The Science of Discworld IV

Ptolemy’s crystal spheres, and like that theory, it explained a great deal and was completely wrong.
    Pseudoscience got in on the act in a big way in 1818, when John Symmes advanced a similar model, in which the outer shell was 1300 kilometres thick with huge circular openings at both poles. Inside were four more shells, also with polar apertures. You have to remember that this was seventy-seven years before the Norwegian explorers Fridtjof Nansen and Hjalmar Johansen reached latitude 86˚ north in 1895, and ninety-one years before Robert Peary reached the North Pole in 1909 – or, as now seems plausible, got very close but maybe didn’t quite make it. Symmes agitated for a polar expedition, and his follower James McBride seems to have convinced the US President John Quincy Adams to authorise and fund it. But the incoming President Andrew Jackson put a stop to the idea.
    In 1826 McBride published
Symmes’ Theory of Concentric Spheres
, and a flurry of similar theories and books quickly followed. Among them were the 1906
Phantom of the Poles
by William Reed, which abolished the secondary shells inside, and Marshall Gardner’s 1913
A Journey to the Earth’s Interior
which sported an interior Sun. As late as 1964 the (probably pseudonymous) Dr Raymond Bernard’s
The Hollow Earth
proposed that our planet’s open interior is the source of UFOs. It also explained what happened to Atlantis, and indeed was where the Atlanteans fled to when their continent disappeared. Rather desperately, the book referred to the Ring Nebula as proof that hollow worlds exist. This structure, just over one light year across and 2300 light years away, is an expanding shell of gas expelled by a red giant star on its way to becoming a white dwarf.
    Mapmaking can’t distinguish the geometry of the interior of a sphere from that of the exterior, although differences arise as soon as the surface extends into the third dimension. The peaks of mountains would be closer together if they were inside the Earth. Not surprisingly, there are some big problems with Teed’s theories. Many can be resolved by special pleading, such as strange refractions of light, but these extra features come very close to reformulating conventional physics in an inverted frame of reference, and have no serious substance. Centrifugal force doesn’t work as a substitute for gravity, because it always acts at right angles to the planet’s axis of rotation. The perceived force would be zero at the poles, and only at the equator would it act in the observed direction, at right angles to the surface. The oceans would migrate to form circular pools at the poles, hundreds of kilometres deep. A central Sun would lead to rapid overheating. A large open interior would block seismic waves from earthquakes, contrary to observations. Smaller caverns would not be a problem in this regard, though. Satellite measurements of gravity wouldn’t work, and neither would satellite orbits.
    Fiction is unconstrained by mere facts, and there are many fictional depictions of a hollow Earth. An early example is
Niels Klim’s Underground Travels
, published by Ludvig Holberg in 1741. The hero falls through a hole in the Earth while caving, and lives on the inside of the outer shell of the planet, and on a separate central ball. In 1788 Giacomo Casanova wrote a five-volume blockbuster
Icosaméron
about a brother and sister who discover a race of hermaphrodite dwarves inside a hollow Earth. Symmes’s pseudoscience found afictional outlet in Captain Adam Seaborn’s 1820
Symzonia: a Voyage of Discovery
. The most familiar story of this subgenre is Jules Verne’s 1864
Journey to the Centre of the Earth
, which has inspired a number of movies, only loosely related to the original. The novels that come closest to a genuine hollow Earth are those in Edgar Rice Burroughs’s
Pellucidar
series, beginning with
At the Earth’s Core
in 1914, where the Earth’s surface is an 800 km thick shell illuminated by a central Sun, with numerous species of quasi-intelligent and intelligent beings living on the inside surface. The hero ends up in Pellucidar when his mechanical mole refuses to turn and burrows directly downwards into the Earth.
    In recent times, hollow worlds have turned up in the media and in computer games.
    We promised an unorthodox but solid proof that the Earth is round. Not satellite photos: those are fakes, you understand – NASA never managed to get satellites in orbit, or if they