___________, working at a library in Alexandria, computed the size of earth by measuring altitude of the sun on summer solstice.

Eratosthenes, a Greek scholar and chief librarian at Alexandria, calculated Earth's circumference around 240 BCE by measuring the sun's angle at noon on the summer solstice at two locations (Alexandria and Syene). His method was remarkably accurate, yielding a value close to the actual circumference. Hipparchus came later and developed trigonometry.