A Brief History of the Universe -(Illustrated Edition)

Author: (English) Stephen Hawking / Country: Mainland China
Publisher:
Publish Date: 2006-05-01
Features: As early as 340 BC, Aristotle, in his work On the Heavens, already presented two compelling arguments to support the belief that the Earth is a sphere rather than a flat plate. , he argued that lunar eclipses occur because the Earth comes between the Sun and the Moon. The shadow of the Earth on the Moon is always round, which is only possible if the Earth is spherical. If the Earth were a flat disc, its shadow would be elongated into an ellipse, and lunar eclipses would always occur when the Sun is directly above the center of the disc. Second, the ancient Greeks had learned from travel that the position of the North Star in the sky, as seen from the south, is lower than that seen from the north. Aristotle even estimated the circumference of the Earth based on the difference in the apparent position of the North Star between Egypt and Greece, arriving at a value of 400,000 stadia. A stadion is an ancient Greek unit of length, the exact length of which is now unknown, but it is known to be approximately 200 yards. If so, Aristotle's estimate was roughly twice the currently accepted value. The ancient Greeks had a third argument to support the idea that the Earth must be spherical: why people first see the sail of a ship appearing on the horizon, then the rest of the ship. Aristotle believed that the Earth is stationary, while the Sun, Moon, planets, and stars all move in circular orbits around it. He was firmly convinced of this because, for mysterious reasons, he felt that the Earth is the center of the universe, and circular motion is perfect. His concept was refined into a complete cosmological model by Ptolemy in the 1st century AD. The Earth is at the center, surrounded by eight celestial spheres that carry the Moon, Sun, stars, and the five known planets at the time: Mercury, Venus, Mars, Jupiter, and Saturn. Each planet moves along a smaller circular orbit within its respective celestial sphere, which explains the relatively complex paths observed in the sky. Beyond the outermost celestial sphere are the so-called "fixed stars," which always maintain the same relative positions but rotate together in the sky. What lies beyond the outermost celestial sphere has never been clearly explained, but it was certainly not part of the observable universe. Ptolemy's model provided a fairly accurate system for predicting the positions of celestial bodies in the sky. However, to accurately predict their positions, Ptolemy had to make an assumption: the distance of the Moon's orbit from the Earth at certain times is twice that at other times. This meant that the Moon sometimes appeared to be twice as large as usual. Ptolemy recognized this as a flaw, but his model, though not without criticism, was accepted by the majority. It was also adopted by the Catholic Church as a description of the universe that was consistent with the Bible. Its greatest advantage was that it placed the celestial sphere beyond the stars.