three o'clock
`` In spherical trigonometry , '' he concluded , `` to solve the problem three elements must be known . I knew four . Therefore I could take each of the known , treat it as unknown , and have four ways to check my result . I find that the time might have been either three o'clock , twenty-one minutes and twelve seconds , in the afternoon , or 3:21:31 , or 3:21:29 , or 3:21:33 . The average is 3:21:26 , and there can therefore be no appreciable error except for a few seconds . For that date must have been one of two days , either May 22 or July 22 . Between these two dates we must decide on evidence other than the shadow .