Conservation of angular momentum explains why an ice skater spins more rapidly as she pulls her arms in. As her arms come closer to her axis of rotation, her speed increases and her angular momentum remains the same. Similarly, her rotation slows when she extends her arms at the conclusion of the spin.
As an interstellar cloud collapses, it fragments into smaller pieces, each collapsing independently and each carrying part of the original angular momentum.
The rotating clouds flatten into protostellar disks, out of which individual stars and their planets form. By a mechanism not fully understood, but believed to be associated with the strong magnetic fields associated with a young star, most of the angular momentum is transferred into the remnant accretion disk.
Planets form from material in this disk, through accretion of smaller particles. In our solar system, the giant gas planets Jupiter, Saturn, Uranus, and Neptune spin more rapidly on their axes than the inner planets do and possess most of the system's angular momentum.
The sun itself rotates slowly, only once a month. The planets all revolve around the sun in the same direction and in virtually the same plane. In addition, they all rotate in the same general direction, with the exceptions of Venus and Uranus. Most Recent. Is speed of light the same everywhere?
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When our solar system was nothing but a cloud of gas and dust, what was likely a shock wave from a nearby supernova bounced up against it and caused it to collapse. As it collapsed, its own gravitational forces pulled it into a flat, spinning disk. And since everything in our solar system was formed from that same disk, its momentum sent nearly everything spinning in the same direction.
Notable exceptions include Uranus and Venus, whose odd spins probably stem from subsequent collisions with asteroids. Our planets have continued spinning because of inertia. In the vacuum of space, spinning objects maintain their momentum and direction — their spin — because no external forces have been applied to stop them. And so, the world — and the rest of the planets in our solar system — keeps spinning.
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