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Comment by boomboomsubban | Hacker News Reader

Tides rely on the sun's gravity. Geothermal might be independent, but also don't seem very renewable.

If you want to look at things on that timescale, then the tides aren't renewable either. They're gradually slowing our rotation. Once the earth's rotation has slowed to once per year, we'll no longer be able to extract energy from the tides.

Then again, if you want to look at things on that timescale, there's no such thing as renewables.

On long enough scales nothing is renewable. Also, the tides rely on the moon's gravity far more than the sun's AFAIK.

Tides don't rely on the sun's gravity. It does affect them, but if the sun disappeared there would still be lunar tides, which are the majority of tidal activity. Well, I suppose the subsequent frozen oceans would be detrimental. The gravitational tidal forces would still exist though.

And why do you think geothermal power isn't renewable?

They do actually. I learned in a marine navigation class that the tides are on a cycle of 18.6 years. If only the moon affected the tides, the cycle would be about 28 days.

https://en.m.wikipedia.org/wiki/Tide > Nineteen years is preferred because the Earth, Moon and Sun's relative positions repeat almost exactly in the Metonic cycle of 19 years, which is long enough to include the 18.613 year lunar nodal tidal constituent.

> It does affect them, but if the sun disappeared there would still be lunar tides

C'mon mate, it wasn't even two sentences. Cool about the Metonic cycle, though.

Sorry, that was very lazy reading on my part

That shows that the lowest-frequency component of tidal motion is driven by the sun's gravity. But the highest-amplitude component by far is the effect of the moon's gravity, which has a period of 12 hours and 25 minutes. So yes, tides are affected by the sun, but they don't depend on it.

>And why do you think geothermal power isn't renewable?

Though small scale energy production does seem to be renewable, using just geothermal to reach our needs would likely overuse the resource.

Tides depend on the moon’s gravity, not the sub’s (mostly, anyway). Besides, if the sun suddenly blinked out, I think having a few fusion reactors in operation wouldn’t do us much good...

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