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What If The Moon Were Bigger?

The questions kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places adults forget to explore. With that in mind, we’ve started a new series called Science Question From a Toddler, which will use kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. I want the toddlers in your life to be a part of it! Send me their science questions and they may serve as the inspiration for a column. And now, our toddler …

Q: How big is the moon? What if it were bigger? — Hagen G., age 5

The first part of your question is easy-peasy. The moon has a circumference of 6,783.5 miles, about 27 percent that of Earth. Imagine setting out from Boston and walking to Peshawar, Pakistan. (Don’t do this. Among other hazards, there is an ocean in the way.) That same walk would take you all the way around the moon at its equator. Here’s another way to think of it: If Earth is a softball, then the moon is a shooter marble. (The circumference of the sun, in this analogy, is represented by the General Sherman sequoia, one of the largest trees in the world.)

But what about a bigger moon? This part of your question took me from a simple Google search to sitting on the telephone with a planetary scientist while we both made thinking sounds and waved our hands around in an attempt to gesture our way through logical speculation about gravitational physics. So, thanks for that, Hagen.

The scientist, Matthew Siegler, is a research assistant professor at Southern Methodist University and an associate research scientist at the Planetary Science Institute.1 He told me that the question of what would happen if the moon were bigger matters because the moon and Earth are a system. Our gravitational pull affects the moon. The moon’s gravitational pull affects us. We’re linked to each other by the push and pull of invisible hands. And that has some big impacts on our planet.

I’m talking about bigger impacts than the flowing and ebbing of tides in a bay.2 For instance, the tilt of Earth changes, but it doesn’t change by much. Earth fluctuates between an angle of 22.1 degrees and 24.5 degrees over 41,000-year cycles. Mars, in contrast, bobbles back and forth like a galactic Weeble. “Mars varies from between 10 and 50 degrees on pretty short time scales, you know, hundreds of thousands to millions of years,” Siegler said. That difference could be because of moons. Mars has two, but they are tiny. The biggest, Phobos, is an oblong lump of rock that’s only about 16 miles long. You could hike it in a day. Because of this, they don’t have much gravitational impact on Mars. In contrast, our moon is big enough to matter. If you could see gravity, Siegler told me, you’d see Earth as this mass with a ring around it — imagine the moon giving our planet a giant bear hug. That hug is a gravitational tug on Earth and makes it difficult to change Earth’s tilt. Because it’s difficult to change Earth’s tilt — that is, the angle at which heat from the sun hits it — this planet has a remarkably stable climate. Because Earth has a remarkably stable climate, it has us.

So without a moon, we might be more like unstable Mars. (And with a smaller moon, we might be less stable than we are.) But a bigger moon probably wouldn’t affect Earth’s stability much, Siegler said. It would make the planet’s tilt harder to change, which would mean a more stable climate, which “just means maybe ice ages wouldn’t happen as often,” he told me.

But there are other ways a bigger moon could make a big difference.

Scientists think the moon started to form about 4.5 billion years ago, when a different planetary body smashed into Earth. Originally, the moon was a lot of debris from that collision, like broken glass left on the street after a fender bender. But Earth is big enough that its gravity pulled some of those shards into orbit. While those bits and pieces were still hot, gravitational forces smushed them all together to make the moon. It’s been orbiting us (and moving away from us at a rate of about 1.5 inches per year) ever since.

Back when all of this was happening, billions of years ago, Earth was probably spinning so fast that a day was only four hours long, Siegler told me. It’s the interplay of gravity between Earth and the moon that has given us the 24-hour day. Imagine trying to walk forward through a crowded shopping mall while holding the hand of an easily distracted toddler. You can get where you’re going, but you’re not gonna get there as fast. (No offense, Hagen.)

And an easily distracted adult would slow you down even more. Earth’s rotation is still slowing, Siegler told me. Billions of years from now, Earth will have a 30-hour day, all on account of the moon. So if the moon were bigger, we’d probably rotate even more slowly and have an even longer day.

We can also get an idea of what a bigger moon would be like by looking at other planets in our solar system. Take Pluto. Everybody’s favorite dwarf planet has at least five moons, but one of those, Charon, is almost half Pluto’s size, much bigger, relatively speaking, than the moon is compared with Earth. (If Earth is a softball and the moon is a marble, then the relative sizes of Pluto and Charon are closer to that of a softball and a racquetball.) Pluto and Charon are tidally locked to each other — that is, each is always facing the same side of the other. Pluto does not spin independently of Charon; they spin together as a unit, like figure skaters holding hands.3

Now, the moon is tidally locked to us. We only ever see one side of it. Hence the “dark side of the moon” — the “dark” here is metaphorical and refers to the side we never see. But we are not tidally locked to the moon. That’s because Earth is so much larger. If the moon were bigger enough, it might be able to “pull a Charon” and lock us in as well. “It might be that you’d have to take a vacation to the other side of the Earth to ever see the moon,” Siegler said. More than that, becoming tidally locked with the moon would change time. Our day and our month would become the same thing. There wouldn’t be a “dark side of the Earth” exactly, Siegler told me. All of Earth would still get access to sunlight. But the length of time that sunlight lasts would change drastically, depending on the size of the moon and how that affected the length of that month-day.

And that could have real, practical consequences. “It might be bad for life if there’s no sun for 15 days,” Siegler said. Of course, on the other hand, life does find a way. “I guess we know in the Arctic Circle things do fine. Things survive six months of darkness. It’s just that life at the equator would be more like life at the Arctic Circle.”


  1. In March, he published a paper in Nature where he used the presence of ice at the moon’s poles to show that the tilt of its axis had changed by roughly 5.5 degrees over the last billion years. That may not sound like much, but it’s the equivalent of Earth’s South Pole moving from Antarctica to Australia. If it weren’t for that tilt, what we see when we look at the moon would be very different.

  2. Seriously, though, take 15 minutes and watch this PBS video on tides. Among other things, you will learn that lakes don’t have tides for the same reason that little pimples are harder to pop than big ones. WORTH IT.

  3. I had to watch a video simulation of this to understand what it looked like. Here’s a good one.

Maggie Koerth was a senior reporter for FiveThirtyEight.