The person sees Earth flat, but for a long time is established that Earth is a sphere. People agreed to call this celestial body the planet. From where did such name undertake?
The Ancient Greek astronomers watching behavior of celestial bodies put two terms, opposite on sense, into practice: planetes asteres - "wandering stars" are the celestial bodies similar to stars moving for a year; asteres aplanis - "fixed stars" are the celestial bodies remaining motionless within a year. In beliefs of Greeks Earth was not mobile and was in the center of the Universe therefore they referred it to category of "fixed stars". Venus, Mars, Jupiter and Saturn, obvious were known to Greeks Mercury, but they called them not "planets", but "the wandering stars". In Ancient Rome the astronomers already called these bodies "planets", having filled up this list with the Sun and the Moon. Idea of a seven-planetary system remained before the Middle Ages. Nicolaus Copernicus in the 16th century turned views of space device, having noticed its geliocentrichnost. The earth which was considered earlier as the center of the world was reduced to the provision of one of the planets rotating around the Sun. In 1543 Copernicus published the work under the name "About Addresses of Heavenly Spheres" in which stated the point of view. Unfortunately, the church did not estimate the revolutionary nature of views of Copernicus: its sad fate is known. By the way, according to Engels, "release of natural sciences from theology" begins the chronology with the published Copernicus's work. So, Copernicus replaced the geocentric system of the world heliocentric. The name "planets" was assigned to Earth. Definition of the planet, in general, always was ambiguous. Some astronomers claim that the planet has to be rather massive, others consider it an optional condition. If to approach a question it is formal, it is possible to call Earth safely the planet, at least because the word "planet" came from the Ancient Greek planis meaning "mobile", and the modern science does not doubt mobility of Earth.