Raising the question of the possibility that the Earth might be hollow at its center is not a crazy idea after all. At its most elementary level, the presence of caverns and caves all around the world leads us to ask ourselves how deep they can be, and even the possibility of finding a sufficiently big underground chamber crossing the whole planet from one side to the opposite side.
Let us recall that, at the beginning of the modern atomic theory, the atoms were still mentally conceived as some kind of microscopic solid spheres until Niels Bohr introduced the atom model of a small positive charge surrounded by electrons traveling in orbits around it. Until then practically nobody imagined that all the atoms are essentially hollow spheres; nobody imagined that every atom is almost "empty".
On the other hand, the solar system is also an almost empty structure. So it is easy to carry this model to other systems as if the nature were obliged to stick to one working system and to use the same bodies configuration everywhere.
Among the recent philosophers and scientists, Edmond Halley (1656-1742) is one the most renowned for having at some time during his life being an advocate and supporter of the notion of a hollow Earth. However, such and idea has a long trail because in somewhat rudimentary ways the same conception was already proposed by Plato, Aristotle, Lucretius, Seneca, and Dante.
Halley is best known for his prediction of the return of a comet, now familiarly recognized as: the Halley's comet. Throughout all history comets have made their unexpected appearances frightening people, and in most cases fearing for bad omens. But possibly, nobody thought that some of those bad portents were one and the same comet. It doesn't take to be an astronomer to have realized that the same comet was visiting us periodically. Maybe an acute-minded historian could have done the same prediction; at least a rough approximation.
It took the analytic mind of Edmond Halley to realize that among all the comet stories and observations, one in particular was the same comet returning after a 75-76 years period. This is a landmark in itself, however, it was also an extension of the fact that the planets are also periodic, making them sometimes visible and sometimes not.
What make comets to return is that they follow an elliptic orbit; some kind of path like the other planets follow, but greatly "flattened". During a complete orbit, they come near the Sun and that's the reason why they are visible for a short time. Once they go away from the Sun, it may take hundreds of years to come again. However, it was not Halley the first to propose that the comets move in elliptical orbits; this was proposed by Giovanni Borelli and Robert Hooke in 1665, and earlier than that, by Sir William Lower in 1610.
At right is shown a portion of the famous Bayeux tapestry (a story 230 ft long), first exhibited at the Bayeux cathedral in 1077, that includes the passing of a comet (shown at top near the center). The Bayeux tapestry is the story of a battle, but in its details is embroidered the moment that a comet is crossing over the sky (Click on the figure to see it larger). The comet was possibly visible during several nights. Note that the embroiderers that worked with the tapestry did not matter including the comet as if it were visible during the daytime. This may be due to the fact that, under some circumstances, and specially a big one as this, a comet can be visible in daytime. There are some spectacular historical comets, like the Great Comet of 1811 (Flaugergues), that was observed for 17 months.
The cometary phenomena has been recorded in many ways through history by many civilizations. The inclusion of a passing comet in the Bayeux tapestry is an unusual example of this.
The word "comet" comes from the Greek kometes that means "hairy one". In the Bayeux Tapestry, the comet is depicted as such: the comet is a star with hairs; making more honor to the tradition than to the spectacle, because surely the comet tail was very long. The comet we see in the artwork is the same one that later Halley recognized as a recurring comet.
Halley became instantly famous for his prediction, but he was a man of many fields of investigations, and not only astronomy science. Among his other interests, he was a geophysicist. As geophysicist, he was interested in the magnetic field of the Earth. While compiling data for some geomagnetic plots, and because the data near the Earth's poles were difficult to obtain, and because of his compass abnormal readings, he arrived at the wrong conclusion that the Earth interior was not fully solid (or at most, of uniform density). Because of this, he put forth the idea that the exceptional and unusual compass readings were caused by some interference from the interior of the Earth. Somehow, there could be other spheres within the Earth, with their own magnetic poles that were intervening with the two Earth poles:
The Globe of the Earth might be supposed to be one great Magnet, having four Magnetical Poles or Points of Attraction.
Here we have the case of a brilliant scientist jumping to a weird conclusion in order to justify the compass readings he was collecting.
