How Galileo’s telescope changed our view of the Solar System forever
From the geocentric to heliocentric model.
The year was 1608 and the first telescope had just been invented, intended to be used to see far away objects across land. But when word of this invention reached the Italian scientist Galileo Galilei, he thought to use it to look at the skies.
Galileo was quick to master the art of making his own telescope, which could magnify objects by up to 20 times. The general consensus at this time was that the Sun orbited around the Earth, but with the help of Galileo’s observations, our cosmic perspective changed forever.
The idea that the Sun and other cosmic bodies orbited around the Earth was called the geocentric model. Many of the ideas behind the geocentric model came from the ancient philosopher Aristotle and ancient astronomer Ptolemy.
Aristotle believed all of the heavenly bodies were perfect spheres, with perfectly smooth surfaces and without blemishes. This made them different to the Earth, which he believed was imperfect as it had mountains, valleys and ridges.
The discoveries that Galileo made using his telescopes helped to prove that Sun was the centre of the Solar System and not the Earth. His observations strongly supported a Sun-centred model known as the Heliocentric model, previously suggested by astronomers like Nicolaus Copernicus.
An imperfect Moon
In 1609 Galileo pointed his telescope skywards to study the phases of the Moon and created very accurate sketches of his observations.
These sketches showed that the lunar surface was rough rather than smooth, full of different areas of dark and light.
But if the Moon’s surface wasn’t smooth, this contradicted Aristotle’s notion that all heavenly bodies were perfect spheres. It meant that objects in the sky were just as varied and ‘imperfect’ as the Earth.
We now know that the surface of the Moon is cratered by billions of years of meteorite impacts, and is marked by mountains and ancient lava flows.
Astronomers believe that the Moon was created from the Earth after a giant impact, suggesting its origins are even more terrestrial than Galileo supposed.
The phases of Venus
When Galileo turned his telescope to the inner planet Venus, he observed that Venus goes through phases, similar to the Moon. Depending on where it was in relation to the Sun and the Earth, it was either a crescent, half lit, or full.
But if the geocentric model was true, and the Earth was in the centre with the Sun further out, these phases of Venus would not be seen. A Sun-centred model was a far simpler way of trying to explain why the phases occurred.
Although the extreme crescent of Venus can be seen using the naked eye, there are no historical recordings of the crescent before Galileo observed them.
Before Galileo pointed his telescope at Jupiter, it was thought that our Moon was the only one that existed. But in 1610 Galileo found that not only did Jupiter also have a moon, it had four!
The discovery of these moons around Jupiter showed that celestial bodies could orbit around objects other than the Earth.
Since Galileo’s time many more moons of Jupiter have been discovered, and at the time of writing there are 79 known moons of Jupiter. Most are named after either the lovers, daughters, or other mythological characters linked to the Roman god Jupiter.
The Galileo mission, named in the scientist’s honour, arrived at Jupiter in 1995 and studied the planet and its moons. Many of the key discoveries made by the mission were related to the four moons discovered by Galileo, which are now collectively known as the Galilean moons.
Among other highlights, the Galileo mission is famous for discovering that the Galilean moon Europa is likely to have a salt water ocean under its icy surface.
When Galileo turned his attention to Saturn in 1610, he found something else unexpected. Saturn appeared to have a band around it, showing yet another example an imperfect heavenly body.
Of course, we now know that this band is Saturn’s impressive ring system, however Galileo was unaware of the true nature of this structure and described these rings as the planet’s ears.
Saturn is tilted 27 degrees with respect to its orbit around the Sun. Because of this, the amount of Saturn’s rings we can see from Earth varies as it goes around the Sun.
Approximately every 15 years, Saturn’s rings turn edge on to the Sun and the rings seem to disappear. The last time this happened was in 2009. However, it also happened in 1612, just two years after Galileo had discovered the rings. Galileo announced that Saturn had swallowed his children. This is a fitting description because Saturn was the Roman version of the Greek god Cronus, who is infamous for eating all his children.
We now know that all four of the Gas Giants in the Solar System have rings around them. However, Saturn’s are still the most impressive. They are made of small pieces of ice and rock. They are thought to have be pieces of comets, asteroids or broken up pieces of moon.
Spots on the Sun
Although Galileo was not the first person to observe spots on the Sun, he used his telescope to make detailed observations of them.
Sunspots are areas of the Sun’s surface that appear darker because they are cooler. Some of Galileo’s contemporaries believed the sunspots could be small satellites between Mercury and the Sun but Galileo showed that the spots changed shape and size and sometimes disappeared, therefore couldn’t be satellites.
Galileo suggested that sunspots were instead either on or very close to the surface of the Sun almost like clouds in the Earth’s atmosphere. This prediction suggested the Sun was imperfect, which again contradicted the Aristotelian view that the heavens were perfect.
Although Galileo was not the only scientist making these discoveries, he wrote his work in Italian, rather than Latin or Greek. This meant that his work was far more widely available to the public. This allowed for his work to spread further and faster, and is a key way that the heliocentric view of the Solar System spread.
During Galileo’s lifetime, the heliocentric model was considered heretical in the eyes of the Catholic Church and Galileo was banned from continuing to write on the subject.
However, in 1632, Galileo published a book exploring the difference between the heliocentric and the geocentric models. He then was tried for going against the Church, was placed under house arrest, and was forced to recant his beliefs.
About the author: Elspeth Lewis is a physicist and a member of the Education Team at the National Space Centre.