History of Astronomy

Why did ancient cultures study astronomy? Several cultures kept very exact records of astronomical events (and could even predict events) such as the Mesopotamians (Babylonians) and especially Chinese. WHY?????? timekeeping agriculture (planting crops or hunting and fishing cycles) astrology/religion and to keep accurate historical records. But, there were no real models of why astronomical events happened…that’s what the Greeks did.

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Greek Astronomy

General Information of Historical Models Models tried to explain motion planets and why they move through the fixed stars. Models tried to explain motion of moon and sun as well. Models tried to account for retrograde motion of planets (this was the biggest problem in finding a model that worked accurately). Astronomy and mathematics advanced side-by-side because it gave cultures problems to solve and calculate GEOcentric--Earth centered universe HELIOcentric--Sun centered universe

A Timeline of World Events and Famous People (left) and Notable Greek Astronomers (right)

The Pythagorean Model of the Solar System

Pythagorean Model Earth moved around a “central fire” but we couldn’t see it because it was blocked by a “Counter Earth” made music as it spun Why it’s important: one of first models planets are spherical and orbit in circular paths

Eudoxus’ Model Tried to account for retrograde motion of planets opposite motion of spheres produced figure eight motion of planet Notice: still circular Problems: 1. Not exact 2. Planets should be same brightness all the time and they aren’t

Aristotle’s (Aristotelian) model ~350 B.C. Geocentric model of the universe Problem: Not very exact

Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius Angular distance between Syene and Alexandria: ~ 7 0 Linear distance between Syene and Alexandria: ~ 5,000 stadia  Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

Ptolemy’s Model--tried to account for retrograde motion using epicycles (still not exact, but better and was accepted for 1500 yrs !)

The Complete Geocentric Model

From Ptolemy into the Renaissance Ptolemy’s model was relatively accurate and stuck around for 1500 years before another replaced it. Note...Ptolemy’s was geocentric. Aristarchus proposed a heliocentric model around 300 B.C., but no one accepted it because they thought the position of stars should shift if Earth were moving. Not until 1500’s (only 500 years ago !) did we see that the Earth wasn’t the center of the universe with Copernicus. Coming up next… Copernicus, Tycho, Kepler and Galileo in the Renaissance.

A Timeline of Notable People and Events (left) and the Four Most Prominent Renaissance Astronomers (right)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Renaissance Astronomy

Copernicus First accepted fully developed heliocentric model (sun at center) published in 1543 Explains diurnal motion of stars, explains motion of sun Important: Retrograde motion occurs whenever Earth passes or is passed by another planet Difference between epicycles (Ptolemy) and Copernican model epicycles (Ptolemy) Copernican model

The Position of the Sun Against the Backdrop of Stars throughout the year

The Retrograde Motion of Mars according to the Heliocentric Model The retrograde motion of mars would occur over a couple of months. The movement is apparent—mars doesn’t really move like that in space. The position of mars shifts against the backdrop of distant stars.

Tycho Brahe Took very detailed and accurate measurements of stars and planets from his observatory Also discovered a supernova in 1572 (a brilliant “new star”) that suggests that Aristotle’s starry sphere was not perfect and it changes

Tycho and Kepler Kepler took accurate measurements from Tycho and devised his own mathematical laws. Kepler’s 1st law: planets move in ellipses and not in circles (the sun is at one of the foci) Kepler’s 2nd law: planets move faster when close to sun and slower when farther away. Kepler’s 3rd law: P 2  a 3 (P=sidereal periods and a=semimajor axis)

Kepler’s Second Law

Galileo used telescope (but wasn’t the inventor) found Milky Way is really a lot of stars showed sunspots on sun—sun isn’t a perfect celestial body showed moon has mountains and valleys like Earth (suggests Earth is a celestial object…part of the “heavens”) showed 4 satellites orbiting Jupiter (proof that Earth wasn’t the center of everything) showed phases of Venus (which crippled Ptolemaic model)…this can’t be explained in the geocentric model

Some of Galileo’s Drawings of the Moon

Phases of Venus—can’t see this in the geocentric model

Galileo’s drawings of Jupiter and the 4 main moons