Question

If Ptolemy were right about Venus, how would it appear to earthlings?

Answer 1

The Ptolemaic system is a geocentric cosmology, which assumed that the earth was stationary and at the center of the whole universe. It is important to remember, though, that even during Galileo's time, most people believed that the earth was the center of the universe and the sun rotated around our planet.

The expectation for the old civilizations was that the sun, moon, planets and stars were supposed to travel in an uniform motion. This motion was supposed to be a circle which was considered the closest to a perfect path for the heavenly bodies.

However, when studying the paths of those heavenly bodies, they could observe that, at least looking from Earth, the paths were not circular.

Therefore Ptolemy had a hypothesis that explained the 'supposed imperfection'. In his model, he suggested that the apparently irregularity in the paths of the moons and starts were a combination of perfect paths (or circular motions) seen in perspective from the stationary body of the observer.

His suggestion was that, the planets were rotating around a center like the moon rotates the earth and they were doing this rotating movement around the earth like the moon rotates the sun fixed at Earth's rotation therefore giving us the illusion that they were not in a circular/perfect path.

So, in Ptolemy's system, Venus would rotate around our planet the same way that our moon rotates around the sun.