Early Models of the Solar System

Reading the ancient sky

Long before telescopes, people watched the Sun, Moon and stars to know when to plant crops, when to hold festivals, and how to keep a calendar. Some even aligned huge monuments to the sky. To explain what they saw, early astronomers built models with the Earth at the centre. This module explores those models, their clever fixes, and the enormous distances astronomers now measure.

Old-model words

Four terms unlock this topic.

Geocentric and epicycles

At the centre

In a geocentric model, what is at the centre?

• The Earth
• The Sun
• The Moon
• A distant star

Ptolemy's clever fix

A pure Earth-centred model could not explain why planets sometimes seem to loop backwards (retrograde motion). Ptolemy's solution was the epicycle: each planet moves on a small circle (the epicycle), whose centre moves around a larger circle around the Earth. With enough epicycles, the geocentric model could predict the planets' positions surprisingly well - which is why it lasted for well over a thousand years.

The advantage of epicycles

Why were Ptolemy's epicycles useful?

• They let the Earth-centred model predict planetary motion, including retrograde loops, quite accurately
• They proved that the Sun is at the centre
• They measured the exact distance to the stars
• They had no real practical use

Measuring vast distances

Space is huge, so astronomers use bigger units than the kilometre. Match the unit to the scale.

Units of distance

Select the TWO that are units of distance.

• The astronomical unit
• The light year
• The degree
• The hour

Convert to AU

An object orbits 4.5 x 10^8 km from the Sun. Using 1 AU = 1.5 x 10^8 km, how many astronomical units is that? Give your answer as a number.

How ancient people used the sky

Smallest to largest

Put these distance units in order, from the smallest to the largest.

• Kilometre
• Astronomical unit (about 1.5 x 10^8 km)
• Light year (about 9.5 x 10^12 km)
• Parsec (about 3.26 light years)

Explain the sky

Three questions about early astronomy. Pick the correct explanation each time.

• Why did the Earth-centred model with epicycles last for over a thousand years?
• A monument aligned to the solstice sunrise 4000 years ago is slightly off today. Why?
• Which unit would an astronomer use for the distance to a far-off star?

Your turn: explain

Explain why the ancient geocentric model, using Ptolemy's epicycles, was accepted for so long, and how it dealt with the motion of the planets.

• State what "geocentric" means (the Earth at the centre)
• Explain what an epicycle is (a small circle a planet moves on)
• Say what the epicycles achieved (predicting planetary motion, including retrograde loops)
• Conclude why the model lasted (it matched observations well enough)

Geocentric, then epicycles

Tie the topic together with a few anchors. The early model was geocentric - Earth at the centre - and epicycles were the clever fix that let it predict planetary motion, which is why it survived for over a millennium. For the sky over time, remember precession: the slow wobble of the Earth's axis that shifts where stars rise and drifts ancient monument alignments. And keep the distance units in order - kilometre, AU, light year, parsec - with the AU pinned to 1.5 x 10^8 km.