Aristarchus’ solar system

Demonstration

Illustrating Aristarchus’ model of the solar system.

Apparatus and Materials

• Umbrella
• Earth globe, small
• Sun globe, large

Health & Safety and Technical Notes

Make sure the umbrella is in good condition and that no ribs are exposed at its edges.

Read our standard health & safety guidance

Procedure

1. Draw a star pattern on the umbrella.
2. Improvise the small Earth globe with a 5 cm polystyrene or wooden sphere mounted on a thin knitting needle and driven halfway into the sphere.
3. Hold the small globe, representing Earth, still at the crook of the umbrella. Spin the umbrella to show how the stars would move across the sky.
4. Hold the umbrella still and spin the globe in the opposite direction.

5. Move the Earth sphere out a short distance, and place another sphere at the crook of the umbrella to represent the Sun. Carry the Earth sphere round the Sun, in an orbit, by hand. This is easier if you remove the umbrella and just hold the Sun and Earth in your hands or place them on a table. You must move the Earth round the Sun with its spin-axis always pointing in the same direction.
6. Walk round the laboratory and point out how, moving in an orbit, you see the pattern of the students seated in the class change as you move nearer or farther away from various groups of them.

Teaching Notes

• In Aristarchus's time, the accepted model was the Pythagorean system, which had the Sun and planets located on a concentric spheres, spinning round the Earth. Aristarchus made two simplifying suggestions: the Earth spins (accounting for the daily motion of stars); the Earth and other planets move round the Sun in a yearly orbit (accounting for the apparent motions of the Sun and planets across the stars' patterns).
• Point out how, in steps 3 and 4, the daily rotation of the stars across the sky can be explained if the stars are still and the Earth spins. The motion in 4 is Aristarchus’ explanation of the Sun’s yearly motion.
• The Greeks objected to Aristarchus' model because: it would remove the Earth and its people from the centre of the Universe; objects would be flung off a moving Earth; the Earth in its orbit would travel nearer and farther from the stars so that the pattern of the stars would change; there would be parallax motions as the Earth moved through the star pattern and none were observed (until the nineteenth century).
• The absence of parallax effect in the starry pattern was a reason why Aristarchus’ scheme was unacceptable to the Greeks (see handout below). Step 6 explains what parallax means. The parallax effect for stars is incredibly small because stars (apart from the Sun) are so very distant from the Earth.

This experiment was safety-tested in July 2007.