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History/Mooon |
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 thought 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. Eratoshenes was not the only ancient astronomer that made incredbile measurements of the Earth, Sun and Moon system. Aristarchus, another Alexdrian astronomer, believed that at first and last quarter moon the angle between the Earth, Sun and Moon must be 90o. Aristarchus also assumed that the Moon orbited the Earth at a uniform rate, something that we have learned is incorrect. However, he did make the claim that the Moon that must take longer to travel from first quarter to last quarter than it does to traverse its orbit from last quarter to first quarter. In fact, Aristarchus believed that the moon took takes roughly one more day to travel from first to last quarter than it does to go from last quarter to first quarter. In the end, Aristarchus concluded that the Sun is approxmately 20 times further away from us than the Moon. The actualy value is 390 times. He also tried to measure the realtive sizes of the Sun and the Moon. He concluded that the Sun is 20 times larger than the Moon. We now know that the Sun is much, much larger than the Moon (about 400 times as large). What can we conclude from all of this? Well, that science is an evolving entity. Everything we know is built upon the work of others and to them we owe much thanks for taking the first difficult steps in attempting to gain an understanding of the universe around us. |
