The Strange Origin of the Hollow Moon Conspiracy Theory

Jessica Coulon
Tue, October 18, 2022 

Smartshots International - Getty Images

The hollow moon conspiracy theory came about during the Apollo missions in 1969.

Conspiracy theorists misinterpreted the results of the astronauts’ seismic experiments, leading them to believe the moon was hollow.

Scientists said the moon rings “like a bell.” That’s because the vibrations from the moon’s seismic events, known as moonquakes, last much longer than those on Earth.

Conspiracy theorists once believed that the moon was hollow. Though that’s more likely than the moon being made out of cheese, it still seems pretty ridiculous by today’s standards. So where did that hollow moon theory—or rather, conspiracy—come from?

Surprisingly, it isn’t based in folklore, and the tale isn’t very old, either. The hollow moon theory first came about in 1969 during the Apollo 12 moon-landing mission.

NASA researchers sought to learn more about the composition of the moon. During the Apollo 12 mission, astronauts Pete Conrad and Alan Bean set up a Passive Seismic Experiment (PSE) at the landing site as part of larger set of moon experiments known as the Apollo Lunar Surface Experiment Package (ALSEP).

Once the Apollo 12 astronauts were safely back in the command module, they crashed the lunar module into the moon’s surface. The impact was the equivalent of detonating one ton of TNT and triggered what’s known as a “moonquake”—the first human-made moonquake to take place. The PSE seismometers recorded the resulting vibrations, which were much bigger and lasted much longer than the scientists had anticipated. They were far different from the earthquake vibrations we’re familiar with.

Photo credit: Bettmann - Getty Images

NASA continued its moonquake experiments during the Apollo 13, 14, 15, and 16 missions, with similar results.

At the time, the findings were surprising because they pointed to the moon being much less dense than Earth, and it is: the moon is only 60 percent as dense. That doesn’t mean the moon is hollow, but as with many things—like the moon landing itself—conspiracy theorists perpetuated that misinformation.
What Are Moonquakes?

The Passive Seismic Experiment seismometers placed during the Apollo 12 mission remained active until 1977, recording both natural and human-made moonquakes alike. In fact, moonquakes happen fairly regularly, as space debris like asteroids hit the moon more frequently than Earth, because the moon’s atmosphere is much less dense.

Scientists have pinpointed four types of moonquakes: deep sub-700-kilometer quakes, meteorite-caused quakes, thermal quakes, and shallow quakes occurring only 20 kilometers to 30 kilometers deep. Shallow moonquakes, like those triggered by NASA, last the longest and have the most devastating effects—some even measured up to 5.5 on the Richter scale. Shallow moonquakes do occur naturally on the moon, too, though scientists haven’t pinpointed what causes them yet.
Why Does the Moon “Ring” Like a Bell?

Here’s where things got lost in translation. “The moon was ringing like a bell,” Clive R. Neal, professor of civil engineering and geological sciences at the University of Notre Dame, says of the experiment results in a NASA writeup. And that’s true from a scientific standpoint. Similarly, the writeup also compares moonquake vibrations to those of a tuning fork, which is a kind of acoustic resonator. “It just keeps going and going,” Neal says.

However, the moon does not literally sound like a bell ringing, nor is it hollow like one. But conspiracy theorists interpreted it in that manner.

Photo credit: NASA

On Earth, vibrations from earthquakes typically last only 30 seconds or so and no more than two minutes. That’s largely due to the amount of water present on the planet. As Neal explains, “Water weakens stone, expanding the structure of different minerals. When energy propagates across such a compressible structure, it acts like a foam sponge—it deadens the vibrations.”

Meanwhile, the NASA-induced moonquakes all lasted over ten minutes. The Apollo 12 moonquake’s shockwave took close to eight minutes to peak after impact and around an hour to fully cease. But we now know there’s a very good, scientific explanation for it. There isn’t much water on the moon that we know of—it’s mostly in the form of ice; and the moon is drier and a lot more rigid than Earth. So, the moon’s composition allows vibrations to “ring” and continue on for a much longer period of time.

The results were surprising at the time of the Apollo missions, but we now know more about the moon’s composition. Though we’ve ruled out the moon being hollow, we have a lot to learn still.

Terry Hurford, a NASA geophysicist, is working on the new Subsurface Lunar Investigation and Monitoring Experiment (SUBLIME), which would “map the moon’s core” and gather even more data on moonquakes for the Artemis program, for instance. “Our understanding of the moon’s interior remains rudimentary and is limited,” he says in a NASA article.

Our moon has been slowly drifting away from Earth over the past 2.5 billion years

Joshua Davies
Mon, October 17, 2022 at 3:00 PM·6 min read


An illustration of the Earth as seen from the surface of the moon.

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Joshua Davies, Professor of Earth and atmospheric sciences, Université du Québec à Montréal (UQAM)

Margriet Lantink, Postdoctoral research associate, Department of Geoscience, University of Wisconsin-Madison

Looking up at the moon in the night sky, you would never imagine that it is slowly moving away from Earth. But we know otherwise. In 1969, NASA's Apollo missions installed reflective panels on the moon. These have shown that the moon is currently moving 3.8 cm away from the Earth every year.

