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Earth, like the other planets, follows its path,
or orbit, around the sun. The period of time that the earth takes to go
around the sun is called a year. The other planets have orbits larger or
smaller than the earth's.
How this solar system came to be and how the
planets came to have the size, location, and orbits they have, astronomers
cannot fully explain. But they have two main types of theories. One
type of theory suggests that the formation of the planets was a part of the
gradual change of the sun from a whirling mass of hot gas to its present size
and brilliance. The planets formed as small whirling masses in the giant
gas and dust cloud as it turned.
No matter which theory is right, the solar
system came to be as it now is more or less by chance. Why does it stay
this way? Kepler's Laws of Planetary Motion state that all planets travel
about the sun in an elliptical (oval) path; that a planet moves faster in
its orbit as it nears the sun; and that there is a relation between its distance
from the sun and the time it takes to make an orbit. Newton's Law of
Gravitation, of which Kepler's three laws were an indispensable part, explained
how two objects attract each other. So the solar systems remains as it is
because certain laws of nature maintain the relationship of the sun and the
planets.
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