Charting the planets and stars
By about 3500 BC (and maybe long before that), people thought of these moving things in the sky as living beings – gods, with their own human-like personalities. If the moon and the planets were gods, they could affect the lives of people, and so the Mesopotamians and the Egyptians began to chart the movements of the planets and the moon to try to predict the future.
They identified hundreds of constellations of stars, drew star maps, and created the idea of horoscopes and the signs of the Zodiac. These early astronomers all thought that the earth was the center of the universe, and that the moon, the sun, the planets and the stars all went around and around overhead in the sky.
Earth is round
The Greeks named their planets the same way the Mesopotamians had, just translating the names. Around 600 BC, the Greek astronomer Thales rejected this idea that the moon and the stars were gods. Instead, Thales suggested that the earth was a round ball, and that the moon was lit by light reflecting from the sun.
If the earth was round, then you could think of the moon and planets and stars and sun as going all the way around the earth. In 585 BC, Thales used this idea to become the first astronomer to successfully predict an eclipse of the sun. By about 430 BC, Anaxagoras had followed up on Thales’ ideas to show exactly what caused eclipses.
Earth goes around the sun
Two hundred years later, about 250 BC, the African astronomer Eratosthenes calculated the circumference of the earth, using the different shadows cast at noon in two different places – in northern Egypt and southern Egypt. Eratosthenes also figured out how far away the moon and the sun were from the Earth.
And shortly after that Aristarchus, also working in Egypt, figured out that the earth went around the sun, instead of the other way around, by considering the curved shadow of the earth on the moon during an eclipse of the moon.
Aristarchus also figured out that the sun had to be a lot bigger than either the earth or the moon, and that the stars must be much, much farther away than the moon or the sun. Even though Aristarchus was right, though, most scientists thought he must be wrong – how could the stars really be that far away? It just seemed unlikely.
Astronomy in China
Meanwhile in the Mediterranean, astronomers were still debating this whole question. Roman astronomers named the planets using the old Mesopotamian system. And they followed some of the old Mesopotamian ideas. The Roman-Egyptian astronomer Ptolemy, for example, still thought that the sun and stars went around the earth. But other Roman astronomers thought Aristarchus might be right.
But Arya Bhata followed Ptolemy and the Mesopotamians in thinking that the earth was at the center of the universe and the moon and the planets, the sun and the stars all moved around the sky.
Even so, Arya Bhata did some important mathematical work on sines and cosines, and on time-pieces like water-clocks, that would help to measure the distances to the planets and stars.
A Chinese star chart from the 600s AD shows even some very faint stars that are hard to see with just your eyes.
How big are the sun and the moon?
Soon afterward, Islamic glassworkers used medical research on the way the human eye worked to figure out how to to make glass lenses that would focus the heat of the sun in one spot. By 1100 AD, Al Ghazali was able to understand again what caused eclipses of the sun and the moon.
The Milky Way
About 1260 AD, another Islamic astronomer, Al Tusi, used an observatory to make very accurate measurements of the movements of the planets.
Al Tusi also realized that the Milky Way was really not a cloud but a lot of very far away faint stars. In the 1470s, the Ottoman astronomer Ali Qushji revived the idea that the earth went around the sun.
Copernicus and Galileo
By 1514, the Polish astronomer Copernicus published a small book proving that the earth actually went around the sun, as Aristarchus had thought more than a thousand years earlier.
Still many people – most people – did not believe Copernicus, and his cause had to be defended in the later 1500s by Galileo, and again by Kepler, and again in the 1700s by Isaac Newton. Even though some astronomers have known for 2300 years that the earth and the planets go around the sun, there are still people today who don’t believe it.
(this article lacks information on Native astronomy in the Americas – we’re working on it!)
Ancient Science: 40 Time-Traveling, World-Exploring, History-Making Activities , by Jim Wiese