Calculating the Earth’s circumference by Al Biruni in Middle Ages | Jim Al Khalili (EN)
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Abū Rayḥān Al-Bīrūnī (913-1050 AD) is a Muslim scientist who figured out a truly remarkable and ingenious method for calculating the Earth's radius (and subsequently its circumference) with less than 1% (321 Km) margin of error. This method was very simple yet accurate, requiring just four measurements in all to be taken and then applying a trigonometric equation to arrive at the solution. What Biruni figured out with unprecedented accuracy and precision in the 10th century was not known to the west until the 16th century.

Short video taken from Jim Al-Khalili's 3-part documentary "Science & Islam"