Like the tilted axis of a spinning top or gyroscope, the Earth’s axis is slowly but continuously changing its orientation, causing an imaginary extension of that axis to trace a repeated circular path in space. It takes the Earth’s axis an estimated 25,772 years to complete one full circle, also known as a Great Year.
To an observer on Earth gazing at the stars year after year, the change is almost imperceptible. A star's location in the sky shifts just slightly more than one degree every 72 years. But time adds up. It takes each zodiac constellation approximately 2,000+ years to pass completely through the location known as the “house of the rising sun” at spring equinox. In other words, over the course of a Great Year, each zodiac sign is imagined to rule over its own 2,000+ year astrological age while located in that "house." Because the constellations appear there in reverse order through the centuries, it is known as the precession of the equinoxes. And, because the direction of the Earth’s axis changes over time, different stars become the “pole star” in different eras.
Civilizations both ancient and modern have attached great importance to astrological ages. Each age appears to have its own overriding theme. The Age of Taurus (the Bull), which began approximately 6,000 years ago, was said to have been the age of agriculture. The Age of Aries (the Ram), which began approximately 4,000 years ago, was considered the age of war and nations. Appropriately enough, visitors heading toward the entrance of Egypt’s great Karnak Temple, built during that astrological age, pass along a row of stone ram sphinxes.
Many believe the Jesus fish symbol signifies the astrological Age of Pisces (the Fish), which is seen as the age of religious faith that began around the time of Jesus.
Because there is no precise boundary line between one zodiac constellation and the next, there is no precise year in which one astrological age ends and another begins. But, as the song goes, we are now thought to be at or near the dawning of the Age of Aquarius (the Water Bearer), an age of technological convergence and humanitarianism.
The Great Sphinx is a lion statue, and some believe it predates ancient Egypt by millennia and was created 10,000-12,000 years ago during the Age of Leo (the Lion), sometimes referred to as the Golden Age.
One would think ancient astronomers would have had great difficulty detecting the very slight changes in the stars’ positions year over year, as well as understanding the phenomenon of the precession of the equinoxes – but it is clear from ancient myths, legends, and other works that they grasped it all perfectly. How they managed to do this is a mystery.