Precession of the Equinoxes, page 4
For the last two thousand years or so, the constellations that carried the sun into the dawn sky have been:
However, precession, as we have noticed, is changing the relationship between the sun and its dawn constellations by 1° (two Moon widths) every 72 years.
The constellations that the sun passes through on its annual journey round the heavens are those of our familiar zodiac. Although the constellations of the zodiac vary in length, by convention we say that each zodiac constellation dominates a 30° segment of the sky. For the sun to move by 30° in relation to the dawn constellation will take just over 2000 years, by which time a different constellation is carrying the sun into the dawn sky.
We call this long period, when the sun rises under the same constellation, an 'age', and name the age for the constellation that carries the sun on the spring equinox. Thus we are living in the Age of Pisces, and are about to enter the Age of Aquarius. (Yes, this really is the dawning of the Age of Aquarius, though we have a couple of centuries to go yet.) The position of the equinoxes relative to the age of Pisces are shown in Figure 7 for 2000 years ago and Figure 8 for our time.
In former times, they may not have had this precision, so marking the actual beginning of an age would be a matter of dispute and contention. Some constellations overlap, for example Libra and Virgo. There is a large gap between Gemini and Taurus, and Scorpio is hardly touched by the sun at all. We can imagine there may have been considerable dispute amongst ancient priest-astronomers about the actual starting point of the new age. Signs in the sky would therefore be important. Santillana and von Dechend determined that conjunctions of Jupiter and Saturn were significant to the ancients who may then have used this as a marker. These transitions between ages saw the world enter a cataclysm when all the old gods died, and new ones had to be found.