|One year is the time required for the earth to travel around the sun.
|For most of history, however, people did not know that the earth moved around the sun.
|Instead, people believed that the sun moved around the earth.
|The old idea that the sun moved around the earth is known as the “geocentric” theory.
|This idea was first made famous by an ancient Greek scientist, Ptolemy,
|who lived in Egypt nearly 2000 years ago.
|Some other Greek philosophers had suggested instead that the earth might travel around the sun.
|However, this idea, which is known as the “heliocentric” theory, was not widely accepted.
|For centuries, people in Europe did not seriously question Ptolemy’s geocentric theory.
|During the early fifteenth century, however,
|a Polish astronomer named Nicolaus Copernicus
|began to think that the heliocentric idea was right,
|and that the geocentric idea was wrong.
|Shortly before his death,
|Copernicus wrote a book that described the mathematical details of his theory that the earth revolved, or traveled, around the sun.
|Later astronomers came to agree with Copernicus’s view.
|One of the most famous of these was Galileo, an Italian scientist.
|Galileo was famous for demonstrating that light and heavy objects fall at exactly the same speed,
|unless an object is so light that it is slowed by the air.
|Galileo was also the first astronomer to use a powerful telescope to observe the sky.
|He discovered many unknown features of the moon, the sun, and the planets.
|When Galileo announced that he believed in Copernicus’s theory that the earth revolved around the sun,
|some officials of the Roman Catholic Church were angry.
|They argued that this theory was against the beliefs of the Church.
|Some church officials disagreed with this view,
|but the authorities decided that Galileo should be punished by “house arrest”
|In other words, Galileo was not allowed to leave his house.
|Also, Galileo was forced to make a public statement that renounced his belief in the heliocentric theory.
|At about the same time that Galileo supported the heliocentric idea,
|another astronomer did some important work that supported Copernicus’s view.
|This astronomer was Johannes Kepler, who lived in Germany.
|Kepler used the observations of previous scientists to figure out the motion of the planets around the sun.
|He realized that the planets did not travel in circular paths,
|but instead, in paths that were elliptical, or oval, in shape.
|Kepler’s discoveries showed mathematically how the planets would revolve around the sun.
|Today, everyone knows that the earth moves around the sun;
|but for a long time, it seemed more reasonable to believe that the sun moved around the earth.
|We can thank the great scientists of the past who discovered the surprising truth.