The thing that caused everyone to freak out because their astrological signs had changed is one of the more fascinating stories in the history of intellectual evolution. That thing is called precession of the equinoxes, and precession is one of those phenomena that is simultaneously invisible and obvious, observable and hidden.
Let’s start with the technicalities and move to the history of it.
In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body’s rotational axis. In particular, it refers to the gradual shift in the orientation of Earth’s axis of rotation, which, like a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years. The term “precession” typically refers only to this largest secular motion; other changes in the alignment of Earth’s axis — nutation and polar motion — are much smaller in magnitude.
So, precession is essentially the planetary equivalent of the wobble in a top as it spins.
If you carve the horizon into twelve roughly equivalent sections, each year, at the equinoxes, the sun will appear to rise in one and set in its opposite. Because of the wobble in the axis of the earth, the section of the sky the sun appears to rise and set in will shift very slowly over a period of roughly 2,160 years. This is the basis of astrology, as various civilizations applied meaning to the constellations they saw in each section. More interestingly, I think, our tracking of it appears to be the basis of astronomy.
To begin to notice that tracking takes time. To fully understand the cycle, and be able to project it forwards and backwards, to mark the passage of time in the relative movement of the stars, would take hundreds, if not thousands, of years — observation, measurement, notation. Once a culture had an awareness of that pattern, no matter on what scale, it could begin to find a place for itself, and make a story out of it, and because we are human, of course, that is what we did.
If you are interested in this subject, and are comfortable with an approach equal parts academic and poetic, you might enjoy Giorgio de Santillana and Hertha von Dechen’s Hamlet’s Mill. It shows glimpses of precession’s possible influence throughout the history of art, an astronomical code for our place in the universe embedded in language.