At a certain time of day the sun is positioned such that the length of an object's shadow is equal to its height. Since the height of any object on the earth is insignificant compared to the distance of the sun from the earth this time of day is essentially the same for all objects. In this picture a=b and c=d. If Thales observed this phenomenon he could have determined the height of a pyramid by measuring the length of its shadow at the right time of day.