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The Copernican revolution
Modern astronomy emerged from its Greek roots when Copernicus put forward a simpler model of the solar system to account for planets' apparent motion.
Practical Activity for 14-16
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 long
Health & Safety and Technical Notes
Be careful not to hit lights, ceiling panels, and so on.
- 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.
- 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.
- The pole represents a
sight linefrom the Earth to a planet, say, Mars. You need to practice to get each arm moving at a different speed.
- 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.
- 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.
A (very) brief history of astronomy
Teaching Guidance for 14-16
Early astronomers, in different civilizations, used the observed motion of the stars, the Sun, Moon and planets as the basis for clocks, calendars and a navigational compass. The Greeks developed models to account for these celestial motions.
Copernicus, in the 16th century, was the first to explain the observed looping (retrograde) motion of planets, by replacing a geocentric heliocentric model of the Universe with a heliocentric model. Modern planetary astronomy really began in the 17th century with Kepler, who used Tycho Brahe’s very accurate measurements of the planetary positions to develop his three laws.
Galileo contributed to the development of astronomy by teaching the Copernican view, and by devising a telescope which he used to show Jupiter’s moons as a model for the solar system, among other things.
Newton built on earlier insights with his universal law of gravitation and its fruits: predictions or explanations of Kepler’s laws, the motion of comets, the shape of the Earth, tides, precession of the equinoxes and perturbations in the motion of planets which led to the discovery of Neptune. He also had to invent the mathematics to do this: calculus.
Planets in the Copernican system
Teaching Guidance for 14-16
Copernicus did not only offer an alternative model that looked simpler than the heliocentric model. He also extracted new information from his heliocentric scheme: the order and relative sizes of the planetary orbits.
But he did not know the real values of planets' orbit radii. For that he needed an accurate measurement of one of the distances. All he had was a Greek measurement of the distance of the Earth from the Sun.
Estimating the size of the planets themselves would have to wait until telescopes had been invented. A rough model of the solar system known to Copernicus would then be:
- Sun - beach ball
- Mercury - a grain of sand, 16m from the Sun
- Venus - a pea, 29m from the Sun
- Earth - a pea, 40m from the Sun
- Mars - an apple pip, 61m from the Sun
- Jupiter - a ping pong ball, 210m from the Sun
- Saturn - a ping pong ball, 380m from the Sun