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Can someone ELI5 why it's at that temperature?

I mean, because it's in the dark I'd expect it to reach equilibrium with space background thermal radiation which is around 3K. Yet its 100K. Where does that heat comes from? It radiates from earth? Conduct through the floor coming from the inner of the moon itself? (Is there some kind of geothermal gradient on the moon BTW?)

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There are multiple factors. The biggest ones are reflected sunlight; infrared and thermal conduction from surrounding rocks; and the Moon's internal heat (the region between the core and the mantle has a temperature of over 1,300 C).

Also, little atmosphere means the sun doesn't heat up the area like in your house. Once you're out of the sunlight, it's darn cold.

The question isn't “why is it cold” but the opposite “why is it so hot” (100K when the universe radiation is 3K)

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