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Geothermal, which at this point in geological history mostly comes from decaying radioactive elements. It's of minor industrial importance, but it fuels undersea vent ecosystems, and does see some use in traditional cultures.
Speaking of radioactive elements, our nuclear generators all run on energy trapped from ancient cosmic catastrophes. Probably colliding neutron stars, for the most part. Hydrogen fusion has been made to happen for research and in atomic bombs - although interestingly we can't use the same kind as the sun does.
Tidal energy is used for some power generation, and it comes from the kinetic energy left in the Moon, and to a lesser degree the Earth itself, from the formation of the solar system.
Does that mean that one day the moon will stop revolving and we will be tidal locked? If so, does that theoretically happen before the sun consumes us?
It gets both slower and further away (to stay in orbit) every year, by like 2 cm IIRC.
If you could go back a couple billion years it would be huge in the sky. There was even a period, called the Jatulian, where you might not have asphyxiated in the early atmosphere. There wouldn't be much else to look at, though, and just your skin germs would be futuristic enough to completely change the course of life on Earth, once they get into the environment.
So it will eventually escape it's orbit?
Uhh, I actually don't know the answer to that. Orbital mechanics is hard; see me being bamboozled elsewhere in the thread. At some point I'm guessing tidal forces from the sun would start having a major impact, since in reality they're both in it's gravitational well at the same time as they orbit each other. Usually that doesn't make orbits more stable.
Also, the sun will go red giant in 4 or 5 billion years, and will eat Venus for sure. The Earth and Moon may well suffer the same fate, we're kind of right on the projected edge.
Do you have any source for the radioactive decay part? I always thought the Earth was hot inside simply because it hasn't finished cooking down from when it formed as a ball of molten stuff. Like a hot potato.
Uh, I'll find one, and edit. It confused the shit out of Victorian scientists, because they had a good guess how old the Earth is from biology, and had thermodynamics, but it was telling them volcanism shouldn't still be happening.
Edit:
Wikipedia mentions it in the geothermal article, but the source is a textbook, and who has time and/or money for that? There's also the article on the age of the Earth. Ah, here we go, in the article on Earth's internal heat budget. Somebody also linked a paper on it elsewhere in this thread.
These give slightly different numbers from each other, but the gist is that radioisotopes (Uranium, Thorium and Potassium being the primordial ones) account for at least half.
Oh interesting. It turns out it's more like a demonically possesed hot potato, that is still cooling down but also gets warmer from demonic activity under the earth.
More like a hot banana, going by the potassium.
I actually had no idea radioactive potassium was primordial until I read this. That's neat.