# Copernicus explains the motion of planets

##### Demonstration

A simple model to show how Copernicus explained the looped paths of the outer planets.

#### Apparatus and materials

Smooth pole or bamboo cane, 2 m to 3 m lon

#### Health & Safety and Technical notes

Be careful not to hit lights, ceiling panels, and so on.

#### Procedure

a Hold one end of the pole in the right hand (to represent the Earth), and let the pole run loosely through a ring made by the finger and thumb of the left hand. Hold your right hand close to your body with your left arm extended sideways from the shoulder.

The line of the pole runs on out to the 'stars', imagined to be on the walls and ceiling of the room.

b Move the Earth (your right hand) quickly in a tight circle whilst Jupiter (your left hand) moves slowly. The pole will wag to-and-fro, as well as making general progress across the sky.

#### Teaching notes

1 The pole represents a 'sight line' from the Earth to a planet, say, Mars. You need to practice to get each arm moving at a different speed.

2 An alternative way is to imagine the Sun just in front of your chest. Move your right hand in a vertical circle, fairly quickly, to represent the Earth in orbit around the Sun. Move your left hand round a larger circle, more slowly, to represent Mars.

3 Copernicus put forward a simple view of the solar system to account for the observed motion of planets in orbits with loops. He placed the Sun at the centre with the planets, including the Earth, revolving around it. He explained the looped pattern of planetary motion through the stars by combining the simple motion of the planet in a circular orbit round the Sun with the Earth’s simple motion in its orbit around the Sun. The loops are due to the Earth’s motion.

Copernicus accounted for the epicycloids of Mars, Jupiter and Saturn by making them move in orbits greater than the Earth’s orbit. He made Venus and Mercury move around in smaller orbits nearer the Sun than the Earth’s. This accounted for their observed behaviour; they keep close to the Sun and swing to and fro each side of it.

Copernicus also predicted the phases of Venus, which were not observed until the telescope was invented.