People have wondered what happens inside the Earth for probably as long as there have been people. Throughout history, many cultures believed that the Earth’s interior featured great hollows that hid mystical realms — things like lost civilizations, lakes of fire, or hungry dinosaurs. But most of these ideas were rooted in tradition, not science. Then, in the 17th century, an English scientist came along, and using some evidence, proposed that the Earth was, in fact, hollow.
Today, we know this idea was pretty wrong, but that doesn’t mean it wasn’t important. This work sparked interest in Earth science, and it helped pave the way for some of the great scientific theories that rose up during the Age of Enlightenment. Also, some parts of it were actually kind of right. The scientist who proposed this was Edmond Halley. He was born in 1656, and was into all things science. Halley was an astronomer, geophysicist, and mathematician, and he’s most famous for calculating the elliptical orbit of a comet — one that was later named after him. He also helped his friend Isaac Newton publish his groundbreaking work now often called the Principia, which showcased Newton’s laws of motion. But all that aside, Halley was also really interested in magnetic declination: the difference between the Earth’s magnetic north and true north.
Also called geographic north, true north is where the lines of longitude meet in the Arctic Circle — the real, northernmost point of our planet. Magnetic north is different. It’s the most northerly part of Earth’s magnetic field, and it’s where a compass points when at rest. Because our magnetic field changes all the time, the magnetic north moves, so these norths usually don’t line up. Nowadays, you can use charts or maps to figure out the difference. But in the 17th century, no one knew why the Earth’s magnetism changed, so they couldn’t account for it. If someone could, that would have saved countless ships from getting lost. And Halley wanted to be that someone. He spent decades studying magnetic readings taken from places across the world — ranging from Paris, France to St.
Helena in the south Atlantic. In general, these measurements were pretty solid, and like others before him, Halley found that Earth’s magnetic field seemed unpredictable. Then again, he also concluded that our planet has four magnetic poles — two in the north, and two in the south. But, you know, it was the 1600s. To explain his observations, Halley concluded that something had to be moving deep underground and affecting Earth’s magnetism. So he started building a full hypothesis.
He finished what came to be called his hollow Earth theory in 1692. But it was a little… unconventional. Halley proposed that the Earth actually contains three hollow, concentric shells. These shells rotate independently on the same axis as the Earth, but they have magnetic poles that are a little misaligned. And as the shells spin, that misalignment disrupts the Earth’s magnetic field on the surface. Halley figured that the shells were comparable in size to Venus, Mars, and Mercury, and that they didn’t crash into each other because they were held in place by gravity, kind of like Saturn’s rings are.
He also suggested that the shells had to be lined with magnetized metals or materials, since the resulting magnetic force would prevent them from caving in. You have to admit, it was well thought-out. Halley even got the idea about the Earth being hollow from his good pal Newton. In the Principia, Newton wrote that that Moon is significantly more dense than the Earth. Today, we know that was based on some bad physics, but back then, Halley interpreted it to mean the Earth was hollow. After all, that would explain why it was less dense, even though Earth is much bigger than the Moon. Except, this was wrong. During the 18th century, the mathematician Charles Hutton proved that the Earth was solid by calculating its mean density. Using data from previous research, he found that it was about times as dense as water — which wasn’t far off. Today, we know it’s about times as dense. But either way, that still made the Earth too dense to be hollow. Sorry, Halley. Now, even though he was wrong about… a lot… Halley was at least partly on the right track: The Earth does have layers.
They’re not magnetized shells or anything, but earthquake research in the 1920s and ‘30s did reveal that our planet has a solid inner core and a fluid outer one. That liquid core — which features a whole bunch of nickel and iron — churns over time. And that movement is what causes disruptions to the magnetic field. So, even if he was wrong about the details, Halley was partially right: Something is moving down there, and there are layers within the Earth. That’s kind of impressive, considering all the things we didn’t know in the 1600s. Also, while Halley’s idea can seem pretty silly to us now, his work inspired others to explore the Earth even further, and led the way to modern theories and discoveries. It all goes to show that science is a process, and the stuff that seems ridiculous in hindsight help pave the way for where we are now. Thanks for watching this episode of SciShow! We believe that understanding the world is a great way to understand ourselves — which is why it’s cool to cover old hypotheses like this one.
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