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-------------------- 2491 - ASTRONOMY - how the Greeks started it?
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- The Greeks were the ancient astronomers. They were scientists. The Babylonians were great observers and recorders. The Egyptians were great technicians and technologists. But, the Greeks were scientists.
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- Eratosthenes was born in 276 B.C. in Libya. He worked as a librarian in the great city of knowledge, Alexandria. The Greeks realized the Earth was a sphere because their ships would disappear over the horizon with the ship’s mast still in sight.
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- During the Lunar eclipse they could see the circular arc of the Earth’s shadow on the Moon. And, the Moon and the Sun were spheres. Eratosthenes was the first to calculate the circumference of the Earth.
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- The city of Syene was located on the Tropic of Cancer and on June 21, noon on the summer solstice, the sunlight shone to the bottom of a deep well. At the same time in the city north, in Alexandria, a stick in the ground would cast a shadow of 7.2 degrees. The travel distance between Syene and Alexandria was 5,000 stades.
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- A stade was a standard distance over which races were held. Eratosthenes was an athlete who participated in the pentathlon and was very familiar with stades.
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- A stade was 157 meters. The Sun’s rays were parallel to the Earth’s surface so the 7.2 degrees was the same angle as the angle of radii from the center of the Earth to the two towns at its surface.
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-------------If 7.2 degrees was 157 m * 5,000 = 785 kilometers,
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-------------then 360 degrees was 360/7.2 * 785 km = 39,250 kilometers for the circumference of the Earth.
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-------------Today’s best measurements are 40,100 km. His calculations were within 2% of being dead on.
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- The Greeks knew that every circle had a circumference that was 3.14 times its diameter. Therefore the diameter of the Earth was 12,500 kilometers. ( Earth’s actual diameter = 12,750 km)
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- The Greeks calculated the diameter of the Moon by observing a Lunar eclipse. It took 50 minutes for the Moon to first touch Earth’s shadow and than to be fully covered. It took 200 minutes for the Moon to pass completely through Earth’s shadow.
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- So, 50 minutes represented the diameter of the Moon and 200 minutes represented the diameter of the Earth. the Moon was ¼ the diameter of the Earth. ¼ * 12,500 km = 3,125 km (Moon’s actual diameter = 3,480 km)
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- Once Eratosthenes knew the diameter of Moon he could calculate the distance to the Moon. During a full Moon stretch out your arm and note that your thumbnail just covers the diameter of the Full Moon. The ratio between the length of your arm to the width of your thumbnail is 100 to 1.
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- This observation forms two equilateral triangles so the distance to the Moon must be 100 times its diameter, 100 * 3,125 km = 312,500 km (Actual distance to the Moon = 384,000 km).
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- The Greeks realized that moonshine was reflected light from the Sun. A quarter Moon (really a half-moon) must occur when the Sun, Moon and Earth form a right triangle. Aristarchus measured the angle to be 87 degrees and calculated the Sun to be 20 times further away than the Moon.
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- This method was sound but the measurement was off. The correct angle is 89.85 degrees. Using trigonometry, the hypotenuse of a right triangle = cosine of the angle * the adjacent side. Cosine of 89.85 degrees = 0.0026. 384,000 km / 0.0026 = 147,000,000 km. ( Actual Earth-Sun distance = 150,000,000 km).
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- Philolaus, a student of Pythagoras, in 500 B.C. suggested that the Earth orbited the Sun. Aristarchus in 310 B.C. took Philolaus’ idea and diagramed the solar system out to Saturn. He had the Earth and planets orbiting the Sun and the Earth spinning on its axis every 24 hours.
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- It took 1,700 years later for the Europeans to rediscover this idea. Until then the Sun-centered universe ran counter to common sense.
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- Einstein said common sense was a collection of prejudices acquired by the age of 18. “The Earth must be stationary other wise we would feel it move“.
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- It took Nicholas Copernicus , 1,700 years later, to finally reinvent Aristarchus’ Sun-centered Universe. He published only 20 pages, called the “Little Commentary“, but it contained these 7 axioms:
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-------------- 1. The heavenly bodies do not share a common center.
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------------- 2. The center of the Earth is not the center of the Universe.
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------------- 3. The center of the Universe is near the Sun
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------------- 4. The distance from the Earth to the Sun is insignificant compared with the distance to the stars.
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------------- 5. The apparent daily motion of the stars is the result of the Earth’s rotation on its own axis.
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------------- 6. The apparent annual sequence of movements of the Sun is a result of the Earth’s revolution around it. All the planets revolve around the Sun.
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--------------7. The apparent retrograde motion of some of the planets is merely the result of our position as observer on a moving Earth.
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- For the next 30 years Copernicus worked on his document to collect 200 pages of evidence. In 1543 his book, “On the Revolutions of the Heavenly Spheres” was finally printed, 700 copies.
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- He died that same year after setting eyes on the first copy. It still took many years after that before most people were convinced that the Earth was not the center of the Universe. The Earth is not flat.
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- November 20, 2019 2491 758
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--------------------- Wednesday, November 20, 2019 --------------------
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