English: The cusp lines and dates of this calendar are all based on Universal Time in the year 2024 and calculated to the second the Earth and Sun’s orbital axis point along the Ecliptic, crosses any one of the 12 Zodiac constellation boundary lines standardized in 1930 by the International Astronomical Union. The illustrations of the Zodiac constellations on this calendar reflect their appearance next to each other in counterclockwise order as that is how they actually appear in the sky which is why the calendar reads counterclockwise as well.

Why it changes:

The reason it changes is something called Axial Precession or the Precession of the Equinoxes which was first described by the Greek astronomer Hipparchus who lived between 190-120 BC. The precession of the equinoxes is a cycle where the Equinoxes move very slowly through the Zodiac constellations of the Zodiac and takes an estimated 26,000-years to come around full circle. A good example of this is the ancient Babylonian calendar which has the March 21 equinox, (the time point when Earth’s equator lines directly up with the Sun) in the constellation Aries. From there it would continue to line up in Aries until crossing the cusp line into Pisces sometime around the year 319 B.C. From there it will continue to line up in Pisces where it is now until crossing into Aquarius sometime around the year 2601 A.D. From there it would continue to line up in Aquarius until sometime around the Year 4,401 A.D. when the March equinox will cross over the borderline from Aquarius into Capricorn and so on.