The Size and Shape of the Earth
Date: 2015-10-07; view: 371.
The size and the shape of the earth were not always calculated accurately. Most ancient peoples thought the earth was flat, but there are many simple proofs that the earth is a sphere. For instance, as a ship approaches from over the horizon, masts or funnels are visible. As the ship comes closer, more of its lower parts come into view. Final proof, of course, was provided by circumnavigating the globe and by photographs taken from spacecraft.
The Greek geographer and astronomer Eratosthenes was probably the first (about 225 B.C.) to measure successfully the circumference of the earth. The basis for his calculations was the measurement of the elevation of the sun from two different points on the globe. Two simultaneous observations were made, one from Alexandria, Egypt (Point B), and the other from a site on the Nile near the present Aswan Dam (Point A). At the latter point, a good vertical sighting could be made, as the sun was known to shine directly down a well at noon on the longest day (June 23) of the year.
Eratosthenes reasoned that if the earth were round, the noonday sun could not appear in the same position in the sky as seen by two widely separated observers. He compared the angular displacement of the sun (Y) with the distance between the two ground sites, A and B.
Eratosthenes measured the distance (X) between Points A and  as 5.000 stadia (about 575 miles – 925.175000 klm.). Although the observer at Point A saw the sun directly overhead at noon, the observer at  found the sun was inclined at an angle of 7° 12' (Y) to the vertical. Since a reading of 7° 12' corresponds to one-fiftieth of a full circle (360°), Eratosthenes reasoned that the measured ground distance of 5.000 stadia must represent one-fiftieth of the earth's circumference. He calculated the entire circumference to be about 28.750 miles (46.258750 klm.).
The following measurements are currently accepted:
Avg. diameter 7.918 mi. – 12.740062 klm.
Avg. radius 3.959 mi. – 6.370031 klm.
Avg. circumference 24.900 mi. – 40.064100 klm.
Exercise 7. Answer the following questions:
Questions:
- Were the size and the shape of the earth calculated accurately in ancient times?
- Most ancient people thought that the earth was round, didn't they?
- Are there any simple proofs that the earth is a sphere?
- The photographs of the globe were taken from spacecraft, weren't they?
- Who was the first to measure successfully the circumference of the earth?
- What was the basis of Eratosthenes' calculations?
- The distance between points A and B was 5.000 stadia, wasn't it?
- How long was the earth's circumference according to Eratosthenes' calculations?
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