Before the invention of the telescope in the early seventeenth century, astronomy was based on observations made by the naked eye.
First, people mapped the positions of stars and planets in the sky.
Most cultures had their own systems for mapping the sky, but astronomy as we know it today has its roots in classical Greek theories.
In A.D. 150, the Greek astronomer and mathematician Claudius Ptolemy wrote an important treatise on astronomy.
In it, he listed forty-eight groups of stars called constellations, with names, such as Orion and Perseus, taken mostly from mythology. In the same way that we can imagine shapes of familiar objects when we look at clouds, so Ptolemy saw traditional figures in groupings of stars.
Ptolemy also noted that the stars seemed to move across the sky. He said that all celestial bodies revolved around Earth, which stood still in the center of the universe.
This was the accepted scientific theory for centuries.
Ptolemy’s view of the universe is called a geocentric model, because Earth (geo refers to Earth) stands at its center (centric means center).
