Thus while the fact of Precession is very clearly defined by these observations, the amount of it is not accurately determined. Later observations with more perfect means of measurement have enabled the official astronomers to give a true value to the Precession of the Equinoxes, which is 50·2453´´ plus O·0002225´´ _t_ where _t_ is the number of years from 1850. Thus while Hipparchus and Ptolemy found it to be 1° in 100 years, Albatani found it to be only 1° in 66 years, and modern observations seem to point the fact that it has been accelerated up to our own time. By taking a mean of all observations during the last eighteen centuries, the amount appears to be 50·062´´ and by extending the observations we can work it down to 50´´ nearly. Now if we divide the circle of the zodiac, or 360° by 50´´ we shall arrive at the figures 25,920 comprised in the Great Year. There are two theories in existence as to the cause of this Precession. At the point of time when the Sun crosses the Equator and comes to the first point of Aries, let the Sun be in line with a star in the heavens. When the Sun returns to the same position on the Equator it is found not to be in line with that star but 50´´ in advance of it. The first theory is that this is due to the change in the obliquity of the ecliptic, or, what is the same thing, in the inclination of the earth’s axis to the plane of the ecliptic, a change caused by the action of the Moon and Sun on the equatorial bulge. The whole effect is taken to be about 50´´ as said, and of this 35´´ is due to the Moon and 15´´ to the Sun. Various observations have shown this change of axial inclination to be about 50´´ per century, or one one-hundredth part of the Precession of the Equinoxes. Of this I shall have something to say later. For the moment we may consider it as part of the theory of Precession. The other theory is that Precession is caused by the proper motion of the Sun through space, whereby it completes an arc of 50´´ of its vast orbit every year. Then the gradual increase of the Precession already noted would in terms of this theory be due to the fact that the earth is getting nearer to the Sun and consequently the angle of parallax would be greater. Here we may leave the theorists with their own material and turn to the significance of this Great Year. As the whole Year consists of 25,920, during which the Equinoxes precess an entire circle, there will be a period of 2160 occupied in traversing one sign, that is to say, a twelfth part of the circle. Now this period is 72 periods of the planet Saturn, otherwise known as Kronos, the god of Time. From this again we derive the foundation of the great Yugas or Ages known to the Indians and referred to in the Vishnu Purana as the Four Ages, Satya-yuga, Treta-yuga, Dvapara-yuga, and Kali-yuga. They correspond with the four ages known as the Gold, Silver, Copper and Iron ages. Thus— 72 × 6 equals 432 multiplied by 1000 = 432,000 years.