The Hollow Moon and the closely related Spaceship Moon are pseudoscientific hypotheses that propose that Earth's Moon is either wholly hollow or otherwise contains a substantial interior space. No scientific evidence exists to support the idea; seismic observations and other data collected since spacecraft began to orbit or land on the Moon indicate that it has a thin crust, extensive mantle and small, dense core, although overall it is much less dense than Earth.
The first publication to mention a hollow Moon was H. G. Wells' 1901 novel The First Men in the Moon. The concept of a (partially) hollow Moon has been employed in science fiction multiple times. In 1970, two Soviet authors published a short piece in the popular press speculating that the Moon might be "the Creation of Alien Intelligence". Since the late 1970s, the hypothesis has been endorsed by conspiracy theorists like Jim Marrs and David Icke.
Introduction
The Hollow Moon hypothesis is the suggestion that the Moon is hollow, usually as a product of an alien civilization. It is often called the Spaceship Moon hypothesis and often corresponds with beliefs in UFOs or ancient astronauts.
The suggestion of a hollow moon first appeared in science fiction, when H. G. Wells wrote about a hollow Moon in his 1901 book The First Men in the Moon. The concept of hollow planets was not new; The first discussion of a Hollow Earth was by scientist Edmond Halley in 1692. Wells borrowed from earlier fictional works that described a hollow Earth, such as the 1741 novel Niels Klim's Underground Travels by Ludvig Holberg. Both Hollow Moon and Hollow Earth are now considered to be fringe theories or conspiracy theories. The concept of the Moon as a spaceship is often mentioned as one of David Icke's beliefs.
Claims and rebuttals
Density
The fact that the Moon is less dense than the Earth is advanced by conspiracy theorists as support for claims of a hollow Moon. The Moon's mean density is 3.3 g/cm3, whereas the Earth's is 5.5 g/cm3. Mainstream science argues this difference is due to the fact that the Earth's upper mantle and crust are less dense than its heavy, iron core.
The Moon rang like a bell
Between 1969 and 1977, seismometers installed on the Moon by the Apollo missions recorded moonquakes. The Moon was described as "ringing like a bell" during some of those quakes, specifically the shallow ones. This phrase was brought to popular attention in March 1970 in an article in Popular Science.
On November 20, 1969, Apollo 12 deliberately crashed the Ascent Stage of its Lunar Module onto the Moon's surface; NASA reported that the Moon rang 'like a bell' for almost an hour, leading to arguments that it must be hollow like a bell. Lunar seismology experiments since then have shown that the lunar body has shallow moonquakes that act differently from quakes on Earth, due to differences in texture, type and density of the planetary strata, but there is no evidence of any large empty space inside the body.
Vasin-Shcherbakov "spaceship" conjecture
In 1970, Michael Vasin and Alexander Shcherbakov, of the Soviet Academy of Sciences, advanced a hypothesis that the Moon is a spaceship created by unknown beings. The article was entitled "Is the Moon the Creation of Alien Intelligence?" and was published in Sputnik, the Soviet equivalent of Reader's Digest. The Vasin-Shcerbakov hypothesis was reported in the West that same year.
The authors reference earlier speculation by astrophysicist Iosif Shklovsky, who suggested that the Martian moon Phobos was an artificial satellite and hollow; this has since been shown not to be the case. Skeptical author Jason Colavito points out that all of their evidence is circumstantial, and that, in the 1960s, the atheistic Soviet Union promoted the ancient astronaut concept in an attempt to undermine the West's faith in religion.
"Perfect" solar eclipses
In 1965, author Isaac Asimov observed: "What makes a total eclipse so remarkable is the sheer astronomical accident that the Moon fits so snugly over the Sun. The Moon is just large enough to cover the Sun completely (at times) so that a temporary night falls and the stars spring out. [...] The Sun's greater distance makes up for its greater size and the result is that the Moon and the Sun appear to be equal in size. [...] There is no astronomical reason why Moon and Sun should fit so well. It is the sheerest of coincidence, and only the Earth among all the planets is blessed in this fashion."