*Chow Carson asked the Naked Scientists:*

I was listening to your 14-3-18 podcast and you mentioned that the moon was moving away from the earth because of the tidal forces.

I believe you said that while the earth's rotation rate was slowing down the moon was speeding up and thrown out to a higher orbit.

Actually, the moon's angular speed is slowing down. Recall Kepler's third law that the period of the orbit squared is proportional to the radius cubed.

That means the angular velocity, which scales as radius/period scales like the inverse of the square root of the radius.

However, the angular momentum increases as the square root of the radius. Hence, the earth loses angular momentum to the moon but it's not speeding up the moon per se.

It's actually slowing it down while pushing it higher. I know you probably know this but your listeners may misunderstand.

Great show.

cheers,

Carson

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Carson C Chow, PhD

Laboratory of Biological Modeling

NIDDK, NIH

*What do you think?*