But how would a second moon affect the ocean tides? Would months on Earth become longer or shorter? And would these two satellites inevitably collide to cause a massive explosion?

Here’s what would happen if the Earth had two moons.

Despite what you think, the Earth isn’t really the monogamous type. Along with the Moon, Earth has another moonlike companion – a small asteroid wandering around it. But it’s so far and so small, that it can only be considered a quasi-satellite to our planet.

In our scenario, we’re talking two real moons. Just like the Earth had it during its formative period. Yep, we likely had two satellites once. Both of them formed when a Mars-sized protoplanet slammed into Earth and shattered into small pieces.

So what exactly would happen if the Earth had another moon parked in orbit around it?

Let’s assume our second moon would be around 1,000 km (620 miles) wide and about one-thirtieth the mass of our current Moon. It would be about the same distance from Earth, too. Just like the sister satellite our Moon had 4.5 billion years ago.

Would we see this double-moon from Earth? Absolutely! The second moon would just appear about three times smaller. Still, it would be a spectacular view.

Along with two moons would come some big tides. The newly captured moon, at just one-third of the Moon’s size and mass, would have a much smaller tidal effect than that of the Moon. But the tidal force from both satellites combined would make waves, literally.

Although those waves would be bigger, they wouldn’t be devastatingly big. In the end, I think surfers will be pretty happy.

The second moon might stay in Earth’s orbit for tens of millions of years. Until eventually, as the Moon retreated further from the Earth, it would destabilize the other moon’s orbit.

Then, just like they did over 4 billion years ago, the two satellites would collide in slow motion. Because they’d be moving slowly towards each other, the debris from the collision wouldn’t shower back on Earth.

The small moon would splatter itself across our Moon as an extra layer of solid crust. the new mountains of crushed rock visible from the near side of our Moon would be the only reminders of its existence.

Now, what if the second satellite happened to be the same size as the Moon and orbited the Earth at half the lunar distance?

This time things wouldn’t be so pretty. The second moon alone would produce tides 8 times greater than what we see right now.

That would force people away from the coastal areas because the difference between high and low tides would now reach 300 meters (1,000 feet).

Constant tidal flooding would shrink the habitable area on Earth. But once we adapted to this new world, we’d get to enjoy a beautiful double-moon. This time, the second satellite would look bigger from Earth due to its closer orbit.

The moons would also have different phases, which means we’d have to come up with another way to measure a month. And then there’s some worse news. The two moons would be slowly moving away from Earth and eventually crash into each other. This collision would send debris back to Earth.

It would be a shower of meteorites of epic proportion. And it could wipe us out entirely. Whatever debris didn’t fall to Earth would form yet another new moon in Earth’s orbit.

At this point, a new, post-human life form may begin to evolve and become Earth’s next civilization.

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