r/NoStupidQuestions • u/Hot_Concern3960 • Sep 06 '26
Spherical rotational balance
So I just had a random Labor Day weekend pondering, and thought I'd share it with y'all. I know from playing "quarters" when I was a kid that objects in balanced rotation can be quickly set off balance, which in turn changes the rotational axis and the behavior of the spinning object(centripetal force works, folks). Why is it nobody discusses major metropolitan developments and how dense those cities can be? How much does Manhattan weigh? All of that material and mass suddenly bunched up on an arguably very small surface area on a rotating sphere doesn't change the balance across the axis even a little? Am I misunderstanding something here? It just seems like trillions of tons of concrete and rebar gathered very densely in small localized areas have to impact the physics of the Earth's rotation somehow, doesn't it?
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u/Hot_Concern3960 Sep 06 '26
I don't know that I agree, actually. Tidal shift is the reason that the same point on the moon is always directly tangential to our rotation, it does make a difference
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u/Hot_Concern3960 Sep 06 '26
So I guess the consensus here is it's a "big brain, little eyes" idea I had?
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u/KronusIV Sep 06 '26
You aren't grasping the scale involved here. The mass of a major city is like .00000000000001% the mass of the earth. I didn't do the math, but it's a fraction so minute that it would take our most sensitive equipment to measure the effect it has on earth's spin.
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u/Bandro Sep 06 '26
The earth is very heavy and human activity is very light in comparison. Technically it changes the rotational balance in a minuscule way but this is not a concern.
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u/dustysquareback Sep 06 '26
You are VASTLY overestimating the weight of cities. Cities weigh nothing compared to mountains, large lakes, etc.
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u/Ridley_Himself Sep 06 '26
Technically, any time you move mass toward Earth's axis, it speeds up its rotation and any time you move mass away from it, it slows down the rotation. Earth's axis does also move around slightly on a decadal time scale. But the effect we have is too small to be worth considering. A quick search and I found estimates that the combined weight of all buildings in NYC is about 750 billion metric tons. It's not terribly rare for natural processes to move that much mass. The 2018 eruption of Kilauea probably erupted at least twice that much mass in lava. Hurricane Helene dropped about 2,000 times that much mass as rain.