The Earth as a planet

THE SOLAR SYSTEM The discovery and description of the planets To appreciate how impressive the night sky must have been to early man it is necessary today to go to a place remote from the distracting lights and pollution of urban centers. Viewed from the wilderness the firmaments appear to the naked eye as a canopy of shining points, fixed in space relative to each other. Early observers noted that the star pattern appeared to move regularly and used this as a basis for determining the timing of events. More than 3000 years ago, in about the thirteenth century BC, the year and month were combined in a working calendar by the Chinese, and about 350 BC the Chinese astronomer Shih Shen prepared a catalog of the positions of 800 stars. The ancient Greeks observed that several celestial bodies moved back and forth against this fixed background and called them the planetes , meaning “wanderers.” In addition to the Sun and Moon, the naked eye could discern the planets Mercury, Venus, Mars, Jupiter and Saturn. Geometrical ideas were introduced into astronomy by the Greek philosopher Thales in the sixth century BC. This advance enabled the Greeks to develop astronomy to its highest point in the ancient world. Aristotle (384–322 BC) summarized the Greek work performed prior to his time and proposed a model of the universe with the Earth at its center.