MadSci Network: Astronomy
Query:

Re: What makes the Earth rotate on its axis?

Date: Sat Feb 21 10:20:36 1998
Posted By: James Steele Foerch, Instructor, Pine Street Creative Arts Academy
Area of science: Astronomy
ID: 887735103.As
Message:

Dear Ms Dolinaj,

Our solar system- the Sun, planets, Kuiper belt and Oort cloud, everything orbiting the sun even many times further than the orbit of Pluto- is thought to have condensed from a vast cloud of interstellar gas some billions of years ago when that cloud was shocked or squeezed by a supernova explosion. The dust and grains that condensed out of the cooling solar gas aggregated to form larger fragments of rock. Chondritic meteorites are basically just such collections of grains and fragments that have been compacted together to form a larger piece of rock and eventually small planetary bodies. Such bodies are termed planetesimals when they become roughly as large as asteroids (several kilometres to a few hundred kilometres in dimension). The larger they grow, the greater the gravitational attraction that the planetesimals exert and hence the more effectively they sweep up additional particles and rock fragments while circling the Sun. (Britannica CD)

Because that initial shockwave came from a particular direction and because the cloud was not of uniform density, the particles had a net rotation around their center of gravity. Humans subsequently identified that direction as "counterclockwise as viewed from above the north pole of the sun". As the bodies of the solar system grew by accretion, particles swept up from different distances from the center of gravity (the sun) had different orbital velocities. Analogously, water swirling down a drain moves slower further and faster closer to the center.

Galileo and Newton showed that rotational energy is conserved: The sun and planets ‘inherited' their spin from the orbits of their constituent particles, so to speak. Today, some five billion years later, all that rotational energy is still conserved in the rotations and revolutions of the bodies of the solar system. Our solar system is not unchanging, though. Because of gravitational interactions between moons and their planets, between the planets, meteors and comets themselves and interactions between the solar wind and the magnetospheres of the planets, rotational energy is still be ‘traded' back and forth: Mercury orbits in a 3:2 resonance with its rotation, our day is getting longer while the moon only shows one face towards us, the rings of the gas giants are temporary phenomena,... But that's another story.

Sincerely yours, James Steele Foerch


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