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Models of the solar system
How ideas about the solar system changed with new evidence: observations by Babylonian, Chinese, Maya, Inca and Islamic astronomers, the geocentric model of Aristotle and Ptolemy, retrograde motion and epicycles, the heliocentric model of Copernicus, and the evidence from Tycho Brahe, Kepler, Galileo and Newton.
Watch Mars for a few months and something strange happens. Night after night it drifts slowly eastward against the stars. Then it stops, goes backwards for about two months, stops again, and carries on as before. For almost 2000 years, explaining this backwards loop was one of the hardest puzzles in science, and solving it meant moving the Earth out of the centre of the universe.
Watching the sky
People in every culture have watched the sky carefully, because it told them when to plant, harvest, travel and hold ceremonies. Many of their observations were astonishingly accurate.
| Culture | Contribution |
|---|---|
| Babylonian (Iraq, from about 1600 BC) | Recorded the movements of Venus and the Moon on clay tablets, discovered that eclipses repeat in an 18-year cycle, and used mathematics to predict them. Our 360° circle and 60-minute hour come from their number system. |
| Chinese (from about 1300 BC) | Kept the longest continuous records of eclipses, comets and "guest stars" (exploding stars). Their record of a supernova in 1054 AD let modern astronomers identify the Crab Nebula as its remains. |
| Maya (Central America, 250–900 AD) | Tracked Venus and predicted eclipses. Their solar calendar of 365 days was very accurate, though it didn't add leap days. |
| Inca (South America, 1400s) | Built stone towers on the hills above Cuzco to mark where the Sun rose and set on key days, timing the planting of crops. |
| Islamic world (800–1500 AD) | Built large observatories, improved instruments such as the astrolabe, measured the length of the year to within minutes and corrected errors in Greek astronomy. Many star names, such as Aldebaran, Betelgeuse and Rigel, come from Arabic. |
Aboriginal and Torres Strait Islander peoples have their own long traditions of astronomy, covered in topic 6G.
The geocentric model
From the ground, the Earth feels perfectly still, and the Sun, Moon and stars all seem to circle around us once a day. So it made sense to Greek philosophers such as Aristotle (around 350 BC) that the Earth sat motionless at the centre of everything. This Earth-centred idea is the geocentric model (geo means Earth).
Around 150 AD, Claudius Ptolemy, working in Alexandria in Egypt, turned it into a detailed mathematical model in his book the Almagest. To explain retrograde motion, he had each planet move in a small circle, called an epicycle, whose centre travelled around the Earth on a larger circle. As the planet looped around its epicycle, it would sometimes appear to go backwards. The model could predict planet positions reasonably well, and it was used for about 1400 years.
The heliocentric model
A Sun-centred model is called heliocentric (helios means Sun). The Greek astronomer Aristarchus of Samos suggested it around 270 BC, but few people took it seriously. In 1543, the Polish astronomer Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres, putting the Sun at the centre with the Earth as one of the planets, spinning daily and orbiting yearly.
The heliocentric model explains retrograde motion simply. The Earth is on an inside track and moves faster than Mars. Every two years or so the Earth overtakes Mars, and for a while Mars appears to slide backwards against the far-away stars, just as a slower car seems to go backwards when you overtake it on the motorway.
- In the Sun-centred view, watch the line of sight from Earth to Mars. When does Mars go backwards on the star strip?
- Where is Earth compared with Mars at that moment?
- Switch to the Earth-centred view and let it run. What shape does Mars trace? How is that like Ptolemy's epicycles?
The evidence piles up
Copernicus's model wasn't accepted straight away. It still used perfect circles, so its predictions were no better than Ptolemy's, and it went against the teachings of powerful religious authorities. What changed minds was new evidence:
- Tycho Brahe (Denmark, late 1500s) measured planet positions, without a telescope, more precisely than anyone before him.
- Johannes Kepler (Germany) used Tycho's measurements to show in 1609 that planets move in ellipses (ovals), not circles, and speed up when closer to the Sun. With ellipses, the heliocentric model finally made accurate predictions.
- Galileo Galilei (Italy) turned a telescope on the sky in 1609–1610. He saw four moons orbiting Jupiter, proof that not everything circles the Earth, and saw Venus go through a full set of phases like the Moon, which is only possible if Venus orbits the Sun. He was put on trial in 1633 and spent the rest of his life under house arrest for supporting Copernicus.
- Isaac Newton (England) showed in 1687 that one force, gravity, explains both a falling apple and the planets' elliptical orbits around the Sun.
Our model has kept changing. Uranus (1781) and Neptune (1846) were added, the first asteroid, Ceres, was found in 1801, and in 2006 Pluto was reclassified as a dwarf planet when many similar icy worlds were found beyond Neptune. That's how science works: models are kept as long as they fit the evidence, and improved or replaced when they don't.
Test yourself
In the geocentric model, the centre of the universe is the
Geo means Earth.
Ptolemy explained retrograde motion using
Small circles riding on larger circles.
In the heliocentric model, Mars appears to go backwards because
It's an effect of viewing from a moving Earth.
Which observation by Galileo showed that not everything orbits the Earth?
Those moons circle Jupiter, not Earth.
Kepler improved the heliocentric model by showing that planets
Elliptical orbits made the predictions accurate.
Name each model.
Put these events in time order.
Compare the geocentric and heliocentric models of the solar system, and explain how new evidence led scientists to accept the heliocentric model.
The geocentric model, developed by Aristotle and Ptolemy, put a stationary Earth at the centre, with the Sun, Moon, planets and stars orbiting it. To explain why planets such as Mars sometimes go backwards, Ptolemy added epicycles, small circles carried around on larger ones. The heliocentric model of Copernicus put the Sun at the centre, with the Earth as a planet that spins daily and orbits yearly. It explains retrograde motion more simply: the faster Earth overtakes Mars on an inside orbit, so Mars appears to slip backwards. New evidence led to its acceptance. Galileo's telescope showed four moons orbiting Jupiter, so not everything circles the Earth, and showed Venus going through a full set of phases, which can only happen if Venus orbits the Sun. Kepler used Tycho Brahe's precise measurements to show orbits are ellipses, which made the heliocentric model's predictions accurate, and Newton's law of gravity explained why the planets orbit the Sun.