The geocentric model

For 1400 years the standard model was Ptolemy's (c. AD 150), building on Aristotle: the Earth is fixed at the centre of the universe and everything — Moon, Sun, planets, stars — orbits it on perfect circles at constant speed.

The problem was retrograde motion. To reproduce the backwards loops, Ptolemy had each planet ride a small circle (an epicycle) whose centre travelled round a big circle (the deferent) centred near the Earth.

• It worked — it predicted planetary positions well enough for centuries.
• But it was complicated, and needed more and more epicycles to stay accurate.
• It also fitted the philosophy of the time (a special, central Earth; perfect, unchanging heavens), which is why it survived so long.

Give Ptolemy credit: a model is judged on whether it predicts observations. Ptolemy's did. It was replaced because a simpler model predicted just as well — and then better.

Copernicus puts the Sun in the middle

In 1543 Copernicus published a heliocentric model: the Sun at the centre, the Earth just another planet, spinning daily and orbiting yearly, with the planets ordered by their orbital periods.

• Retrograde motion needs no epicycles — it is simply what you see when the faster inner Earth overtakes an outer planet.
• It explains why Mercury and Venus never stray far from the Sun (their orbits are inside ours).
• It gives the planets' relative distances — something Ptolemy's model could never do.

Objections at the time were scientific, not just religious: if the Earth moves, why is no stellar parallax seen? (Answer: the stars are far more distant than anyone imagined — parallax was not detected until 1838.) And Copernicus still used circles, so his predictions were no better than Ptolemy's.

The distance trick: for an inferior planet at maximum elongation, the Sun-planet-Earth angle is 90°, so the planet's distance from the Sun in AU is simply sin(maximum elongation).

Your turn — how far is Venus from the Sun?

Your turn — and Mercury?

What were epicycles for?

Galileo turns a telescope on the sky

From 1609 Galileo used the new telescope, and what he saw wrecked the old model:

• The phases of Venus. Venus goes through a full set of phases — including a small full disc and a large thin crescent. In Ptolemy's model Venus stays between us and the Sun, so it could never appear full. This was the decisive evidence.
• Four moons of Jupiter (the Galilean moons). Here were bodies plainly orbiting something other than the Earth — so the Earth was not the centre of all motion.
• Mountains and craters on the Moon, and sunspots on the Sun — the heavens are not perfect and unchanging.
• The Milky Way resolves into countless stars — the universe is far bigger than assumed.

Exam-precise wording: Galileo's observations disproved the Ptolemaic model. They did not, on their own, prove Copernicus — Tycho's hybrid model also survived them. It was Kepler's ellipses and Newton's gravity that finished the job.

The killer observation

Tycho's data and Kepler's ellipses

Tycho Brahe (1546–1601) built huge quadrants and sextants and spent decades measuring planetary positions to about 1 arcminute — the finest naked-eye data ever taken. (He never had a telescope.) He proposed his own hybrid model: the planets orbit the Sun, but the Sun orbits a stationary Earth.

Johannes Kepler inherited Tycho's observations of Mars. Circular orbits missed Tycho's positions by 8 arcminutes — and Kepler trusted the data rather than the theory. That forced him to a radical conclusion:

The moral (and an easy 4-mark question): Tycho supplied the precise observations; Kepler supplied the mathematical analysis. Science needs both. Kepler's laws finally gave heliocentric predictions that beat Ptolemy's.

Your turn — Kepler's third law

Who said what?

Tap a statement, then tap the model or person it belongs to.

Match the contribution to the astronomer

Tap a description on the left, then its matching term on the right.

No parallax?

Judging a model

Before Ptolemy: the ancient astronomers

Astronomy is older than writing. Alignments at monuments such as Stonehenge mark the solstice sunrise, showing that Neolithic people tracked the Sun's yearly cycle carefully enough to build in stone.

• Aristotle (c. 350 BC) argued the Earth is a sphere — from the circular shadow it casts in a lunar eclipse and from the changing stars seen as you travel north or south — but placed it fixed at the centre.
• Aristarchus (c. 270 BC) proposed a heliocentric model, some 1800 years before Copernicus, and tried to measure the relative distances of the Sun and Moon. His idea was rejected — partly because no stellar parallax could be seen.
• Eratosthenes (c. 240 BC) measured the circumference of the Earth to within a few per cent.
• Hipparchus (c. 130 BC) catalogued around 850 stars, invented the magnitude scale and discovered precession.

The lesson for the exam: the geocentric model won not because the Greeks were foolish, but because it fitted the observations available — and the one prediction that would have falsified it (stellar parallax) was far too small to detect.

Ahead of his time

The big ideas to know

That is Early Models of the Solar System covered for Edexcel GCSE Astronomy. Press Finish to see your score.