Two Types of Planetary Motion

An orrery shows the motions of the planets through the solar system.
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All planets, including the Earth, move in two ways. Earth moves around the sun, completing one solar orbit in roughly 365 days. The planet also spins on its axis, rotating completely around every 24 hours. Other planets take different times to do the same things, but they all go through the same types of motions.

Orbit Mechanics

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.

All planets, including the Earth, move in two ways. Earth moves around the sun, completing one solar orbit in roughly 365 days. The planet also spins on its axis, rotating completely around every 24 hours. Other planets take different times to do the same things, but they all go through the same types of motions.

Astronomy History

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.

All planets, including the Earth, move in two ways. Earth moves around the sun, completing one solar orbit in roughly 365 days. The planet also spins on its axis, rotating completely around every 24 hours. Other planets take different times to do the same things, but they all go through the same types of motions.

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.

All planets, including the Earth, move in two ways. Earth moves around the sun, completing one solar orbit in roughly 365 days. The planet also spins on its axis, rotating completely around every 24 hours. Other planets take different times to do the same things, but they all go through the same types of motions.

Rotation Rules

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.

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