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Comment by jaytaylor | Hacker News Reader

"Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which remains locked inside its crust. This heat slowly trickles to the surface, providing a source of internal energy that has endured since the Earth formed. This interior heat will last for billions of years to come, but it’s a puny amount of energy, 3,000 times smaller than the sunlight that blasts the Earth daily."

Do this mean 3,000 times less than the Suns total daily output, or 3,000 times less than the sunlight that physically hits the earth daily?

I suspect the latter, but I still find this surprising. If daily sunshine hitting the earth is so massive then let's make solar work! If we could just collect even a fraction..

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I don't think this sentence is true:

> Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which remains locked inside its crust.

In 19th century, Lord Kelvin estimated the age of Earth using this method, arriving at somewhere between 20 and 400 million years[1], much shorter than 4.6 billion years currently accepted. It turns out radioactive decay keeps the Earth's inside warm for a much longer time. (I.e., assuming Kelvin's calculation was correct, without radioactivity the Earth's core would have frozen over within the first 400M years.)

[1] http://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calculat...

Tidal friction from the moon (and to a lesser extent from the sun) would also have contributed.

You are right - we now know that half of the heat in the core is from fission!

The word "daily" shouldn't have been in the original sentence. That makes it ludicrously wrong. (The total heat energy stored in the earth vastly exceeds the daily solar flux.)

Instead, it should be saying:

"The solar energy flux is 3000 times greater than the surface heat flux from interior heat."

https://en.wikipedia.org/wiki/Earth%27s_energy_budget

https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget

> 3,000 times less than the sunlight that physically hits the earth daily

This one.

The Earth is quite a lot smaller that 1/3000th the volume of the Sun. If it was producing 1/3000th the total energy output of the Sun, it would explode.

I don't doubt that you're right, but to play devil's advocate Earth's surface to volume ratio is also much higher than the sun's so it cools quicker too.

As they're both effectively spheres, don't they have the same surface-to-volume ratio?

The ratio is actually a function of the radius:

v = (4/3) * pi * r^3

s = 4 * pi * r^2

s / v => 3/r

This makes a kind of sense in terms of heat transfer: the sun's got a whole lot more insides for radiation to get through before it can even reach the surface to escape.

EDIT: line breaks unbreak formulas.

Ah, of course. Increase in volume is cubic, area is square.

Thanks.

http://en.wikipedia.org/wiki/Surface-area-to-volume_ratio

>For a given shape, SA:V is inversely proportional to size.

When an object grows in size its volume grows faster than its surface area.

Perhaps, and furthermore Earth's surface-to-volume ratio would increase quite a lot more shortly afterward, as its shattered remains spread throughout the solar system. I imagine that the Earth would cool off rather quickly, or what's left of it anyway.

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