Imagine that you’re looking up at the night sky — and instead of spotting a single moon above you, you see two glowing orbs instead.
In this alternate reality, one of these celestial bodies is about the size and brightness of our present-day moon, but the second appears four times bigger and brighter. From this secondary moon’s surface, fountains of magma erupt from volcanoes, creating space debris that enters our atmosphere to produce meteor showers more spectacular than any we know today.
These are just some of the possibilities for our planet if we had two moons, each roughly the same size as the other, with the second situated about halfway between Earth and our current moon. But what would life on Earth really be like if our planet was orbited by two moons instead of one?
Astrophysicist Neil F. Comins wondered about this very question; in 2010, he wrote a book, What If the Earth Had Two Moons? : And Nine Other Thought-Provoking Speculations on the Solar System, to dig into some possible answers.
In his book, Comins, who teaches physics and astronomy at the University of Maine, provides readers with 10 different scenarios for how life on Earth would be vastly different given alternative conditions.
In the titular chapter, Comins posits a scenario in which a planet similar to Earth, called “Dimaan,” has two moons. The first, called “Kuu,” stands in for our present moon, while “Lluna,” becomes a second moon through orbital capture.
Our present moon is thought to have formed within 200 million years of Earth’s formation, and was likely created when a Mars-sized body slammed directly into the planet. This impact created a ring of debris around Earth that eventually bunched together under its own gravitational attraction to become the moon we know today.
In short, a second moon would likely alter life on Earth in some pretty significant ways. Here are five ways that existence on our blue planet would change with two moons in the night sky.
Our ocean tides result from a combination of the moon’s gravitational force, which pulls oceans towards it, and the force directed away from the moon caused by the Earth’s rotation around the barycenter, the point around which two celestial bodies orbit.
This center of mass for both orbiting bodies is located over a thousand miles beneath the Earth’s surface. Technically, it’s what the moon orbits around, rather than the planet Earth itself. If a second moon were added to the equation, the effect would be dramatic.
“The tides on earth would be eight times higher than the tides created by our moon today,” says Comins.
Erosion issues would also be more pronounced, and would occur far faster than we currently experience. This would drastically affect our civilization’s ability to live and build on coastlines or near the mouths of rivers. In the latter case, deeply-carved tidal channels would cause tidal waves known as tidal bores to run up the mouths of rivers, inundating whatever’s in the way.
Twin moons would create a nighttime surface five times brighter than what we currently experience during a singular full moon. This would be bright enough to easily read a book without additional lighting, Comins writes.
A brighter environment would also likely impact how people and animals behave in predator-prey cycles, he adds. Camouflage might become more refined for those seeking survival, possibly leading nocturnal creatures to develop more refined senses for hunting.
A second moon would also change the nature of eclipses. As Comins writes, the reason eclipses don’t happen monthly is because our moon is tilted five degrees relative to the elliptic, or the plane defined by Earth’s orbit around the sun. As such, solar ellipses only happen when a new moon crosses that plane, and lunar eclipses when a full moon crosses the plane.
“There would be significantly more eclipses,” says Comins. “When the closer moon is particularly close, there would be an eclipse every month, or every orbit of that moon.”
The second moon’s umbra — the shadowy area that darkens the Earth during an eclipse — would also cover twice the area that the first moon does. “It would effectively block the sun every time it orbits, assuming that the orbit is in the plane of the [ecliptic] Earth-Sun system,” adds Comins.
Larger solar eclipses, however, would last slightly less time than those we currently experience (which generally top out at no more than seven-and-a-half minutes). That’s due to Johannes Kepler’s third law of orbital motion, which states that moons orbiting closer to a planet do so faster than a moon that’s farther away.
The moon takes about 29.5 days to complete a lunar cycle. If a second moon existed at half the distance to Earth, it would go through the same cycle of phases but only take about 10 days to complete.
It would be a cycle so rapid that residents of Earth would be able to see the second moon in different phases within the span of one evening or day, Comins says.
And because the time-keeping unit of a month is based on the lunar cycle, civilizations that developed with two moons might have more convoluted calendars as a result. In addition to days, weeks and years, there might be more reliance on the “parmo,” or partial month, Comins writes. A similar unit of time that might see renewed use would be the “fortnight,” which lasts for 14 days.
If Earth’s second moon had a noncircular orbit, the gravitational pulls it would undergo might result in a moon like Jupiter’s Io: in other words, covered with volcanoes.
This activity, Comins says, would likely be the result of the changing distance from Earth, stressing the moon’s surface and causing a tidal effect. The solid part of the moon’s interior would undergo friction, which would create heat-melting internal rock and lead to lava-heavy eruptions on the surface.
There would be so many eruptions, in fact, that a second moon would likely have few impact craters due to frequent covering by fresh lava, Comins writes. Some volcanic debris might leave the moon’s surface and be shot into space, either forming a debris field around Earth or entering our atmosphere.
The latter would result in far more frequent shooting stars — the streaks of light created when bits of space debris burn up inside Earth’s atmosphere — than any meteor shower we currently experience, as well as more possible meteorite impacts.
Our moon is very slowly moving away from Earth at a measurable distance due to the complexities of gravitational push and pull between the two bodies.
If a second moon existed, it too would be affected by these forces. The moon closer to Earth, Comins writes, would likely move away from us faster than the moon that’s farther away, eventually leading to a cataclysmic collision.
Shortly before that collision, gravitational forces exerted on each other would cause the moons to crack open, emitting lots of glowing lava. Finally, the moons would impact, appearing to break apart in slow motion and sending debris in all directions.
While Earth’s atmosphere would likely keep smaller debris from surface impact, it’s plausible that larger meteors would fall to Earth’s surface, causing potential mass-extinction. Still, the remaining debris would likely form a ring around Earth — and in time, a new single moon would again grace the night sky.