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Starts With A Bang

The top 5 things we’d miss if we didn’t have a Moon

Plus a bonus: perhaps the single most important achievement to all of humanity.

“What was most significant about the lunar voyage was not that men set foot on the Moon, but that they set eye on the Earth.” –Norman Cousins

If you look towards the west after sunset tonight, you can catch a small, crescent Moon follow the Sun down to the horizon. But have you ever wondered: what would life on Earth be like without the Moon?

Image credit: Frank Borman, Apollo 8; poem by John Gillespie Magee, Jr.

Our nearest neighboring body in the cosmos has a profound effect on us. It’s helped not only shape our evolution, biologically, but has shaped the entire physical evolution of our planet. Created some 4.5 billion years ago — when our planet and Solar System were still in their infancy — when a roughly Mars-sized planetoid crashed into a young proto-Earth, the Moon has been our companion-in-orbit ever since.

Image credit: Fahad Sulehria of

It’s totally reasonable and conceivable that life would have sprouted and thrived on Earth even without the Moon, but things would be significantly different in detail. Some of them would be obvious, some would be a little more subtle, but there would be a great many impacts that we’d notice if we knew to look for them.

So today, I present to you the top 5 things we’d miss if we didn’t have a Moon!

Image credit: Wadsworth Publishing / ITP (L), Sagredo, via Bob King (R).

1.) There’d be no such thing as eclipses on Earth.

Without the Sun, Moon and Earth, there would be no eclipses. The Sun is constantly shining on Earth, casting a shadow for over a million miles (and over a million kilometers) in its wake. Yet without our Moon — just a few hundred thousand miles (or kilometers) away — there’d be no object that would pass through the Earth’s shadow; there’d be no lunar eclipses.

There’d also be no solar eclipses: no annular, partial, or total eclipses. The Moon’s shadow is almost exactly equal in length to the Earth-Moon distance; without the Moon, no shadow, and no disc to block the Sun’s disk. The next largest object that can pass in between the Earth (after the Moon) is Venus, and while it’s incredibly cool when that happens, that’s the closest we’d get to an eclipse without the Moon.

Image credit: © 2002 By Keith Cooley, via

2.) Our tides would be tiny in comparison to what they are now, and they’d be dominated by the Sun.

Although the Sun is some 400 times larger (in diameter) than the Moon, it’s also, on average, about 400 times farther away. This explains why they appear about the same angular size from Earth. But the Sun is only about 27 million times as massive as the Moon.

Why in the world would I say “only” there? Because it would have to be about (400)^3 times the mass of the Moon, or 64 million times its mass, in order to have the same effect on Earth’s tides as our small, lunar neighbor. As it stands, tides from the Sun are only about 40% as strong as tides from the Moon. When the Sun and Moon line up in either the “new” or “full” Moon phases, we get spring tides, 140% as large as a typical tide, and when they’re at right angles, we get neap tides, only 60% as strong as a standard tide.

Image credit: Arthur Thomas Dodson of Bridgeport, Connecticut, via Wikipedia.

But without any Moon at all, our tide patterns would be much simpler, and only the Sun would contribute anything substantial. See the differences between big-and-small tides on the graph, above? That’s due (mostly) to the Sun, and that effect would be all we’d have for our tides without a Moon. All in all, our tides would only be about 40% as large as a typical tide is today, and much more uniform. Not the biggest of deals, but definitely something we’d notice.

But there’d be some very large impacts on how we experienced life on Earth.

Image credit: user Rutjuga of the forums at

3.) Nights would be much, much darker than we’re used to.

If you’ve ever been outside in the wilderness on a totally moonless night, without any artificial light, you probably noticed two things. First, the night sky is absolutely breathtaking; you can see thousands upon thousands of stars, the plane of the Milky Way, and even dozens of extended, deep-sky objects with your naked eye alone. And second, you can’t see a damned thing in front of your own face.

Image credit: Paul Kinzer of Cambridge University Press.

