Astrophysicist Dr Gemma Lavender from All About Space magazine explains the science behind Galileo’s discoveries
What was geocentrism?
This was a belief, as early as the 4th century, that the Earth was at the centre of the Solar System, while the Moon, Sun and planets orbited around our planet. The idea also extended further outside our solar neighbourhood, with ancient civilisations also thinking that the stars orbited the Earth on circular (not elliptical!) paths.
In the geocentric model, the Earth was pictured as a sphere at the centre of the cosmos that doesn’t move – and doesn’t even rotate on its axis. The system was devised by Greco-roman astronomer and mathematician Claudius Ptolemy who, alongside Greek philosopher Aristotle, was seen as an authority on matters that were encompassed by astronomy and the natural sciences.
Where did this theory originate from?
The idea came about from observations of the motions of the Sun during the day and the stars and planets at night. If you watch these celestial bodies making their way across Earth’s sky, it appears as if they’re orbiting us. We also know our planet to be spinning on its axis and making its way around the Sun on its orbit. Standing on its surface, though – and from our Earth-bound perspective – it feels like it’s stationary. That’s how Ancient Greek, Roman and Medieval philosophers, led by Aristotle and Plato, saw our place in the universe. At the time of their studies of the geocentric model, Aristotle and Plato had also worked out that the Earth is a sphere.
How does heliocentrism work?
Heliocentrism is the model that we use today and explains how Earth and other planets in our Solar System orbit the Sun. The theory was initially proposed by Polish astronomer Nicolaus Copernicus, who also believed that the universe was in orbit around our nearest star. He had been made aware of the rising problems brought about by the geocentric model; it was unable to make accurate predictions during studies of the cosmos. Copernicus proposed, just like the geocentric model, that orbiting bodies moved around the Sun on circular orbits. Of course, today we know these orbits to be elliptical.
Why was this idea so controversial at the time?
By moving the Sun to the centre of the model, displacing the Earth, we – God’s creations – were no longer at the centre of the universe. Copernicus knew that this would greatly upset the Church and so he didn’t publish his theory until near the end of his life.
Later, in 1610, Italian astronomer Galileo Galilei observed the phases of Venus and the moons of Jupiter using a telescope. He used his findings to promote Copernicus’ heliocentric model. Galileo’s discoveries were met with opposition from the Catholic Church and as a result, many educated people still believed that the Earth was at the centre of the universe. Books, teachings and the defending of the heliocentric theory were banned.
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An early illustration of Copernican theory shows the Sun at its heart
Galileo enjoys a night of stargazing from his pioneering telescope, circa 1620