Lesson 17 - Retrograde Movement
Many ancient civilizations, like the Greeks, believed that the Earth was the center of the Universe, and that the Sun, the planets, and all other celestial bodies revolved around the Earth. This was known as the geocentric model of the Solar System. The only problem was that periodically, some of the planets appeared to reverse direction for a while, then go back to their regular movement. This is known as “retrograde motion” and it is very difficult to explain using the geocentric model. Later on, it was discovered that the Sun was, in fact, the center of the Solar System (heliocentric model) and everything revolves around it. In this lesson, students will examine retrograde planetary motion.