The Earth was not "discovered" until the 16th century. That was when people figured out that the Earth was a planet, and was not flat as was believed.
During the 16th century, a scientist by the name of Nicolaus Copernicus discovered that the Earth was a planet and that it revolved around the sun. Before that, people thought the Earth was flat and that it was the center of the solar system.
Copernicus studied the theories published by Ptolemy, who first realized that the Earth was not the center of the universe and did so with the naked eye. Galileo was the first scientist to use a telescope, but this did not occur until 50 years later.