Who was Thales?
Thales was born in what's now Turkey, in the city of Miletus, about 630 BC, during the Archaic period. At that time, Miletus was a Greek city, though it may have paid taxes or tribute to the Lydians. Thales (THAY-lees) travelled all over when he was a young man, and he may have studied with Egyptian or Babylonian scientists. Miletus was very well off at this time, and had colonies in Egypt which would have made this easier.
Thales was the first Greek scientist that we know of who did not try to explain the weather and the stars and planets as things controlled by the gods. Instead, Thales thought there were scientific explanations for these kinds of things. That doesn't mean Thales got everything right! He thought that water was the basis of all living things - that everything living was basically made out of water. Thales was right that everything is made out of the same things, but those things are electrons and protons, not water.
But some things Thales did get right. He understood that the earth was round, and that the moon was lit by light reflecting from the sun. According to Herodotus, in 585 BC Thales was the first person to successfully predict an eclipse of the sun (an eclipse which ended a war between the Lydians and the Persians).
Thales was also an important mathematician. He figured out a way to measure the height of one of the Egyptian pyramids. He waited until a time of day when his own shadow was the same height that he was, and then he measured the shadow of the pyramid. (You can try this for yourself - does it work?)
And he was able to prove several interesting mathematical ideas. Thales is said to have proved that
- a circle is bisected by its diameter,
- the angles at the bases of any isosceles triangle are equal
- if two straight lines cut one another, the opposite angles are equal.
- if two triangles have two angles and a side in common, the triangles are identical.
- if A, B and C are points on a circle where the line that connects A and C is a diameter of the circle, then the angle ∠ABC is a right angle.
Today people call this last proof Thales' Theorem.
Thales may have been Anaximander's teacher, and Anaximander was Pythagoras' teacher. Some ancient writers say that Pythagoras, when he was young, actually visited Thales, and that Thales advised Pythagoras to go study in Egypt. Thales died in 543 BC, only a few years after his city was conquered by the Persians.
Greek and Roman Science, by Don Nardo (1998). Nardo has written a lot of good simple books about the ancient world; this one is no exception.
Ancient Science: 40 Time-Traveling, World-Exploring, History-Making Activities , by Jim Wiese (2003). Activities, as the title says - how to make your own sundial, and so on. The author is a science teacher.
Early Greek Science: Thales to Aristotle, by Geoffrey Lloyd (1974).
History of Greek Mathematics: From Aristarchus to Diophantus, by Thomas L. Heath (1921, reprinted 1981). A lot of Euclid, but also describes who the other major Greek mathematicians were and what they did.
Episodes from the Early History of Mathematics, by Asger Aaboe (1997).