hipparchus concluded that the intersection marking the equinox

slowly crept forward along the ecliptic, and called that motion "the precession of the equinoxes. " The rate is about one full circle in 26,000 years.

• 100B.C. Hipparchus names constellations

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Ptolemy (100-170A.D.)

• Ptolemaic view– The Earth is round, stationary and very small relative to

the celestial sphere of the sky.– The stars are fixed points of light inside the celestial

sphere.– Day and night result from the rotation of the entire

celestial system around the fixed, nonrotating Earth.– The planets moved on small circular paths, called

epicycles. – The centers of these epicycles move around the earth

on other circular paths, called deferents. – Combination of the planetary motions along the

epicycles and deferents produces the observed wandering of the planets among the stars, including the retrograde motion.

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• Earth centered or geocentric.

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Copernicus (1473-1543)

• First to promote the heliocentric system (motion of the Earth and planets in orbit around the fixed Sun)

• He could easily explain the observed retrograde motion of planets like Mars.– The Sun is the center of the Solar System; the Earth and

planets orbit the Sun in circular orbits.– Day and night are the result of the rotation of the Earth

on its axis.– Mercury and Venus are closer to the Sun than the Earth

is.– Only 3 motions of the Earth are needed:

1. daily rotation on its axis 2. yearly revolution around the Sun 3. wobble or libration of the Earth on its axis, accounting for

precession of the equinoxes

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• Has to do with the planets moving at different speeds and our line of sight.

• Venus moves quicker…closer to the sun.

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Tycho Brahe (1546 - 1601)

• He built his observatory which became the finest observatory in Europe.

• For 23 years Brahe, using instruments, made some of the most accurate drawings of the visible universe.

• 1600- Kepler joins Brahe and serves as assistant

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Brahe

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Brahe’s Observatory

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Johannes Kepler (1571 - 1630)

• Kepler's teacher was one of the earliest astronomers to subscribe to Copernicus's heliocentric theory.

• Takes Brahe’s position and data upon his death

• Develops and publishes 3 Laws

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Kepler’s 3 Laws• The First Law: Planets move in ellipses with the Sun at one focus.

– orbits are not circles– Aristotle's cosmology required circular orbits– sun is not the center

• The Second Law: The radius vector describes equal areas in equal times (law of equal areas)

– orbits are not at constant speed– Aristotle's cosmology required constant speed

• The Third law: Period squared is proportional to distance cubed (the harmonic law or law of periods)

– period is the time required for one orbit– distance refers to the average distance from the sun– planets further from the sun move slower– Aristotle's cosmology required all planets to move at the same speed– the relationship (harmonic ratio) is the same for earth and planets, except the moon– Kr3=p2

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Galileo Galilei (1564 - 1642)

• His most famous invention was the telescope. • He was able to look at:

– the moon and describe its rough surface– Discovered the four satellites of Jupiter– Observed a supernova– Verified the phases of Venus– Saw the trenches on Mars– Saw that the milky way was a collection of stars– Discover sunspots

• His discoveries proved the Copernican system which states that the earth and other planets revolve around

the sun. (Heliocentric)

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Isaac Newton (1642 - 1727)

• Among his achievements are:– The laws of motion that are the basis of mechanics. He therefore

explained why the planets follow Kepler's laws.– He showed that gravity not only makes an apple fall to the ground but

also governs the motions of planets and their satellites, and should apply to any bodies (for example, binary stars).

• Newton formulated three important laws:– An object stays at rest if it is at rest or stays in uniform motion in a

straight line unless it is acted upon by some force. (inertia)– When acted upon by a force, an object changes its motion in an amount

proportional to the force acting upon it, and according to the direction of the force. (f=ma)

– When one object exerts a force upon a second object, the second object exerts an equal and opposite force back on the first. (equal and opposite)