Precession of the Equinoxes
The alignment of the winter solstice sunrise with the center of the galaxy occurs because of phenomena called the precession of the equinoxes. Precession refers to a change in the direction of the axis of a rotating object. From Earth's point of view, the sunrise on any fixed day will rise against a background of stars that changes over time. This cycle is called the precession of the equinoxes because the equinoxes have traditionally been used to measure its progression. They are fixed days that are predictable with relation to the solar year.
What is an equinox?
The equinox is when the sun is located vertically over the equator. This happens twice a year. The first one, on March 20 or 21, is the spring or vernal equinox for the northern hemisphere and the autumn equinox for the southern hemisphere. The second equinox takes place around September 22 or 23. With the solstices, these days are the four fixed quarters of the year.
Precession is quite a slow process, where the stellar background against which the sun rises moves approximately 1° every seventy-two years. This also creates what is known as the changing ages of the zodiac in astrology. The stellar background for sunrises and sunsets is always found in this 14° wide belt called the ecliptic. The ecliptic is an imaginary circle projected into the sky from the plane of the solar system. This circle is then divided into twelve constellations, which form the signs of the zodiac.
Most astrologers think we are currently at the end of the Age of Pisces and are on the cusp of entering the Age of Aquarius in the ages of the zodiac, although there is some dispute about the exact timing. Some astrologers put the date for the beginning of the Age of Aquarius as far forward as the twenty-third century.
Together, the twelve signs create a wheel of the year. The cycle of precession, where the sun's rising and setting points gradually move against the sky, means that the zodiac can also be used to mark a greater wheel of the ages. Each of these zodiac ages lasts 2,160 years, where the rising sun at the vernal equinox will stay within one of the signs. Twelve zodiacal ages make one great year of 25,920 years, an approximation of one precessional cycle.
The ecliptic is the path the sun follows as it moves through the year. Each day, the location of the rising sun appears to move against the background of the stars as it gradually moves its way around the circle of the ecliptic. In a year, the location of the sunrise will have returned to almost exactly the same position in the sky. The difference will be around 0.0138°. Measuring this difference is how we detect precession. Over the whole 25,771-year cycle, these accumulated differences will mean that the rising point of the sun, measured on the spring equinox, will have completed a 360° circle and will have returned once more to exactly the same place in the sky.
The Greeks, and possibly other ancient cultures including the Egyptians and the Neolithic builders of Stonehenge, knew about the precession of the equinoxes. Accurate figures for the rate of precession have only been recorded for the last 150 years. There is a debate about whether the Maya knew about precession. Most conventional academics have suggested they did not, but some are now revising their opinions.