By THOMAS STEVENSON.

I’m going to start this episode bluntly. There is no room for delusion here, only cold, hard facts. So here is the fact I wish to share with you. The Earth, that big rocky ball you live on, moves around the sun.

This is accepted by pretty much everybody now. The simple statement that we go around the sun is almost the definition of common knowledge. Don’t be ashamed if this is the first time you’ve heard it though. Sherlock Holmes didn’t know either!

However, there is nothing trivial about that fact. It took centuries of head scratching, number crunching and thumb twiddling to come to this conclusion. There was even a time when suggesting this kind of thing would get you exiled. That’d be like being kicked out of New Zealand for not liking rugby.

Way back in ye olden days (from about 2,000 to 400 years ago), people believed that Earth was at the centre of the universe. It was only logical; in the night sky they could see countless stars that seemed to go in circles around the Earth. There was no reason to suspect any differently. It was also pleasing to religious folks, who claimed that a divine power had placed Earth at the centre of everything.

We call this the geocentric model of the universe. It was neat, it made sense at the time, but it was very wrong.

It took a few brave souls, including Nicolaus Copernicus, to get the truth out there. Through scientific observations of the night sky, they worked out that Earth could not be at the centre. It moved around the sun in an orbit, they said. Earth was just another planet. This is the heliocentric model and its development sparked a major revolution in science.

You see, not everything in the night sky behaves as the geocentric model predicted. The Moon, the sun and the stars appear to move around us like clockwork. But the other planets in our solar system – Mars, Venus and so on – have been trolling us the whole time.

It’s hard to spot, but the planets actually double back on themselves as they travel across the sky. There are composite photos of Mars that show it making loops like a dog chasing its tail. They don’t do this overnight, but over a couple of weeks or a month. That’s why you need to take a composite photo to see the effect, with one snapshot each night.

After Copernicus, astronomers got serious about measuring the motions of the planets. Tycho Brahe was one of them. He wanted to sort out, once and for all, how everything was arranged in the solar system. Brahe is famous today for two things: his incredibly detailed measurements and his pet moose. I’m only going to focus on the measurements here, but if you’re really curious, there is plenty of reading on his moose.

Brahe spent years collecting data on the motion of planets, especially Mars. He did it all with primitive devices like the astrolabe, for the telescope hadn’t been invented! However, at the end of all this effort, he realised that he needed help. The data itself need to be sorted through and analysed to create a new, accurate model of the solar system.

So he hired Johannes Kepler to help with this monumental task. If Kepler were alive today, he’d be the king of maths nerds. His job was basically to go through chunks of Brahe’s data and do lots and lots of calculations with it. All without even the most basic Casio.

In the end, Kepler came up with a solid theory. He wrote down three laws of planetary motion that explain how planets move around the sun. These laws are a bit mathematical for this article, but if you’re interested you can find plenty of videos about them.

It was Kepler who proved beyond all doubt that the Earth orbits the sun, like a normal planet. Its orbit is shaped slightly like an oval, not a perfect circle as Copernicus had assumed. Getting to this point had taken centuries of hard work and bright ideas. The men I’ve mentioned in this article were all amazing scientists who worked toward an understanding of the universe.

Later on, it was down to Sir Isaac Newton to explain why planets move in orbits. Newton is well-known for his theory of gravity, among other things. The story goes that he was sitting under an apple tree one day, when an apple fell and hit him in the head. Thinking about what caused the apple to fall, he had a “eureka” moment and rushed off to write about gravity. That’s the story; personally I don’t think it happened that way.

Newton’s real insight was to realise that gravity also affects the Moon and the planets. It’s always been known that the Moon revolves around the Earth. It is in its own orbit around us. Gravity is the force that keeps it there, moving in a round path for all of time. The sun’s gravity keeps all the planets in their orbits. We still don’t know what creates gravity itself – that’s a topic for another episode. But for now, it looks like we’ve finally sorted out how the solar system works.

That is how we got from the geocentric model to today’s level of understanding. These days the fact that Earth goes around the sun is assumed common knowledge. However, it’s worth taking a moment to appreciate the sheer amount of effort that went into this theory we have.

Even with all our fancy modern equipment, science is still all about hard work. In the next episode I’ll be looking at this year’s winners of the Nobel Prize, and you’ll see just what it takes to be at the top of your field. See you then!

 

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