What if the Moon was twice as close

Neil Cummins, a physicist at the University of Maine, said a flood scenario on our planet isn’t really far away if the Moon suddenly approaches Earth.

The moon’s most famous effect is its gravitational pull on Earth’s oceans, which results in low tides each day.

But if the moon were half the distance from Earth than it is now, the tide would be eight times higher, Cummins told Live Science. He added that some islands will be completely underwater for a large part of the day, and inhabited coasts are likely to become uninhabitable due to high tide.

But rising ocean tides won’t be the only consequence of the Moon’s approach. The moon also has a tidal effect on Earth, Cummins said.

“If the moon suddenly approaches the Earth twice, the effect will be like hitting a bell with a hammer: the energy waves will echo through the planet due to the sudden increasing force of the moon’s gravitational pull,” he explained.

This sudden blow of gravity “will actually affect the Earth’s crust, which means it could lead to more earthquakes, and could lead to more volcanic eruptions,” said Jasmine Scarlett, a historical and social volcanologist at Queen Mary University of London.

Take, for example, Jupiter’s moon Io, the most active volcanic world in the Solar System, caused by the thrust and pull of the gravitational pull of Jupiter and two of its other moons. Earth might see a similar fate if the moon were suddenly about half the distance closer.

Besides all the sudden twisting of the planet’s crust, the Earth’s rotation will slow down over time. As the moon’s gravity attracts the oceans, the friction created between the ocean floor and the water slows the Earth’s rotation. Today, Cummins said, the Earth’s rotation is slowing down by about a thousandth of a second per century. And if the Moon were half the distance away, the Earth’s rotation would slow down even more, lengthening our days and nights.

And if we survive sudden earthquakes, volcanic eruptions, extended days and nights, and tides, at least we’ll see more frequent solar eclipses. Because the moon will cover more of the sky, it is more likely to pass in front of the sun from our perspective, Cummins said. He added that we are still able to see the sun’s corona (the outer atmosphere) shining around the dark silhouette of the moon, but not with the same clarity. Otherwise, the moon and its phases will look the same, only larger in the sky, according to rt.

Scarlett said the planet’s crust and tides will gradually change, and hopefully, life will adapt. The length of the day and nights can alter our climate and drive evolutionary changes in multiple ways. And the animals will have to adapt to a brighter moon at night. For example, prey may have to learn how to hide better at night, when predators may have more light when hunting.

Sorry if this has been asked before, I did a search but couldn't find anything. Basically what the title says. If the moon was further away it would affect tides, but if it was also twice as big would it have the same effect that it does now? Would the lunar cycle be twice as long? What other changes would there be? Thanks

Note: In a weird coincidence, I was working on this post when I got word about the similar Saturn video I wrote about earlier. Since Saturn was at opposition last weekend, I wrote that up and posted it first. The article that follows is based on a video by the same animator, and has some of the same themes but covers different ground and dips into the math a bit more as well.

One of the fun things to do with astronomy (and science in general) is to imagine what it would be like if things were different. For example, right now the Moon orbits the Earth at an average distance of about 384,000 kilometers (238,000 miles). Even though that makes it the nearest astronomical object in the Universe, that’s still pretty far—a four-day ride in a space capsule, for example.

But what if it were closer? YouTube user “yeti dynamics” (real name: Nick) is an animator who created a fantastic video showing what it would look like from Earth if the Moon orbited us as the same distance as the International Space Station. The results are pretty amazing! Make it hi-res and fullscreen for the full effect.

Right off the bat, let me give my kudos to the animator! That was very well done, very realistic looking, and—my favorite bit—really cool, so it’s likely to grab people’s attention and get them thinking about the Moon and space.

And given its intent (just showing what it would look like, without any extrapolation on the physics), it’s pretty accurate, too. Indulge me while I dabble in some (fun) math.

When the Moon Hits Your Eye

If the Moon were that close—420 kilometers (260 miles) over the surface of the Earth—it would be more than 100° in size, literally more than half the sky! Right now it’s a mere 0.5° in size, for comparison (which is actually even smaller than you think). It’s neat how it appears to rotate, too, though that’s really just perspective; it’s the same effect that makes it look like features are sinking below the horizon as you orbit. Yeti dynamics also explains that the color of the Moon is from reflected Earthlight: The blue is from the Gulf of Mexico, and the greenish-tan from the United States. It’s dark in the middle because with the Moon blocking the Sun for so much of the Earth, there’s no light to reflect and illuminate the Moon there!

