How and why does Earth spin?

Imagine that you were an ice scatter on the ice. Normally, to execute a spin you have to launch yourself into it in some way. For Earth, this means that either the matter that formed it was originally moving, or that some large body hit it while it was forming, and that incident imparted the whack to set it spinning. Both of these processes probably happened, because we can still see them going on in the solar system today, but vastly slowed down. We are pretty sure that Earth was bombarded by large asteroids and planetoids hundreds of miles in size because we see cratering from such bodies all across the solar system.

We think that the last major impact with an object nearly as big as Mars, not only tipped Earth to its present inclination, but also tore material out of our planet, and formed a ring around it. A part of this ring condensed to become the Moon within the first 100 million years of Earth's formation. The impact energy liquefied the entire surface of Earth perhaps 4.4 billion years ago. It is amazing that the first fossils are present in rocks 3.8 billion years ago. Life was hell-bent on forming on this planet so soon after it had suffered a catastrophic impact and formed the first traces of a solid crust.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

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