# [LEAPSECS] Far past and far future

Clive D.W. Feather clive at davros.org
Fri May 27 18:08:27 EDT 2011

Rob Seaman said:

> Almost the definition of a moot point, but if the oceans boil into the atmosphere, how much of a change will that make to the Earth's moment of inertia - and in what direction?

Wikipedia says that the current moment of inertia is 8.04e37, which is
close enough to the numbers I get by calculating it, so let's go with that.
The mass of the hydrosphere is, apparently, 1.4e21. It's all close to the
surface, so let's say that the radius is 6.378e6. So the moment of inertia
of the oceans is mr^2 = 5.7e34. That's less than 0.1% of the total.

If it turns into atmospheric steam at, say, an average height of 1km, that
adds a tiny amount to the MoI - about 0.03% of the water's total, or under
0.2 part per million of the earth's total. If it all escapes into space, it
reduces the MoI by 0.1% or so.

[All quantities are in SI units without prefixes.]

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