How ERATOSTHENES calculated the circumference of the Earth ? 

ERATOSTHENES method to calculate circumference of the earth
Eratosthenes calculated the circumference of the earth by using the shadow of the stick and simple geometry.
Eratosthenes also calculate sun to earth distance(150 million Km) and angle of earth spinning axis with vertical.
Eratosthenes knew that sun cast no shadow of vertical things in Syene, Egypt in summer solstice on 21 June at 12.00pm and at the same time sun cast shadow of vertical things in Alexandria.
By using the cast of stick in Alexandria, Eratosthenes calculated that Alexandria and Syene are off by vertically 7.2 degrees and this result showed that Earth has curvature.
Using this angle and distance from Alexandria and Syene calculated the circumference of the Earth (46,250 km).
#Eratosthenes #EarthRadius
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