Heliocentric Model of the Solar System
Electricity and Magnetism | Energy and Thermal Physics

Greek astronomy

for 14-16

In this section we describe simple models to illustrate some of the imaginative and elegant suggestions made by Greek astronomers.

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Thales' model of the Universe

Heliocentric Model of the Solar System
Earth and Space

Thales' model of the Universe

Practical Activity for 14-16

Demonstration

The early Greek philosopher Thales, in about 600 BC, proposed a model to explain the daily motion of the stars. You can demonstrate it using an umbrella.

Apparatus and Materials

  • Umbrella, plain black

Health & Safety and Technical Notes

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

Read our standard health & safety guidance


Procedure

  1. Mark some constellations on the underside of the umbrella and rotate it slowly by the handle in an anticlockwise direction over a flat disc, which represents the Earth. Make sure that students can see the stars on the underside.

Teaching Notes

  • See guidance note

    Early astronomical observations


  • Thales described a simple model of a small flat Earth surrounded by a sheet of water, with a single vast sphere. This sphere carried the stars and revolved daily round an axis through the ‘Pole Star’. The model made no explanation of the extra motions of the Sun, Moon and planets, except that they must crawl backwards on the inner surface of the sphere.
  • The umbrella represents the sphere of the heavens. When the umbrella is opened the ferrule represents the Pole Star. If there are eight ribs they provide a useful marking guide, since pairs of ribs enclose 45°, or three hours of time in a daily revolution.

This experiment was safety-tested in April 2007.

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Model of the early Greek scheme

Heliocentric Model of the Solar System
Earth and Space

Model of the early Greek scheme

Practical Activity for 14-16

Demonstration

Using the flask model of the celestial sphere to demonstrate early Greek ideas about the structure of the Universe.

Apparatus and Materials

  • Flask, large (e.g. 2 litre wide neck)
  • Bung to fit flask
  • Knitting needle, long
  • Wooden washer (about 25 mm diameter)
  • Retort stand, boss, and clamp or Tripod
  • Sun label

Health & Safety and Technical Notes

Read our standard health & safety guidance


Slip the wooden washer over the knitting needle so that it rides freely. Push the needle through the bung so that the point of the needle almost reaches the bottom of the flask when the bung is inserted.

Half fill the flask with water and carefully insert the bung so that the wooden washer floats centrally on the water surface. Turn the whole upside down and place the flask in a ring on a tripod. Alternatively, attach it to a retort stand so that it is inclined at an angle.

Mark stars on the outside of the flask with an alcohol based marker pen. Mark the ecliptic on the outside at 23.5° to the celestial equator (see below experiment) and represent the Sun with a sticky label.

Model of the celestial sphere


Procedure

  1. Rotate the flask around its axis, so that the heavens can rotate about the Earth.
  2. Turning the neck will show the Sun's daily motion. The Sun can be moved to successive positions on the ecliptic, the daily motion being shown for each.

Teaching Notes

This experiment was safety-tested in April 2007.

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Pythagorean model of the Universe

Heliocentric Model of the Solar System
Earth and Space

Pythagorean model of the Universe

Practical Activity for 14-16

Demonstration

An umbrella model of the Pythagorean system.

Apparatus and Materials

  • Umbrellas, 2, black expandable

Health & Safety and Technical Notes

Make sure no ribs of the umbrella are exposed at their edges.

Read our standard health & safety guidance


Procedure

  1. Cut the spike off the inner umbrella and cut a hole in its fabric for the handle of the outer umbrella, such that the two can be inclined at 23 1/2° to each other. Mark the star pattern on the inner umbrella and a planet or the Sun on the outer one. This can be represented by a polystyrene sphere fixed to one of the spokes at the rim.
  2. Spin the inner umbrella, carrying the outer one with it, one rotation representing one day. Adding a slow rotation of the outer umbrella from west to east shows the planet's yearly movement backwards round the ecliptic.

Teaching Notes

See the guidance note

Early astronomical observations


Pythagoras (about 530 BC) developed a more complex model then 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 sphere 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 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 motion 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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Flask model of the Pythagorean system

Heliocentric Model of the Solar System
Earth and Space

Flask model of the Pythagorean system

Practical Activity for 14-16

Demonstration

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


Procedure

  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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Eudoxus' system

Heliocentric Model of the Solar System
Earth and Space

Eudoxus' system

Practical Activity for 14-16

Demonstration

Using an onion as a model of Eudoxus' system of the Universe.

Apparatus and Materials

  • Onion

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

Eudoxus needed four spheres to account for the motion of planets.

  1. An onion can be shown and sliced open, to illustrate Eudoxus' scheme of many concentric spheres.

Teaching Notes

  • In Eudoxus' system the Sun and Moon had three spheres each, the planets had four spheres each and the stars shared one sphere. All the spheres were spinning steadily around different axes. The combinations of these motions succeeded in imitating the actual motions of the Sun, Moon and planets across the star pattern.
  • One of the problems which Eudoxus (about 410-350 BC) was trying to solve was that the planets do not move steadily along a circle amongst the stars. They show retrograde motion with a varying speed. Also the speed of the Sun and the Moon varies around their orbits. Eudoxus was the greatest mathematician of his time. He was very clever at the geometry of solid space and he saw how to imitate the planet's looped paths by giving each planet four spheres.

