When the moon appears to us in its phase of First Quarter or Last Quarter, it is "dichotomized," or half illuminated. At the moment when the dividing line between light and dark exactly bisects the moon's circle, the angle from Earth to Moon to Sun is exactly 90 degrees. Aristarchus determined that at this precise moment the angular separation between the moon and the sun, that is, the angle from Moon to Earth to Sun, is equal to 87 degrees, as he stated in his fourth hypothesis. Using these angles, he determined (without using trigonometric tables or formulae--they weren't invented yet) that the ratio of the distance ES to EM was greater than 18 to 1, but less than 20 to 1.