Ancient Astronomy

Eratosthenes teaching in Alexandria
by Bernardo Strozzi (1581–1644)

Our ideas about the Solar System and beyond have developed over time. Humans have looked to the skies and observed the apparent movement and changes in the Sun, Moon and stars for thousands of years. People with knowledge of the positions of stars, solar eclipses, and the phases of the Moon could create star maps, calendars and measure time. This was useful for farming and navigation.

The first records of people doing astronomy in a scientific way are from the Assyro-Babylonians in around 1000 BCE. Today, this area of the world is in Iraq and Syria.

The ancient Greeks did lots of work on astronomy and mathematics, among other topics. Eratosthenes is thought to be the first person to calculate the circumference of the Earth, which he did around 240 BCE. Hipparchus was the first person to record the Earth’s precession (the wobble of the Earth as it rotates around its axis). He also measured the distance to the Moon.

Want to recreate Eratosthenes' work? Put a date in your diary to try our Solstick experiment at the summer solstice!

The Geocentric Model

The ancient Greeks believed that the Earth was at the centre of the Universe. This idea is called the “geocentric model” of the universe. Geocentric means “Earth-centered”.

The geocentric model explained why the stars appeared to rotate around the Earth once a day and why the planets appear to move faster than the stars.

A problem with the geocentric model was that planets sometimes seemed to move backwards (in retrograde) around the Earth.

Claudius Ptolemy “solved” this problem in the year 140 CE by using a system of circles (or epicycles) to describe how the planets moved. Ptolemy was Roman but lived and worked in Egypt. Even Ptolemy’s geocentric model was complicated and couldn’t always accurately predict the movement of the planets.

Around 400 CE Hypatia was the western world’s leading mathematician and astronomer. Hypatia was Greek but lived in Egypt. She designed a machine, known as an astrolabe, to calculate the positions of stars. Sailors and astronomers used her tables of positions of the stars, called the Astronomical Canon, for the next 1200 years.

The Geocentric versus Heliocentric Models. Credit: NSO

The Heliocentric Model

In 1543, Polish astronomer Nicholaus Copernicus proposed that the Earth and other planets orbited the Sun. This idea is called the “heliocentric model” of the Universe. Heliocentric means “Sun-centered”. Copernicus’s system predicted the movements of the planets as well as Ptolemy’s geocentric model did but without the need for complicated epicycles.

The shift from an Earth-centered view to a Sun-centered view of the Universe is called the Copernican Revolution. People were used to the Earth being centre of everything, so this was a big change in thinking. Many people did not accept it. (Astonishingly, over 1800 years earlier, the Greek astronomer Aristarchus had said that all the planets, including the Earth orbited the Sun.)

Danish astronomer Tycho Brahe developed his own model of the Solar System, known as the Tychonic System. It combined the heliocentric system, with the beliefs behind the geocentric system. Tycho said that the planets orbit the Sun, and the Moon orbits the Earth, but that the Sun also orbits the Earth.

Tycho made observations of the planets five times more accurate than any other observations at the time. In his observations, he discovered several ‘new stars’ that we now know to be supernovae. Tycho also showed that comets were not in the atmosphere of the Earth but must move through the Solar System out towards the planets.

Italian astronomer Galileo Galilei observed the sky using a telescope he had made. In 1610, he discovered that Jupiter had moons orbiting around it. This was the first evidence that objects could orbit something other than Earth and supported the heliocentric model of the Solar System. Galileo’s discovery helped build support for the heliocentric model.

The Chinese astronomer, Gan De may have actually been the first person to describe Jupiter’s moons. He observed two bright objects in 365 BCE without using a telescope.

Copernicus’s model still contained some errors. This was because he assumed the planets orbited the Sun in perfect circular obits. In 1609, German astronomer Johannes Kepler improved the heliocentric model by proposing that the planets moved around the Sun in elliptical orbits. This system was much more successful at accurately predicting the movements of the planets. Kepler used Brahe’s astronomical data to develop his three laws of planetary motion.

Use our Electric Orrery to explore the orbits of the planets and find out where they were in the past, and where they will be in the future!