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Giants of Science• Our understanding of the Universe developed slowly over

centuries.

• Most of the breakthroughs came through careful study of the positions and motions of the Sun, Moon, planets and stars.

• The careful analysis of these large sets of measurements and the use of mathematics led to the creation of physical models that were used to make predictions on the future position of planets, Sun, Moon, etc.

• When these models failed to explain observed phenomena they were abandoned or modified. (eventually!)

• The following is a short summary of several notable philosopher/scientists and their contributions.

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Aristotle (c. 384 - 322 B.C.)• Taught that the

Earth was the unmoving center of the Universe.

• Taught that the Earth was spherical– Lunar eclipses due

to Earth’s shadow are always round.

– Different stars are visible the farther South you go.Two examples proving the Earth is a sphere.

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Aristarchus (c. 310 - 230 B.C.) • Used geometry to measure

the size and distance of the Moon and the Sun. – Moon results were good.– Sun results were bad.

• Did show that the Sun was bigger than Earth and much farther than Moon.

• Taught that the Earth went around the Sun. (Not much proof and not widely accepted).

comparing the size of the Earth’sshadow to the size of the Moon

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Eratosthenes (c. 276 - 196 B.C.)• Used shadows and geometry to measure

the size of the Earth!

• On Summer Solstice Sun was directly overhead in Syene, Egypt.(no shadows at noon)

• On the same day there were shadows in Alexandria, Egypt.

• Using shadow length and distance between Syene and Alexandria and geometry found Earth circumference.

• His value probably around 42,000 kilometers.

• Actual value about 40,000 km.

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Ptolemy (100 - 170 A.D.)• Last of the great Greek

astronomers• Used discoveries and ideas of

others plus his own observations to develop a model of the Universe.

• His work survived from Arabic translation of Almagest (the greatest compilation)

• His model of the Universe was undisputed for almost 1,400 years!

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Ptolemy’s Model of the Universe• A geocentric (Earth-

centric) model

• All the stars, planets, Sun and Moon moved around the Earth

• Motions were perfectly circular and perfectly uniform

• Fastest objects were closest to Earth.

• His model also had to explain Retrograde Motion

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Retrograde Motion

• It was well known that the planets occasionally appear to stop and change direction in orbit, but why?

• It is an illusion based on the motion of the Earth and the planet.

• Ptolemy explained this effect without moving Earth. He used epicycles instead.

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Epicycles• These are small circles to

which the planets are fixed.

• These epicycles are attached to the larger circles which move around Earth.

• Ptolemy adjusted the sizes of the circles and their rate of motion to replicate the motion of the planets.

• Maintained perfect uniform, circular motion.

The large circle is called a deferentthe smaller circle is an epicycle.

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Ptolemy’s Model

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Nicholas Copernicus (1473 - 1543)• Recognized that the predicted

planetary positions using Ptolemy’s model was not very accurate.

• Tried to fix Ptolemy’s model but eventually abandoned it.

• Returned to the idea of Aristarchus 1800 years before.

• Created a heliocentric (Sun-centric) model of the Universe.– Retrograde motion easier to explain.

– Distance of the planets from the Sun could be determined using geometryFinding the distance of the planets

from the Sun using Copernicus’heliocentric model.

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Problems with Copernicus’ Model

• Copernicus maintained that all planets move in perfectly circular orbits at constant rates.

• Didn’t really do much better than Ptolemy at predicting planetary positions. He actually needed to keep epicycles to make the model work out right.

• Several objections– We don’t feel the Earth moving. Went against “common-

sense”. Went against philosophical (Aristotle) and religious beliefs.

– Why don’t we see stellar parallax?

– Not widely accepted at first

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Stellar Parallax• Parallax - the shift in an

object’s position caused by the observer’s motion.

• Since the Earth moves we should see parallax but we don’t.

• No one realized that stars are very far away.

• Only with powerful telescopes can they be observed. Took 300 years to see first parallax.