"Supermoon" - If the Moon actually did grow 20% what then?

[2sense]

@VOW

Humans would have no worries at all.

To achieve 20% greater mass, that probably means the Earth blew up.

The Earth blowing up would be discernible to humans. Hypothetically at least since we’d all be dead.

@srzss05

I may be wrong, but I think this is on the track the OP means. He wasn’t asking what would happen if it got larger, but what would be different if it was larger, and always was that way. I think there is a lot of pedantry going on here.

He’s clearly talking about the Moon changing and everything else staying the same. What exactly the change would entail isn’t exactly clear. I assume he’s concerned with mass since he references tides. In any case explanations of how the change might occur are missing the point. So yeah there are a lot of nonanswers here to this interesting question. (Including this one. )

[Fiendish_Astronaut]

It’s typical GQ - lots of pedants trying to find reasons not to answer the question even though the intention of the OP is fairly clear.

I’m not a physicist so I can’t give accurate answers but a change of mass would surely disrupt the moon’s orbit. If the extra mass was to suddenly appear I suspect the moon might experience a slingshot effect due to a sudden gain in momentum. Or am I wrong about that?

[TheSquirminator]

@Ethilrist

Earthquakes. Tsunamis. Cats and dogs living together. Real biblical stuff.

quoted for truth

[Kamino_Neko]

@OldGuy

When the radius or diameter goes up by a factor of k the surface area and visible disk go up by a factor of k[sup]2[\sup] and the volume (and holding density constant) mass go up by a factor of k[sup]3[\sup]. I’d assume brightness goes up like area.

Thanks. I did know that at one point.

[Emerald_Hawk]

@Fiendish_Astronaut

It’s typical GQ - lots of pedants trying to find reasons not to answer the question even though the intention of the OP is fairly clear.

Seriously. Let’s just assume that someone flew in from outside the solar system and gently deposited 20% extra mass on the moon, after matching the moon’s orbit. I’m not saying it was aliens (but it was aliens).

@Fiendish_Astronaut

I’m not a physicist so I can’t give accurate answers but a change of mass would surely disrupt the moon’s orbit. If the extra mass was to suddenly appear I suspect the moon might experience a slingshot effect due to a sudden gain in momentum. Or am I wrong about that?

I don’t think the orbit would change much, since the added momentum gets cancelled out by the added gravitational pull. That’s why Kepler’s law relates speed and distance independent of mass. The center of mass for the earth/moon system would move but not by much, as the moon would just go from 1.2% of the earth’s mass to 1.4%.

I only minored in physics so I just did some Googling to find some equations, the best I found so far was here: http://mb-soft.com/public/tides.html. Since tidal acceleration is directly proportional to moon mass, and the height of the tides is proportional to the acceleration, I think the tides would only be 20% higher. Since the storm surges that typically flood coastal areas are many times higher than the difference between high and low tide, I think New Orleans and such places would be safe until the next Katrina. And even then, they would only be in extra trouble if the storm surge and high tide synced up together.

According to the link above, if you really wanted to screw up the tides, you should move the moon closer:

If the Moon were only half as high above the Earth, the tidal accelerations would be EIGHT TIMES as great as they are now!

[Dr.Strangelove]

@Darth_Panda

Well, it would ruin eclipses for one.

Why? Lunar eclipses would be a bit less common–though not by much since the moon is so much smaller than the earth–but solar eclipses would be more frequent. I suspect you’d never have the annular kind, which I guess is kind of a loss, even though total eclipses are considered the “cool” kind.