This article is from the In-Depth Report It's Back! Total Solar Eclipse Hits Skies Friday

What would happen to Earth if the moon was only half as massive?

Neil F. Comins, a professor of physics and astronomy at the University of Maine, Orono, explains why an Earth with a half-mass moon would be populated by bug-eyed creatures that would never know the beauty of a total eclipse


We take the moon for granted, but Earth would be a very different place had our nearest neighbor only achieved half of its present mass when it formed some 4.5 billion years ago in a titanic collision. In fact, we might not even be here to appreciate it at all.

Let's start with eclipses. In one of those bizarre cosmic coincidences, our moon today is positioned at just the right distance between Earth and the sun for its diameter to completely block out the sun during a total solar eclipse—the next of which will occur on Friday, August 1. [see's special report on the eclipse]. 

But what would happen if the moon had only grew to half its present mass? Assuming our half-size moon was composed of rock as dense as that of the actual moon, it would still be 80 percent as large across as the full-size version (based on the relationship between a sphere's volume and radius that you learned in grade school).

Most solar eclipses are "annular," meaning that the moon only partially blocks the sun and appears to be framed by a ring of glowing sunlight. Annular eclipses happen on average three to four times a year; total eclipses only occur about once per annum. At its current distance from Earth, if the moon were 80 percent of its current size, there could be no total eclipses—just the annular kind.

A less massive moon would also orbit closer to Earth than the real one. (This means that total eclipses could still happen, although the half-mass moon would have to be at least 20 percent closer to Earth than the actual moon is now, or closer—but that would require a coincidence on top of a coincidence) Our real moon orbits at an average distance of 238,600 miles (384,000 kilometers), but every year it drifts about 1.6 inches (four centimeters) farther away. The cause? Ocean tides.

The moon's gravity, combined with the waltz of Earth and the moon around their center of mass, forces the oceans into an oval shape, with two simultaneous high tides. One high tide is on the side of Earth facing the moon, whereas the other high tide is directly opposite, on the other side of our world. Because Earth spins so rapidly compared with the moon's orbit around us, our planet drags the high tide closest to the moon a little bit ahead of it.

The gravitational pull of the water on the trailing moon imparts energy to it. This makes it spiral a little farther outward with every orbit around Earth. (Each lunar revolution takes about 29.5 days). If the moon were half its mass, then the ocean tides would have been correspondingly smaller and imparted less energy to it. Given the moon's lesser mass, this means that less energy would be required to push it away from Earth; however, it turns out that the half-as-big high tide would actually contain less water than our high tide, therefore it would have less mass to influence the half-pint moon's orbit. So a less massive moon would nonetheless end up closer than the real one to Earth.

The energy given to the moon comes from Earth's rotation—and to compensate, our planet is slowing down. In other words, days are getting longer. Geologists believe that an Earth day was originally five to six hours long. If the moon had been less massive, thereby creating less drag on Earth, our planet wouldn't have slowed down as much. The day would be, perhaps, 15 hours long.

Weaker tides (of a half moon) would also have caused less erosion of Earth's landmasses over the past few billion years—and the continents' shorelines would likely look quite different for it. Less soil and minerals from land leaching into the ocean might have had profound effects on the origin of life, too. Some organic (carbon-based) compounds thought to have seeded life may not have made it into the primordial soup of the early oceans, which would also have mixed less thanks to the reduced tides.

Assuming life had still arisen, it would have had to contend with more frequent ice ages as well more extreme warm snaps. Large moons stabilize planets. Mars, which sports only two tiny moons, wobbles a lot on its axis, and as a result it has bigger climatic swings and seasonal temperature changes than Earth does. Without the full-mass moon to hold us steady, life on Earth might have experienced greater seasonal fluctuations.

The outlook for life would have been dim—literally. A smaller moon means less scattered sunlight at night—that's all moonlight is—which would mean darker nighttimes. Whatever life forms did evolve on this altered Earth would have had to develop bigger or more sensitive eyes to help them navigate, forage and spawn at night under this diminished glow.

Neil F. Comins is the author of several books, including What If the Moon Didn't Exist?: Voyages to Earths that Might Have Been; Heavenly Errors: Misconceptions About the Real Nature of the Universe; and The Hazards of Space Travel: A Tourist's Guide. He teaches astronomy at the University of Maine, Orono, and despite his lunar fascination, he swears he isn't a lunatic.

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