# The earths OLD moment of inertia?

1. Jan 7, 2005

### Dracovich

Now i don't know if this is just a stupid question :) But i came to thinking while reading the news about the earth having slowed down 3microseconds because of the recent earthquake in souteast asia. The explaination i read was that it was because the earth's crust had moved a few meters up and so the moment of inertia of the earth had been changed.

This got me to thinking, that this is no different then building a house (obviously on a much smaller scale), and was wondering if there was any way to calculate the earths moment of inertia if there were no houses on the earth at all, so all bricks/cement/metal used in houses were not raised above the earth in houses, but still in the ground where they were originally harvested.

Is there any way to do this relatively easily? That is without just crunching it the hard way on paper i'm bad enough with just "normal" simple shapes

2. Jan 7, 2005

### Tide

You have the complication that the materials used to build those houses may have come from higher elevations in the first place so that the net effect, in some cases, will be to lower the elevation. Also, the materials will have come from different latitudes than the structures.

3. Jan 7, 2005

### Dracovich

Agh true didn't think of that.

4. Jan 7, 2005

### FredGarvin

I would suspect that even if you could come up with an estimate of the mass of every building on the planet, that total would still be insignificant when compared to the mass of the Earth's crust.

5. Jan 8, 2005

### rcgldr

or the effect of tidal drag slowing the earth's rotation down.

6. Jan 8, 2005

### Garth

or the long term spinning up of the Earth, once tidal drag has been taken into account. Stephenson et al. [ Astronomy & Geophysics, Vol. 39, October 1998. "The Sands of Time and the Earth’s Rotation", also Astronomy & Geophysics, Vol. 44 April 2003. "Historical eclipses and Earth’s rotation"] have found from the analysis of the length of the day from ancient eclipse records that in addition to the tidal contribution there is a long-term component acting to decrease the length of the day, which equals:
△ T/day/cy = −6 x 10−4 sec/day/century.

This component, which is consistent with recent measurements made by artificial satellites, is thought to result from the decrease of the Earth’s oblateness following the last ice age.

However, it is remarkable that this value of △T/day/cy is equal to
H, Hubble's cosmological expansion parameter if
H = 67km.sec−1/Mpc.

The question is; why should this spinning up of the Earth’s rotation have a natural time scale equal to the age of the universe rather than the natural relaxation time of the order of that of the Earth’s crust or the periodicity of the ice ages? This effect may be cosmological rather than geophysical in nature.

Note. Such a spin up is a prediction of http://arxiv:org/abs/gr-qc/0405094.

Garth

Last edited by a moderator: Apr 21, 2017