[♪ INTRO]
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 4.5 times as dense as water
— which wasn’t far off.
Today, we know it’s about 5.5 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.
If you want to help us keep making more episodes
like this, and support free science education
online, you can go to patreon.com/scishow.
And to all of our current patrons — thank
you!
We couldn’t do this without you.
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