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I'm currently reading Atom (Krauss book), and in chapter 11 there is this text:

This collision can explain many things, including the high rate of spin of the Earth (the grazing collision would have twisted it around like two football players who collide while running in opposite directions).

This is in reference to a proto-planet hitting the early Earth, which led to the formation of the Moon.

Why is Earth's spin considered high rate, given that Mars' spin is pretty similar without having a big moon? Or have I misunderstood what is being referred to?

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    $\begingroup$ Perhaps he's pointing out that, even with the measured deceleration due to Moon-tidal forces over the last 10^x years, the current rotation is compatible with Mars and other planets (which do not have a large moon and/or liquid oceans) $\endgroup$ – Carl Witthoft Apr 29 '19 at 15:34
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    $\begingroup$ He has a contact page on the imaginatively named lawrencemkrauss.com so you could ask directly, or try facebook, which he also uses I think - I don't use facebook myself. $\endgroup$ – StephenG Apr 29 '19 at 21:34
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The interactions of the Earth and its moon have slowed the Earth's rotation significantly over the billions of years since their formation. Wikipedia cites a paper that gives a figure of approximately five hours for rotational period in the past. We don't know of a similar method to slow down Mars.

This implies that the Earth must have had a much faster rotation than Mars in the past.

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    $\begingroup$ Can't resist asking whether the Earth's spin is integral or half-integral (Boson or Fermion) :-) $\endgroup$ – Carl Witthoft Apr 30 '19 at 12:48

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