History of Astronomy

History of Astronomy

Ancient Astronomy

Ptolemy

Copernicus

Galileo
Stonehenge

Chichen Itza

Founded by Mayans in 550 AD, conquered by Toltecs in 800 AD, abandoned 1000 AD
El Castillo-78 foot pyramid with 91 steps on each side. Shadow would hit different step each day.
Twice a year makes Serpent shape up side of temple.
Had actual observatory, El Caracol, with holes placed throughout to place certain stars in them at certain times of years.

Why build a Stonehenge or an El Castillo?

Being able to tell the time of year was critically important to ancient cultures.
When to plant, when to harvest, when did the rains come, when would frost start: These were questions of life and death.
In a world with little light, moon’s cycles also critical to night hunting, traveling, etc.
Astronomy was so critical it became intertwined with religion, sacrifice, and leadership.
Leads to astrology, omens, even cultures built AROUND ASTRONOMY!

Greeks: Beginnings of Astronomy the Science

Note: The Chinese actually did some substantial Astronomy, but like a lot of their works, not much was passed to Western culture (although still debated...)
Greeks developed some of our first real cosmologies, that is, working models of the universe.
Simple explanation of motions of sun, moon, stars, and planets.
Most influential: Aristotle (384-322 BC)
He taught a geocentric model: The earth is the center and everything spins around it.

Epicycles and Ptolemy

One small problem with the pretty picture of planets revolving around the earth, is sometimes they went backwards: RETROGRADE MOTION.
Greeks worked around this with EPICYCLES
Basically the planets move on little circles which moved around the bigger circle of their orbits.
System perfected by Ptolomy, a Greek living in Egypt in 140 AD, and written down in the Almagest
Predicted the motions of the planets 15 centuries or so.

Erastosthenes Measures the Earth

While they got the earth at the center wrong, Greeks did realize the Earth was round.
In fact Erastosthenes did a simple experiment to measure the size of the globe.
All he needed was a stick, a protractor, and some sucker who would walk through the desert at a steady pace.
Simple triangle geometry and his estimate was twice as good as that Columbus had.

Astronomy and the Dark Ages

With the fall of Rome, Europe fell into its Dark Ages and not much knowledge of any sort was kept or studied.
Astronomy (and a large hunk of western thought) kept alive in the Arab world. Most math terms and star names are Arabic in origin.
Meanwhile the earth centered view of the universe became transformed from model and theory into a total Dogmatic TRUTH. (Along with just about everything else Aristotle said...)

Copernicus (1473-1543)

A Polish cleric who felt the model for a geocentric system was just too complicated.
A Heliocentric (sun-centered) model seemed much simpler and cleaner to him.
It explained the Inferior planets never getting far from the sun.
If their orbits are within ours, they can never be on the far side of the Earth, i.e. Far from the sun.
And it explained Retrograde Motion without the dreaded Epicycles.

Retrograde Motion

Sometimes the Planets go Backwards along the Ecliptic.
They are NOT actually going backwards in their orbits.
Think of planets like cars going around on tracks.
If you were in one of the cars and you passed another one, it would LOOK like it was moving backwards.

Birth of Modern Astronomy

Copernicus’ theory did not travel far initially.
He published it after his death, in Latin where only scholars would read it.
Galileo Galilei (1564-1642) had to come along and popularize it.
First modern astronomer, who first used telescope aimed at the sky
Experimentalist: Dropped two balls of different sizes and saw they fell at the same rate.

First New Astronomical Discoveries Since Ancient times

Saw four moons orbiting Jupiter
Saw Venus had phases
Saw imperfections, sun spots, on the sun and craters on the moon.
These led him to discount Aristotle world view
Publicly ridiculed colleagues, wrote books in Italian expressing his theory, eventually wrote Dialogue Concerning the Two Chief World Systems, mocking a dumb Aristotelian.
The pope was not amused and he finished his life under house arrest (not burned at the stake as others had been, because he had powerful friends).