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.