This experiment was safety-tested in April 2007.

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Aristarchus’ solar system

Heliocentric Model of the Solar System
Earth and Space

Aristarchus’ solar system

Practical Activity for 14-16

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.

Resource

Download this handout

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An eccentric scheme for the Sun

Heliocentric Model of the Solar System
Earth and Space

An eccentric scheme for the Sun

Practical Activity for 14-16

Demonstration

An Earth-centred model to explain the irregularity of the seasons.

Apparatus and Materials

  • Ball, large (7.5 cm diameter approximately)
  • Ball, small (2.5 cm diameter approximately)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Hold the large ball - representing the Earth - in one hand. With the other hand, holding the smaller ball to represent the Sun, sweep out a large vertical circle. The Earth should be noticeably off-centre with respect to the Sun's circular orbit. This demonstration needs practice!
  2. If an elastic thread is joined between the two balls students can watch the thread and see how the Sun’s apparent speed must change through the seasons if the Earth is off-centre.

Teaching Notes

  • The Sun’s motion around the ecliptic is a little faster in the northern winter than in the northern summer so the interval between mid-winter and the spring equinox is a little shorter than between mid-summer and the autumn equinox. The seasons are not quite the same length.
  • To imitate this uneven motion, the Sun was imagined to move round a circle at constant rate but the Earth was placed a little off centre. Then the Sun, viewed along a line of sight from the Earth, would seem to move a little faster in December than in June.

This experiment was safety-tested in July 2007.

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An epicycle system for planets

Orbits
Earth and Space

An epicycle system for planets

Practical Activity for 14-16

Demonstration

Simple models to show epicyclic and retrograde motion from an Earth centred view.

Apparatus and Materials

  • Ball, large (7.5 cm diameter approximately)
  • Ball, small (2.5 cm diameter approximately)

Health & Safety and Technical Notes

In step 2 make sure that parts cannot fly off and hit observers.

Read our standard health & safety guidance


A suitable turntable for step 3 can be obtained from ASCOL, catalogue number P16-1000.

Procedure

  1. Hold the larger ball stationary in front of you to represent the fixed Earth. Hold the small ball in your outstretched arm so that you can sweep it in a large arc vertically around the Earth as centre. At the same time make your outstretched hand turn quickly around the wrist so that the ‘planet’ turns in a small circle as it moves in its large orbit.
  2. You can use a small electric motor assembly to represent the ‘planet’ moving in its epicycle. Sweep the whole assembly in a large vertical arc about the fixed ‘Earth’ whilst the motor drives the ‘planet’ in its small circle.
  3. The electric motor assembly can be mounted on a rotating turntable. If the turntable is rotated slowly by hand whilst the sphere on the electric motor assembly rotates, you can observe the epicyclic motion. The motor may be tilted a little.

Teaching Notes

  • This model still fixes the Earth at the centre of the star sphere. The radius of that circle acts as an arm to carry, at its end, a small circle (an epicyle). A radius of that small circle carries a planet round its circumference at a steady rate while the arm of the large circle revolves at a smaller steady speed.
  • In this epicycle system for Jupiter, arm EA rotates around the Earth once in 12 years, while arm AJ carries the planet J round once in 365 days. The two motions combine to give the pattern below.

This experiment was safety-tested in July 2007.

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Early astronomical observations

Heliocentric Model of the Solar System
Earth and Space

Early astronomical observations

Teaching Guidance for 14-16

The observations of early astronomers enabled them to determine the following:

  • There is an unchanging pattern of stars, revolving daily round an axis through the Pole Star.
  • Sun, Moon and planets share that daily motion, except they drift slowly backward through the star pattern.
  • The paths of the Sun, Moon and planets fall in a narrow band of the star pattern called the Zodiac.
  • ‘Freezing out’ the daily rotation, we find the Sun travels round the ecliptic, the central line of the Zodiac, in a year.
  • The Moon travels round an orbit in the Zodiac, tilted at some 5° to the ecliptic, in a month.
  • Mars, Jupiter, and Saturn travel round orbits in the Zodiac, making reverse loops, one for each Earth year, as they do so.
  • Mercury and Venus are only visible shortly after sunset and before sunrise – they remain close to the Sun.
  • Jupiter completes an orbit of the Sun in 12 years, Saturn in 30 years, Venus in a fraction of an Earth year

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Greek evidence for the Earth's shape and spin

Earth
Earth and Space

Greek evidence for the Earth's shape and spin

Teaching Guidance for 14-16

A round Earth

Pythagoras' pupils, if not the great man himself, knew that the Earth is round. Traveller's tales of ships disappearing over the horizon and noticing bright stars, such as Polaris, shifting to a higher position in the sky as one journeyed north suggested a curved Earth.

Aristotle (about 340 BC), two centuries later, supported the idea of a spherical Earth, Moon and planets because:

  • the sphere is a perfect solid and the heavens are a region of perfection
  • the Earth's component pieces, falling naturally towards the centre, would press into a round form
  • in an eclipse of the Moon, the Earth's shadow is always circular: a flat disc would cast an oval shadow
  • even in short travels northwards certain stars, such as Polaris, appear higher in the sky.

This mixture of dogmatic reasons and experimental common sense was typical of him and he did much to set science on its feet.

A spinning Earth

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