The Sun is much, much brighter than the Moon; the full Moon is just 1/400,000th as bright as the daylight Sun. Yet Venus, the next brightest object in the night sky, is only 1/14,000th as bright as the full Moon!

We have pretty decent night vision, so long as the Moon is out. But without it, our night vision is, well, not very effective, as anyone who’s been camping without a headlamp or working flashlight can testify. It’s probably safe to say that vision would have evolved somewhat differently without the Moon, and that our nights would provide us with a wildly different world to experience.

But that wouldn’t be the biggest difference, not by a long shot.

Image credit: Tim Thompson.

4.) A day on Earth would be much, much shorter; only about 6-to-8 hours, meaning there’d be between about 1,100-1,400 days in a year!

Our 24-hour-days may seem like they don’t change from one year to the next. In reality, the change is so tiny that it took centuries to perceive, but the Earth’s rotation slows down ever so slightly over time, thanks to the tidal friction provided by the Moon. The slow-down is very, very slow (on the order of microseconds-per-year), but over millions and even billions of years, it adds up!

In about 4 million years, we’ll no longer need leap years to keep our calendars on track. If the Sun would live an infinite amount of time, the Earth would eventually slow down and become tidally locked to the Moon, the same way the Moon is locked to us and always shows us the same face. Instead of 24 hours, a day would last for some 47 current Earth days. (In reality, the Sun will end its life long before that happens, so no worries there.)

But in the meanwhile, we can use what we know to extrapolate backwards in time, and we find that in order to get a 24 hour day today, the Earth had to have been spinning much faster in the past: about three-to-four times as fast more than four billion years ago! If we didn’t have a Moon — if we never had our Moon — the day would be much, much shorter than it is today, and our planet would have a larger equatorial bulge, much more flattened poles, and over 1,000 days in a year!

And finally…

Image credit: Center for Mars Exploration, via

5.) Our axial tilt would vary tremendously over time!

You probably learned that the Earth rotates on its axis, tilted at about 23.5 degrees relative to its orbital plane around the Sun. This is true! But did you ever stop to think what keeps the Earth from changing the tilt of its axis-of-rotation? The same way a spinning top not only precesses but also exhibits more complicated motion over time (some of which you may know as nutation), an entire planet can do this, too. Mars is a perfect example: currently tilted at about 24 degrees relative to the Sun, we know that its axial tilt varies from about 15 degrees to about 35 degrees over time!

Earth is special, though, because we have an external force to stabilize us against that sort of behavior. Know what’s responsible?

Image credit: Mathieu Dumberry of

That’s right, the Moon! Thanks to our Moon, our axis stays tilted between 23 and 26 degrees over time, even over hundreds of millions of years! But without our Moon, there would be nothing preventing catastrophic shifts in our rotational axis. It’s probable that sometimes, we’d be like the planet Mercury, orbiting in the same plane as our rotation, and having practically no seasons due to our axial tilt. At other times, we’d possibly be as extreme as Uranus, rotating on our side like a barrel, having the most extreme seasons imaginable!

And finally, the Moon has had one additional impact on us, as humans, that we’d certainly miss if we didn’t have it.

Image credit: NASA’s Office of Manned Space Flight, Apollo missions.

Bonus.) We wouldn’t have our first stepping stone to the cosmos.

I know that it’s a tremendous disappointment to a great many of us that since the Apollo program came to a close in the early 1970s, no human has ever set foot on another world. But we know we can, and our Moon gave us something relatively close by to reach for. It gave us a lofty but achievable goal, and every human that was ever sent to the Moon, whether to land or orbit, has been successfully returned to Earth.

Image credit: NASA / Apollo 11, of Buzz Aldrin deploying the Solar Wind experiment.

We can reach it, and it’s taught us that if the will is there and the resources are devoted to it, we can go as far as our imaginations (and the laws of physics) can take us. And that’s far!

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

So the next time you take our Moon for granted, think about how different life would be — and how different the entire history of life on Earth would have been — if we didn’t have our Moon.

An earlier version of this post originally appeared on the old Starts With A Bang blog at Scienceblogs.


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