Heliocentric Model of the Solar System
Earth and Space

Flask model of the Pythagorean system

Practical Activity for 14-16 PRACTICAL PHYISCS


Using the celestial sphere apparatus to model the Pythagorean system.

Apparatus and Materials

  • Polystyrene sphere, small
  • Celestial sphere apparatus as used in the Model of the Celestial sphere experiment
  • Knitting needle
  • Sellotape, coloured
  • Yellow Sun disk

Health & Safety and Technical Notes

Take normal care with glassware. Keep the model out of sunlight as flask and water can act as a convex lens and produce localised heating.

Read our standard health & safety guidance


  1. Mount the polystyrene sphere on the needle and insert it through the bung into the flask which is half full of water. Adjust the position of the sphere and the volume of water so that the sphere is at the centre of the flask and in the water surface when the flask is inverted. The sphere represents the Earth.
  2. Invert the whole assembly and place it in a ring on a tripod or attach it to a retort stand. Mark the ecliptic on the flask with a pen or a band of coloured tape (at 23 1/2° to the equatorial plane of the model). Temporarily stick a yellow disc to represent the Sun at various points on the ecliptic.
  3. Show the daily motion of the heavens by rotating the flask about its own axis. Show the progress of the Sun through the seasons of the year by moving it from place to place round the ecliptic whilst the flask is spun for each position.

Teaching Notes

See guidance note.

Early astronomical observations

Pythagoras (about 530 BC) developed a more complex model than Thales' model. The Pythagorian School accepted that the Earth was a sphere. The stars and planets were imagined to sit on an imagined scheme of concentric spheres, like shells of an onion, the Crystal Spheres. The outermost spheres carried the stars with the daily motion. Inside were other spheres, each carrying a planet. Starting from the outermost, they were in this order:

  • Stars
  • Saturn
  • Jupiter
  • Mars
  • Sun
  • Mercury
  • Venus
  • Moon

In later stages, this model of celestial spheres had all the inner spheres attached to the outermost one, which carried them round with the 24-hour motion. Then the Sun's sphere revolved backwards once a year about an axis on the ecliptic. The spheres for the Moon and other planets all revolved slowly backwards about the same axis; one revolution a month for the Moon; one revolution in 12 years for Jupiter. This model imitated the motions of the Sun and Moon fairly well but gave only the general motions of the planets, without their retrograde loops.

This experiment was safety-tested in April 2007.

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