It's complicated, but it has to do with the wobble of the earth. We all know the equinoxes aren't the same day every year, every 2,150 (about) years, the date moves through an astrological sign. I can't find an actual date for the change, apparently it's difficult to calculate but around the year 2000. Astrologers predict the new age may bring world peace and advanced space travel as well as selfishness that could use up our resources, causing disease and famine.
That's what I found in my info, Google might have more answers but I hope it helped :)