You would think that if the Sun is located exactly on the celestial equator as it does on the dates of the equinoxes, that the Earth would have exactly 12 hours of daylight and 12 hours of night time. The problem is that the Earth is constantly moving in its orbit, which means that the Sun constantly moves in the sky. During the 12 hours of daylight, the Sun will move about 0.9/2 = 0.5 degrees. This is enough so that when you try, then, to measure the length of the next 12 hours of night time, your number will not come out to be exactly 12 hours, because you are now 12 hours PAST the Equinox!