But Halley's opinion about a hollow Earth was not due only to his account of the Earth's magnetic field. There was another important factor influencing this theory: it was the computation that Newton made of the Earth to Moon mass ratio in his Principia Mathematica. From the Newton's computation of this ratio, and from the geomagnetic lines deviation was that Halley arrived at the conclusion that the Earth must hollow, because whatever was causing the deviation must ...
turn about the Centre of the Globe, having its Centre of Gravity fixt and immoveable in the same common Centre of the Earth, but detached from the external parts.
Halley made some astonishing computations: the Earth's outer crust thickness must be some 500 miles with an air gap of the same thickness between it and the inner sphere. But how can this happens without the inner sphere colliding with the outer shell? Simple —at lest for him: just see the example of Saturn and its outer rings; see how comfortable they survive for millions of years! By analogy, there could be another concentric globe inside the hollow Earth.
But what about the oceans? Could the seas' water leak away by some earthquake? For how much time the Earth could be hollow? Don't mind about that: within the ground there existed some kind of "vitriolic particles" that can contribute to rocks formation so that water flowing down would soon find its path blocked.
The theory of a hollow Earth that Halley published in 1692 was welcomed by several scientists of his time to such an extent that1:
William Whiston, a mathematician and cleric that served as Newton's assistant and the successor as Lucasian professor at Cambridge, not only accepted Halley's theory, but believed the sun, other planets, and comets to be hollow and inhabited as well.
Yes, for Whiston, and many other "scientists" and philosophers of the time, the Earth was not only hollow, but inhabited as well.
Newton never supported the idea of an onion-like Earth.
About a century before Halley, Johannes Kepler created his famous model for the orbits of the known planets. He inscribed and nested each orbit (sphere) in proportion to the planet's orbit diameter. Thus he inscribed Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Kepler was searching for some kind of divine intervention in the planet's position and orbit size. There must be a geometrical plan for the universe. The configuration he achieved was published in his book: Mysterium Cosmographicum (The Cosmographic Mystery), published in 1595.
If we compare Halley's attempt for explaining the possibility of the Earth being a hollow sphere, with the spheres being in proportion with the Mercury, Venus, and Mars diameters with the work of Kepler, we can immediately see a striking similarity between both. Halley must have know Kepler's Mysterium ..., so what Halley did was a forced continuation of what he found in Kepler's book. But there is a clear difference between both astronomers: Kepler's work is based in true orbits of true planets, while Halley's concentric spheres is not based on anything real, or observable. At least in this aspect Kepler was the genius, Halley the imitator.
Halley's reasoning was not always scientific as we may expect: he used Newton's Principia (published with the backing and support of Halley) for some of his calculations and some other theological reasoning for others.
A hollow planet with a concentric spheres within is more optimal for supporting life; twice the chances for life developing on the outside and on the inside fields. There is more surface to populate with two spheres than with one sphere. The "Almighty Wisdom" —said Halley— have arranged it this way. That's why he said:
I have adventured to make these Subterranean orbs capable of being inhabited.
The matter thus distributed is
to yield as great a Surface for the living Creatures as can consist with the conveniency as security of the whole.
The Polar auroras are beautiful and overwhelming and colorful sky phenomena. On March 6 of 1716, an extraordinary event was visible in the northern skies of Europe, and this time it was not a comet. The Auroras are quite frequent in the regions of the North and South poles, but one visible in the lower latitudes are infrequent. Halley observed the phenomena of this year and lost no time in finding a convincing explanation: the auroras are
luminous vapors escaping from the Earth's center through the thin crust of the polar regions.
When in 1572 a supernova was suddenly visible in the sky, Galileo ventured an explanation for this phenomena saying that its sight was the product of some kind of Earth effluence that will gradually disappear with its distance. So, we see that urge of producing sudden and appealing explanations for daily activities or rare events is part of the search for the truth.
Halley had no proof of what caused the auroras, so venturing an explanation without previous studies and polar explorations was an act of scientific irresponsibility. More than that, he was forcing an interpretation of an event to sustain and feed a theory that lacked any physical demonstration.
Obviously his explanation and rationale was an effort to reinforce his slowly decaying theory of a hollow Earth, because Newton and some other never supported him with this kind of mental divagation.
E. Pérez
10/10
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