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What did Galileo discover

Galileo Galilei was born over 450 years ago in February 1564 in Pisa, Italy. Often called the ‘Father of Observational Astronomy’ he literally changed the way people not only thought about the night sky, but with the invention of the telescope, how people saw the night sky.

The first ancestors of man started roaming the planet around 70,000 years ago. Considering Earth is now 4.5 billion years old, it’s merely as blink of an eye (or 0.00001% of Earth’s age). Since the dawn of mankind, we have been observing the stars, planets and anything else we could see.

In those days of course there was none of the annoying light pollution we have today. Our earliest ancestors view of the night sky from any state or city would be much better than the majority of us will ever see.

It’s astonishing to think that up until the year 1610 we never knew that Jupiter had moons!

But on a cold and dark night, 7th January 1610, Galileo saw 3 ‘small stars’ in line around Jupiter (they are four obvious moons of Jupiter, but we’ll come on to why Galileo only saw three shortly). Originally thinking these are just small distant stars along a similar line of sight, he spent the next few nights observing. He noticed the alignment of these ‘stars’ changed, and that one disappeared from view.

He concluded that these ‘stars’ must be orbiting Jupiter and therefore these were not stars at all, but in fact moons of Jupiter, dancing around the planet just as our moon does to Earth.

Why though couldn’t Galileo see the main four moons of Jupiter? – because on that winter night, 7th January 1610, the fourth moon was hidden behind Jupiter, so out of sight.

Spot Jupiter through any small telescope, and you will see at least two or three of its largest moons, and if you’re lucky, you’ll see all four!

The largest four visible to us all, through a telescope or strong binoculars, are Io, Ganymede, Calisto and Europa. Jupiter has a total of 79 known moons, but new small moons that have been grasped by the huge gravity of Jupiter, are regularly being discovered.

Along with the telescope, and the discovery of Jupiter’s four largest moons, here is a list of all the discoveries made by Galileo Galilei.


Discoveries made and inventions created by Galileo


Refractor Telescope

Galileo is often credited with inventing the telescope. Whilst this is not strictly true, and credit for this should rightly go to Dutch lens maker, Hans Lippershey in 1608, what Galileo is credited for is enhancing on the concept and design of the telescope. He enhanced the structure and design of the lenses and significantly increased its magnification – allowing him to see the moons of Jupiter (as we’ve already mentioned) along with craters and mountains.



Rings of Saturn

Through Galileo’s modifications to the telescope he saw, for the first time in human history, the rings around Saturn. The positioning of the rings at the time towards Earth wasn’t great – almost head on – and what Galileo saw confused him.

No-one had thought of such a concept before. There are no rings around our moon, or the Earth or any of the closer planets such as Mars or Venus, so no one was expecting anything of the kind around Saturn.

Originally thinking the rings were additional planets around Saturn he continued to observe until the shape of the rings changed slightly, blowing the theory of additional planets out of the water.

It took a number of years before the conclusion that these were dust and debris particles in orbit around the large planet.

If you have yet to see the rings of Saturn for yourself, I would enthusiastically recommend you go do so any way you can!


Moons of Jupiter

We have touched upon the moons of Jupiter already in this article, so I won’t go in to too much depth here, but for those that have skipped the beginning and ventured straight to the discoveries, suffice to say that Galileo was the first person to see Jupiter’s four largest moons – Io, Ganymede, Calisto and Europa.



Moon Craters and Moon Mountains

Although Galileo wasn’t the first person to see the moon through a telescope, he was the first person to conclude that the uneven surface and shadows from the sun must be the result of craters – caused by impacting comets and asteroids – and also mountains, just like we would have here on Earth, and those discovered on other planets such as Mars and on dwarf planet Pluto.

By noticing the shape and transit of the sun across the Moon’s surface, he could determine not only which shapes were crates and which were mountains, but also relative sizes of each.



Venus Phases

Armed with his telescope he was able to make out all sorts of amazing and wonderous things in the sky that were not known of before. It must have been such an exciting time. To see and try to make sense of things no person ever had.

With his telescope he could now start seeing detail, and one such detail was the phases of Venus. Think of our own moon. As it travels around the Earth, different sides of the moon are illuminated. The moon is what is known as ‘tidally locked’ with the Earth.

What this means is that here on Earth we only ever see one side of the moon. It’s crazy to think about considering the moon is spinning. The moon spins relatively slowly on its axis. It slowly spins on its axis as it slowly orbits Earth – and the duality means we only ever see one side of the moon.

Therefore, as the Sun sees and shines on different parts of the moon, and we always see the same side, we see the different phases of the moon.

From a full moon, where the side facing us is also facing the Sun – to a new moon, where the sun is only shining on the part of the moon we can’t see. Each night different parts of the moon are illuminated, and these parts are called Phases.

Just as we can easily see this on the moon, the same thing happens on the inner solar system planets such as Venus. Venus isn’t tidally locked, but the Sun will shine on different parts of Venus (i.e. day and night) as it rotates and depending on which part of Venus is facing us we may see Venus fully illuminated or just part illuminated.



Milky Way

If you’ve looked at the night sky from a reasonably dark location, you can’t fail to see the Milky Way. The spiral arm of our home Galaxy arcs across the sky.

Of course Galileo didn’t discover the Milky Way, as it’s visible to the naked eye so has been looked upon and fascinated over for thousands of years, but using the telescope he was able to discover the light wasn’t just a wispy column of light in the sky, it was made up of millions of stars




Its no wonder Galileo Galilei was, and still is, a celebrated man. Although many of these discoveries have been with us for hundreds of years it’s still amazing to think how much we have grown is such a short space in time.

It took mankind 70,000 years to build the technology to see and know Jupiter has moons, and then just another 400 years to visit and land on it using spaceships.

A thought-provoking idea is where we will be in the next 400 years!

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