AKA: Worlds within worlds.
Theory born: Ancient/1692
Parent(s): Edmond Halley
Edmond Halley, Astronomer Royal, b. 1656, d. 1743
Description: The idea that the Earth is hollow and that there exist on the surface passages to take explorers below is an ancient myth, cropping up in places as far apart as the underworld of the ancient Greeks to Patala of Hindu mythology to the Native American peoples. Many of them too suggest that people or creatures live within the Earth and periodically emerge onto the surface, sometimes with benign aims and other times not.
In 1692, Halley developed his theory that the Earth was made of an 800km thick outer shell, two concentric inner shells and an inner core, similar to the diagram below.
Halley's Hollow Earth
As ridiculous as it sounds to us now, Halley was a brilliant man and had evidence to support this startling theory. When Isaac Newton, a friend of Halley's, published his Principia, Newton made some unwarranted assumptions about the effect of the Moon on the Earth's tides when determining the Moon's relative mass and came to the conclusion that the ratio of the Moon's mass to the Earth's mass as 1:26 which, given their sizes, gave them relative densities of 9:5. In other words, the Moon was much denser - more solid - than the Earth. Halley suggested this could be explained by the Earth being hollow,
He extended the theory further. It was well known that the Earth's magnetic field lines wandered and fluxed over time. If the Earth was solid, then the cause of that magnetism would have to be moving through solid rock - clearly impossible. What if then, that the Earth contained other spheres, also moving, and it was their influence that caused the fluctuations of the Earth's magnetic field. He also posited that the gap between the spheres was filled with a luminous gas and it was the leakage of this gas through a hole in the Earth's north pole that caused the aurora borealis.
Halley himself was somewhat unsure of his theory and wrote “If I shall seem to advance anything that looks like Extravagant or Romantick, the Reader is desired to suspend his censure, till he have considered the force and number of the many arguments which concurr [sic] to make good so new and so bold a Supposition.”
His uncertainty was justified as the theory had to be made more complex to withstand scrutiny. The spheres within the Earth did not collide because gravity held them all perfectly in place; gravity didn't cause the Earth's outer shell to crumble into the spaces below because its inward face was lined with 'magnetic materials'. Why didn't the oceans leak away below? Well, the inner spaces contained "Vitrolick Particles" which would plug the leak.
It's safe to say that the theory didn't become popular, but Halley never really abandoned it, When he sat for his portrait as Astronomer Royal at age 40, he included a drawing of this theory for all to see.
Theory death: 1774
Cause of theory death: Incompatible with the results of the Schiehallion experiment.
The Schiehallion experiment took place at the Schiehallion mountain in Scotland. Essentially, the idea was to hang a pendulum near the mountain. As all masses exert a gravitational pull and large masses like a mountain exert a larger pull, the theory was that a pendulum hung near a mountain should show a tiny deflection from true 'down'.
Schiehallion was chosen because of its isolation from other mountains and its relatively symmetrical shape. The mass of the mountain was then determined by approximating its volume (by painstakingly surveying the mountain and dividing its slope into vertical prisms) multiplying it by the density of the rocks found there (which assumed that the mountain had uniform density, of course). The mass of the mountain and the deflection of the pendulum from true were then used to calculate the mass of the Earth.
It was found to not only be too high to support the theory of a hollow Earth, but also gave a density for the Earth much higher than that of the mountain. Clearly, there was something else aside from just rocks in the Earth beneath their feet.
Schiehallion mountain, Scotland
Attributions
Halley's Hollow Earth - Rick Manning, Wiki Commons
Schihallion mountain photograph - Andrew2606, Wiki Commons
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