The motion in the video is sped up; at that distance the Moon would orbit the Earth in about 90 minutes or so. It would cross the sky in very roughly five minutes. Note: Nick sent me a note saying that, accidentally, he rendered the Moon upside-down! I’m generally pretty good at spotting these things, but I’m having a hard time picturing that even with a map in front of me. Maybe you can do better: At 35 seconds, Mare Crisium dominates the view near the bottom (and rotates back into view at 45 seconds), then at 1:00, right after the Moon blocks the Sun, the dark crater Plato can be seen to the upper right. Does that help orient you?

All in all, that would be a very impressive sight! Unfortunately, if you saw it, you’d be very, very dead.

Why?

The answer is gravity, specifically, tides.

The Pull of the Moon

The force of gravity you feel from an object depends on how massive and how far away it is (measured from its center). The Earth has about 80 times the mass of the Moon, so if you could situate yourself exactly halfway between them, the Earth would pull on you 80 times harder than the Moon. But it’s worse than that; gravity drops as the square of the distance, and the Moon is pretty far away. Right now, the center of the Earth is roughly 6,400 kilometers below you, and the Moon’s center is about 380,000 kilometers above you (actually that can vary depending on where the Moon is in its orbit, but let’s ignore that). Take the ratio and square it, and you see that the Earth pulls on you 3,500 times harder just because it’s closer. Add in the fact that the Earth is more massive, and you’ll find it pulls on you about 300,000 times harder than the Moon!

That’s why you don’t notice the gravity of the Moon. It’s only 0.0003 percent as strong as what you feel from the Earth.

But what if the distance were closer? In the video, the Moon is 420 kilometers (260 miles) from the Earth—in this case, that’s measured from the surface of the Earth to the surface of the Moon. The center of the Moon would then be an extra 1,738 kilometers away (the distance of the Moon’s surface to its center, in other words its radius). So now the center of the Moon is 2,158 kilometers (1,340 miles) away, which is close.

If you redo the gravity calculation, you’d find the force of gravity from the Moon on you is 1/10th that of Earth! When the Moon passed overhead, you’d weigh 10 percent less. I’ll note this depends on your latitude and other factors, but again I’m trying to keep it simple here. You’ll see in a moment why worrying over details isn’t important.

The Tide Waits for No Moon

Weighing a little less every time the Moon goes over you might sound like fun, but then you have to remember about tides. The force of gravity gets weaker with distance. For example, right now the far side of the Earth feels less gravity from the Moon than the side facing it. The difference isn’t much, but it’s enough to stretch the Earth a little bit (it’s actually more complicated than that, of course, but for now that’s close enough). That’s what we call the tidal force. Right now, the Moon’s tides on Earth pull water up and down by roughly a meter or two between high and low tide as the Earth rotates under the Moon every day.

But if we bring the Moon in really close, suddenly one side of the Earth is a lot closer to the Moon than the other: The Earth’s near side is 2,158 kilometers from the Moon’s center, and the far side is nearly 15,000 kilometers away. That’s a huge difference, and the tides felt by the Earth would be amplified enormously—nearly 100,000 times what we experience now! There would be global floods as a tidal wave kilometers high sweeps around the world every 90 minutes (due to the Moon’s closer, faster orbit), scouring clean everything in its path. The Earth itself would also be stretched up and down, so there would be apocalyptic earthquakes, not to mention huge internal heating of the Earth and subsequent volcanism. I’d think that the oceans might even boil away due to the enormous heat released from the Earth’s interior, so at least that spares you the flood … but replaces water with lava. Yay?

In the video, you’re standing in a pastoral park enjoying the view as the Moon passes silently overhead. In reality you’d be drowned, vaporized, and shaken to bits. So, yeah.

And it would be even worse for the Moon. The Earth is more than 80 times more massive than the Moon, and so the tides the Moon feels would be even bigger. In fact, at that distance the Moon would be well inside the Earth’s Roche limit, the distance from the Earth where its tides could break another object apart. In other words, the tides from the Earth would literally rip the Moon to pieces! So we wouldn’t even have a Moon; we’d have a thick debris ring composed of ex-Moon. That would be cool to see, too, except for the whole everyone being dead thing.

I can understand why Nick left that part out. It might distract from what he was trying to show. Animating it might have been something of a chore, too.

As far as even more pedantic scientific nitpickery goes, there are tons of details to consider here—the Earth’s rotation, dependence on latitude, whether the Moon suddenly appeared much closer, or if it spiraled in over a few years—but I’ll leave that as exercise for the ambitious reader. As for me … I think I’ll go listen to some Debussy.