Understanding our Solar System-Ptolemy

Claudius Ptolemy was a Roman astronomer who was born in 90 AD and died in Alexandria, Egypt in 168 AD.

He refined the earlier Greek model of the solar system and heavenly bodies and produced a model that accurately explained the observed motion in the night sky of heavenly bodies.

This Earth centred model became the focus for understanding the structure of the solar system. Ptolemy assumed that the Earth was, not only, the centre of the solar system, but the universe as well.

Ptolemy's model tried to account for the apparent wobble of planetary orbits that were observed at the time. The animation on the right shows how Saturn's path, across the night sky, appears to have a wobble as it goes backwards and forwards.

As Mars' moved across the sky it was also observed to be moving backwards (retrograde).

Ptolemy assumed that each planet rotated around a circle, called an epicycle, which in turn rotated around a larger circle, called a deferent, as shown on the right.

It was assumed that the stars moved on a celestial sphere around the outside of the planetary orbits

The solar system, according to Ptolemy, consisted of the Earth as the central body with all other heavenly bodies rotating around it, as shown on the right.

Ptolemy positioned the planets to explain observations made by astronomers at the time. It was observed that Venus and Mercury never appear to move far from the Sun, hence, Ptolemy placed the two planets between Earth and the orbiting Sun. In fact, Ptolemy could have put the planets, Saturn, Mars and Jupiter, in a number of alternative positions and still be able to explain observations of planetary motion.

1) What observation were made by early scientists that convinced them that the stationary Earth was the centre of the Solar System?

2) What are epicycles?

3) What was Ptolemy trying to explain when he introduced epicycles?

4) How did Ptolemy explain the observation that Venus and Mercury are always observed close to the Sun?