It has been observed that each year at the time when the sun reaches his equinoctial point of Aries 0, when throughout the earth, the day and night are equal in length, the position of the earth in reference to some fixed star is nearly 50"1/3 of space farther west than the earth was, at the same equinoctial movement of the previous year. This slight increase -retrograde motion of the equinoxes- is known as the Precession of the Equinoxes.