Earth

The Greeks were clever scientists. By the time of Aristotle they had already figured out that the Earth was round. They came to this conclusion through a variety of increasingly sophisticated means. While watching lunar eclipses, they noticed that the Earth threw a circular shadow on the Moon. Seafarers, the Greeks noticed small hints of the curvature of the Earth by watching the horizon as it slowly revealed the shape of approaching islands. By recording the differences between the distribution of the stars in the night sky at different locations, including the varying height of the Pole Star, the Greeks discovered the idea of latitude, which implied different locations on the surface of a sphere. In the third century BC, Eratosthenes found the radius of the Earth quite accurately by simultaneously measuring the different lengths of two shadows located in two separate places located along a straight north-south line, and using geometry to figure out the rest.

Yet despite their general scientific acumen, the Greeks, along with Aristotle (384-322 BC), perhaps the most respected of classical physicists and astronomers, believed that the Earth was at the center of the universe. Such belief was shared by many major civilizations, from the Chinese to the Maya. The origin of this common misconception about the nature of universe is not hard to understand. Each of these civilizations based their beliefs on common-sense observation and understanding of the laws of physics and astronomical phenomena. Limited as they were technologically, the Greeks and others could only experiment, or even conceive of, physical phenomena in reference to the Earth. Such experimentation clearly showed that the natural motion of things on Earth was clearly vertical with respect to the ground: a rock fell to the ground, fire leapt into the air, bubbles moved upward through water. It seemed only natural that since everything on earth acted in reference to the Earth's center, so would everything in the universe. After all, any movement perpendicular to the vertical axis between the Earth's center and its sky always petered out; of course, today we correctly attribute such a phenomenon to friction.

The belief that the Earth stood at the pivot of the universe remained a keystone of human thought for over two thousand years, the stability of this incorrect notion no doubt augmented by the vanity inherent in the idea of the Earth's centrality, and the theological importance of the Earth's centrality for Judeo-Christian religious beliefs which held that God himself created the Earth.

Yet as religious influence began to wane in the seventeenth century, and as scientists began to pay detailed attention to the movement of the planets, Sun, Moon, and stars, it became clear that these objects did not seem to obey the same rules as earthly things. They apparently moved in circles around the Earth, their motion recurrent and unaffected by 'earthly' physics laws. The only way to conceive of these objects within the old "four element" system was to see them as made of a "fifth" element, eternal and perfect, called ether. Because of their 'perfect' motion, the skies were perceived as a reflection of the world of the Gods. Yet after numerous scientists such as Copernicus, Tycho, Kepler, Galileo, and Newton began to build upon each other's work, the modern view of the solar system as heliocentric began to take hold (for an extended discussion of the history behind the evolution of the modern view of the universe, see the Scientific Revolution SparkNote).

As modern science evolved, so did our understanding about our planet and the time scales of geological phenomena. Even in the 19th century western world, it was widely believed that our planet was just a few thousand years old. Though formalized by the literal interpretation of some passages in the Bible, this belief was consistent with the beliefs that preceded it, and it was shared by other civilizations. The modern scientific view of an ancient Earth 4.5 billion years old is only the product of the science of the last century, and is based on strong and consistent geological, biological, and astronomical evidence.