The Early History of Astronomy and Cosmology

Szydagis02.02.2015

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Flammarion engraving (1888)

Pondering an Ultimate Question• Originally domain of religion and philosophy alone, followed by

science and religion mixing, then slowly separating• A common theme of order from chaos seen in ancient times to

explain the origins of the universe around us• Babylonian creation myth: goddess of chaos killed by other gods and

split in half, to create the heavens and the Earth• Question of what happened in the beginning is one thing, but the

ancients also considered the current setup• India: animals support half-dome Earth. Egypt: mountains prop up sky

• Many ancient cultures studied the motions of the stars and the planets and came up with very accurate calculations and tables, allowing them to predict exact dates of eclipses, sunrise and sunset, moonrise and moonset, the rising and setting of particular stars and constellations, locations of planets within constellations, conjunctions -- without even having the correct model• Astronomy also practical: navigation, timekeeping, agriculture

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The Aristotelian Universe• Greeks get the credit as the first cosmologists in the West• Aristotle, fourth century B.C., basing off work of Eudoxus• Prime mover (creator) keeps the static heavens (made of

“aether,” the fifth element) in perfect rotation for all eternity. No “space” as we know it outside of them (meaningless)

• 8 solid crystal nested spheres (“most perfect” shape) for Sun, Moon, 5 planets (Mercury through Mars), and fixed stars, around Earth with different axes, rotation directions, speeds

• Heavens (non-empty) domain of unchanging perfection, while “base” Earth (made of earth, air, fire, water) subject to decay

• Circular motion in the sky, while straight or random on Earth. Natural state rest here, natural state movement up there.

• Earth itself neither rotates, nor revolves around anything, and is the natural place of earth-like particles; heaven adobe of aether

• Some modified version of this picture of the cosmos (sadly) held sway for over a millennium and a half in Europe

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The Ptolemaic Universe• Claudius Ptolemy, second century A.D. Greco-Egyptian• Correction upon Aristotle’s cosmological model• Still geocentric (Earth = the center of the universe)• Addressed anomalies like planet course changes, tilted paths,

observed changes in brightness with extra circles (epicycles)• Earth already known to be round (since ~ 500 B.C. thanks to

Pythagoras, but we don’t know for sure)

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Activity• Explain every single proof that you can think of that the

earth is round (specifically spherical) instead of flat• Either ancient or more modern

• ALSO: come up with a way to estimate its size. What are the possible sources of (systematic) error for your technique?

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• Some good answers• Shadows and sticks in different cities (next slide)• View from high-up places, or airplanes: can see a lot• View from outer space (rockets, probes, satellites)• Appearance of ships on the horizon on water• Round shadow of the Earth on the Moon during a lunar eclipse• Changing constellations as you cross the equator• Other planets (inside and outside of our solar system) ~spheres• Existence of different time zones• Center of gravity: consistent weight

The Earth’s Circumference• Worked out in between the times of Aristotle and Ptolemy

by jack-of-all-trades Eratosthenes (third century B.C.)• https://www.youtube.com/watch?v=G8cbIWMv0rI (excerpt

from the original Cosmos, with famed astronomer Carl Sagan)• Correction: we know today not sphere but spheroid (rotation

“squishes” the Earth, which is bulging at the equator)• The Greeks did not just philosophize (a stereotype). They

actually also made empirical measurements• They also estimated the sizes of the other planets and the

Sun and Moon, and their distances from the Earth, but they lacked precision instruments, so missed by orders of magnitude, but had good qualitative ideas

• But they figured shapes out: for instance, full moon always looks like a circle, so must be sphere not disk

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www.geo.hunter.cuny.edu

Eratosthenes

Library of ancient Alexandria

The Medieval Worldview

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Dante, The Divine Comedy, mid-13th century A.D.

• Earth at center of everything theologically sensible for Church• Though Hell and Devil

at actual center• Significant historical

misunderstanding (Columbus cartoons): even the “backwards” people of the Middle Ages knew the earth wasn’t flat for sure

• Greek (Aristotelian and Ptolemaic) cosmos

Separation from Astrology• Astronomy broke off from astrology (the two were once one) as

the reasons behind the motions of the planets were de-mystified, largely due to Kepler (~1600) with his three laws of planetary motion, and Newton’s three laws, plus his (separate) law of gravity

• Astrology still studied, but…• Same horoscope given to multiple people, but all agree fits them

“perfectly.” Different horoscopes, but wrong signs. Same result.• Horoscopes of brutal serial killers or other criminals given to ordinary

people. Same result as above. Horoscopes vague, unpredictive. Moreover, twins can have very different lives.

• Only 12 signs of zodiac: so 1/12 of planet should have same or similar set of experiences every day (given daily horoscope)

• Horoscope will refer to “big dividends” or profits one day, for an Aquarius for instance, but tell that to an Aquarius dying of hunger in a third-world country

• Earth wobbles on its axis, so zodiac no longer even aligned• Astrology ends with Saturn, because other planets not known in

antiquity (Uranus, Neptune)

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Homework

• Scanned copy of “Discovering the universe: an introduction”• Read introduction (pp. 15-17) of http

://www.amazon.com/The-Astronomical-Revolution-Copernicus-Borelli/dp/0415474892 (click on “Look Inside”)

Finis

“The thirteenth-century monarch King Alfonso (“the Learned”) of Castile is said to have remarked, upon being briefed on the Ptolemaic model, that if this was really how God had built the universe, he might have given Him some better advice.”

- Timothy Ferris, Coming of Age in the Milky Way