If we take the moon’s current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. However, the moon was formed around 4.5 billion years ago, meaning that the current recession rate is a poor guide for the past.

Along with our fellow researchers from Utrecht University and the University of Geneva, we have been using a combination of techniques to try and gain information on our solar system’s distant past.

We recently discovered the perfect place to uncover the long-term history of our receding moon. And it's not from studying the moon itself, but from reading signals in ancient layers of rock on Earth.

Related: How was the moon formed?

Reading between the layers

In the beautiful Karijini National Park in western Australia, some gorges cut through 2.5 billion year old, rhythmically layered sediments. These sediments are banded iron formations, comprising distinctive layers of iron and silica-rich minerals once widely deposited on the ocean floor and now found on the oldest parts of the Earth’s crust.

Cliff exposures at Joffre Falls show how layers of reddish-brown iron formation just under a meter thick are alternated, at regular intervals, by darker, thinner horizons.

The Joffre Gorge in Karijini National Park in western Australia, showing regular alternations between reddish-brown, harder rock and a softer, clay-rich rock (indicated by the arrows) at an average thickness of 85 cm. These alternations are attributed to past climate changes induced by variations in the eccentricity of the Earth’s orbit.

The darker intervals are composed of a softer type of rock which is more susceptible to erosion. A closer look at the outcrops reveals the presence of an additionally regular, smaller-scale variation. Rock surfaces, which have been polished by seasonal river water running through the gorge, uncover a pattern of alternating white, reddish and blueish-grey layers.

In 1972, Australian geologist A.F. Trendall raised the question about the origin of the different scales of cyclical, recurrent patterns visible in these ancient rock layers. He suggested that the patterns might be related to past variations in climate induced by the so-called "Milankovitch cycles."
Cyclical climate changes

The Milankovitch cycles describe how small, periodic changes in the shape of the Earth's orbit and the orientation of its axis influence the distribution of sunlight received by Earth over spans of years.

Right now, the dominant Milankovitch cycles change every 400,000 years, 100,000 years, 41,000 years and 21,000 years. These variations exert a strong control on our climate over long time periods.

Key examples of the influence of Milankovitch climate forcing in the past are the occurrence of extreme cold or warm periods, as well as wetter or dryer regional climate conditions.

Rhythmically alternating layers of white, reddish and/or blueish-grey rock at an average thickness of about 10 cm (see arrows). The alternations, interpreted as a signal of Earth’s precession cycle, help us estimate the distance between Earth and the moon 2.46 billion years ago.

These climate changes have significantly altered the conditions at Earth’s surface, such as the size of lakes. They are the explanation for the periodic greening of the Saharan desert and low levels of oxygen in the deep ocean. Milankovitch cycles have also influenced the migration and evolution of flora and fauna including our own species.

And the signatures of these changes can be read through cyclical changes in sedimentary rocks.

Recorded wobbles

The distance between the Earth and the moon is directly related to the frequency of one of the Milankovitch cycles — the climatic precession cycle. This cycle arises from the precessional motion (wobble) or changing orientation of the Earth's spin axis over time. This cycle currently has a duration of ~21,000 years, but this period would have been shorter in the past when the moon was closer to Earth.

This means that if we can first find Milankovitch cycles in old sediments and then find a signal of the Earth’s wobble and establish its period, we can estimate the distance between the Earth and the moon at the time the sediments were deposited.

Our previous research showed that Milankovitch cycles may be preserved in an ancient banded iron formation in South Africa, thus supporting Trendall's theory.

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The banded iron formations in Australia were probably deposited in the same ocean as the South African rocks, around 2.5 billion years ago. However, the cyclic variations in the Australian rocks are better exposed, allowing us to study the variations at much higher resolution.

Our analysis of the Australian banded iron formation showed that the rocks contained multiple scales of cyclical variations which approximately repeat at 4 and 33 inch (10 and 85 cm intervals). On combining these thicknesses with the rate at which the sediments were deposited, we found that these cyclical variations occurred approximately every 11,000 years and 100,000 years.

Therefore, our analysis suggested that the 11,000 cycle observed in the rocks is likely related to the climatic precession cycle, having a much shorter period than the current ~21,000 years. We then used this precession signal to calculate the distance between the Earth and the moon 2.46 billion years ago.

We found that the moon was around 37,280 miles (60,000 kilometres) closer to the Earth then (that distance is about 1.5 times the circumference of Earth). This would make the length of a day much shorter than it is now, at roughly 17 hours rather than the current 24 hours.

Understanding solar system dynamics

Research in astronomy has provided models for the formation of our solar system, and observations of current conditions.

Our study and some research by others represents one of the only methods to obtain real data on the evolution of our solar system, and will be crucial for future models of the Earth-moon system.

It's quite amazing that past solar system dynamics can be determined from small variations in ancient sedimentary rocks. However, one important data point doesn’t give us a full understanding of the evolution of the Earth-moon system.

We now need other reliable data and new modelling approaches to trace the evolution of the moon through time. And our research team has already begun the hunt for the next suite of rocks that can help us uncover more clues about the history of the solar system.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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