Eratosthenes measured the circumference of earth to within 4% of the correct value eratosthenes measured the circumference of earth to within 4% of the correct value noting how much more of the celestial sphere came into view when walking south. using the detailed reports of far-sailing mariners. looking at stick shadows on the solstice. sighting the stars from within the deepest well in egypt on the day of the solstice.