While Copernicus was the first to widely promote the discovery that planets orbited the sun, Johannes Kepler was first to explain just how they did it with any accuracy. Working during the late 16th and early 17th centuries, the German Kepler made the key discovery that the planets move in elliptical, not circular, orbits. Using astronomical data gathered from the observations of his mentor and rival Tycho Brahe, Kepler came up with the Three Laws of Planetary Motion, for which he is now famous. The first law said that the planets' orbits are ellipses, with the sun as a stationary point of focus. The second law says that a line connecting the planet to its focal point (the sun) will sweep over equal areas in equal amounts of time. This principle applies even though the distance between Earth and the sun changes as the Earth moves through its elliptical orbit. Basically, the implication of this principle is that a planet moves faster when its orbit takes it nearest the sun, and it moves more slowly when the orbit swings it farther away. The third law lays out an equation that explains the relationship between the distance of a planet from the sun and the length of its revolutionary period. At its core, the third law says the time it takes for a planet to make a revolution around the sun is proportional to its distance from the sun, so planets that are farther away have longer years.
While all of these laws are still considered fundamentally true, they required some tweaking. Isaac Newton used his Laws of Motion and Gravity to demonstrate inconsistencies in Kepler's laws. For instance, Newton knew that the sun could not occupy a fixed position. Instead, the sun itself must move because, thanks to gravity, the planets pull on the sun just as the sun pulls on them. This means that the second part of the first law -- that the sun sits motionless -- cannot be true. Newton adjusted the third law to accommodate for the mass of the planet as well as its distance from the sun.
Today, scientists are still questioning and probing how the planets move. They're even trying to decide if it is orbit or size or some other criterion that makes a body of matter in space qualify as a "planet." In 2006, the International Astronomical Union demoted Pluto, which does indeed orbit the sun, to dwarf planet status because of its small size [source: Matson]. So if size makes a planet, what do we call the perhaps billions of planet-sized bodies that are adrift in our galaxy, seemingly unattached to any stars? They are as large as planets -- some bigger than Jupiter -- yet they don't orbit any star directly, as far as astronomers can tell [source: Nature].
Jupiter, its great Red Spot three of its four largest satellites (NASA)
There are eight planets in our solar system (Pluto was reclassified as a "dwarf planet"). While each planet is unique, they do share a number of similar features. Each planet has an imaginary axis running through its center and joining its north and south poles. Like Earth, each planet rotates on this axis and, in most cases, the time it takes the planet to complete a full rotation equals the length of its day. This is called a rotation period. In addition to rotating on its own axis, each planet also revolves around the sun. One revolution around the sun equals a year. However, each planet takes a different amount of time to complete its year.
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