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| History/Mooon Moon's Orbit Moon's Phases Sideral/Synodic time Moon's Orbital Properties Size of the Earth Size of Earth (cont) Eclipses Eclipses (cont) Causes for Eclipses |
Over two thousand years ago Greek
astronomers knew that the Earth was sphereical. During lunar eclipses the Moon passes
through the Earth's shadow. The Greeks noticed that the shadow cast by the Earth was
always circular, and the only object that always casts a circular shadow is a sphere.
Therefore, the Greeks concluded that the Earth was a sphere. However a nagging question was how big was the Earth? About 200 BC. Eratosthenes, a Greek astronomer, discovered a way to measure the circumference of the Earth. He had heard reports from the city of Syene Egypt, which was on the equator, that the Sun shown directly down vertical wells on the first day of summer. Eratoshtenes did not observe such phenonmenon at his home, thus he concluded that the Sun never reaches Zenith at his home in Alexandria 7o north of Syene. Eratosthenes measured the Sun to be about 7o south of his local zenith on the first day of summer (the summer solstice). Based upon this observation is concluded that distance from Alexandria and Syene must be 7/360 or 1/50 that of Earth's circumference since 360o make up a complete circle. At the time the standardunit of measurement was called a stade and is guessed to be about 1/6 of a kilometer. The distance from Syene to Alexandria was about 5000 stades. Thus, Eratosthenes estimated the Earth's circumference to be about 50 x 5000 = 25,000 stades = 42,000 km. The modern value for the circumference of the Earth is 40,000 km. So Eratosthenes was correct to within 5% of the actual value. |
