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How Do Planets Orbit in Space?

The three laws of planetary motion, formulated by Johannes Kepler in the seventeenth century, describe the motions of the planets in our solar system to a high degree of accuracy. Historians of science credit these laws with helping Sir Isaac Newton arrive at the universal law of gravitation. Because the laws of physics are universal, the planets of other stars must also obey these laws. Interestingly, the modern theory of gravity, Einstein's General Theory of Relativity, describes gravity not as a "force" operating between masses, but a consequence of the curvature of space caused by the presence of mass. This theory has explained certain gravitational phenomena, such as the tiny shifts in Mercury's orbit, that the laws of Newton and Kepler could not.

Orbital Plane Of The Planets
- The eight planets in our solar system orbit the sun in one direction and in nearly the same plane. All the planets remain within eight degrees of the "ecliptic," the apparent path of the sun across the sky. The periodic movements of the planets away from the ecliptic plane form the band in the sky called the Zodiac. Modern planetary theory holds that the sun and planets formed from a vast "solar nebula", some 4.6 billion years ago. The laws of physics require that the nebula must have been rotating, and, over time, flattened into a rotating disc. The coplanar (same plane) orbits are attributed to this single disc of rotating primordial material.
Kepler's First Law
- Kepler's first law states that planetary orbits around the sun are mathematical shapes called ellipses, the sun occupying one of the two foci of each ellipse. Because an ellipse is a continuous curve, a planet's direction is continually changing. Therefore, each planet remains in a state of continual acceleration with respect to direction, but not with respect to velocity. According to Newton's second law of motion, the force due to the acceleration is directed toward the sun. With the exception of Mars, the elliptical orbits of the planets are nearly circular.
Kepler's Second Law
- Kepler's second law states that each planet revolves in such a way that the line joining it to the sun sweeps over equal areas in equal lengths of time. The orbital speed of each planet varies inversely as the square-root of its distance from the sun. With the exceptions of Venus and Uranus, the planets rotate in the same direction as their motion around the sun.
Kepler's Third Law
- Kepler's third law states that the squares of the periods, or revolution times, of any two planets are in the same proportion as the cubes of their average distances from the sun. From the mathematical expression of this law, it can be shown that the average distances of all the planets can be expressed in terms of the distance of only one planet. The natural yardstick for this is the Earth's average distance from the sun, which is called the "astronomical unit."

Astronomy