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Eratosthenes: Earth
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Eratosthenes will always be remembered for the calculation of the Earth's circumference circa 240 BC, using trigonometry and knowledge of the angle of elevation of the Sun at noon in Alexandria and Syene (now Aswan, Egypt). The calculation is based on the assumption that the Earth is spherical and that the Sun is so far away that its rays can be taken as parallel.
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Eratosthenes' experiment was highly regarded at the time, and his estimate of the Earth’s size was accepted for hundreds of years afterwards. His method was used by Posidonius about 150 years later.
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The circumference of the earth calculated by Eratosthenes was only two hundred miles off from the modern day figure, and considering the magnitude of the numbers, it can be considered the first accurate measurement. After his discovery, he provided the first mathematically based map of the world. His Geographica, geography book of the world, was finally complete.
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In the illustrations you can see, as Eratosthenes did, that the string- and post-lines at Alexandria formed the same angle as the alternate angle formed by the post- and sunray-lines at the center of the earth. Both represent angles of 7*12'. With this fact and simple arithmetic Eratosthenes was able to calculate the approximate circumference of the earth.
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