While it's common to think of the solar system's planets moving in circles, orbits are actually ellipses -- oval rather than perfectly round. If they were circles, the sun would be at the exact center of the orbit. That's not true for ellipses, though for most of the planets, it's pretty close. The orbit is the result of two competing forces, the straight-line motion of the planet through space pushing against the sun's gravity.
German astronomer and mathematician Johannes Kepler crafted laws to describe the motion of planets in orbit. Most of his laws -- for example that planets move fastest in the part of their orbit closest to the sun -- were based on observation. Isaac Newton later developed three laws of motion and showed that by applying math to them, he could develop all of Kepler's laws in theory, as well as by observing the planets' movements.
Around and Around
Planetary orbits and rotation were the result of the solar system's creation. The sun and planets were born from the collapse of an interstellar gas cloud. When the vast cloud shrank and solidified, it did so with enough force that it put the bodies of the solar system in motion. Cornell University says online that a cloud that didn't have enough energy would create a single rotating star, with no planets around it.
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Just as planets orbit around the sun, they rotate around their axis, a line running through the center of the planet. Where orbital movement is restrained by gravity, rotation is limited by the tendency of bodies to hold together. If a planet rotated fast enough, it wouldn't be a planet because the speed with which it spins would break it apart. The Earth's axis of rotation is angled roughly 23.5 degrees from its orbital plane. This difference is what causes the seasons rather than an even climate year-round.