Lunar error

Noting Brian Perry’s letter [FT440:66], the implication seems to be that there is something mysterious about the slowing of the Earth’s rotation due to interaction with the Moon. This is certainly news to me, as the effect of tidal forces on a planet by its moon is well understood. Essentially, the bulges created on the planet (particularly the oceans in our case) by the Moon’s gravity are subject to torque because the Moon is not stationary relative to the planet’s surface where the bulges occur. Since the Earth rotates faster than the Moon orbits, the overall effect is a transfer of momentum between the two bodies, so the Earth’s spin slows, the Moon picks up speed in its orbit and given enough time the two bodies eventually become tidally locked so that the same side of the Earth always faces the Moon, just as the same side of the Moon always faces the Earth. The whole process also releases energy, some of which – as Mr Perry suggests – dissipates as heat (or can be exploited in tidal power stations). An extreme example of the heating effect that tidal forces can inflict is seen on Jupiter’s moon Io – the surface of which is plagued by volcanic eruptions to the point that the mining colony in the (excellent) movie Outland would not be a practical proposition.

Regarding the comments on infrared emissions by CO2 molecules – I am by no means sold on the wisdom of reducing our emissions, and would caution

FT against kicking a hornet’s nest by indulging discussions on this topic. However, I do fear that your correspondent has misunderstood the mechanism: water vapour does indeed absorb radiation, but it does not significantly absorb infrared wavelengths in the ~8 to 14 micron range, which allows for a transmission window through the Earth’s atmosphere, and the leakage of infrared energy back into space. CO2 absorbs across part of this range (and re-emits at wavelengths outside the window). So the window gets smaller, and theoretically the dynamic equilibrium of ‘sunlight in’ to ‘infrared out’ gets shifted. I say ‘theoretically’ because the atmosphere is a complex beast where unexpected effects are the only reasonable expectation. Ian I’Anson

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