Detail of the Dendera temple ceiling showing some of its many constellations.

In this astronomical timepiece, the vernal equinox would serve as the pointing hand, marking the Sun’s position against the background constellations on the first day of spring when day and night are of equal length. Due to the precession of the Earth’s poles, the vernal equinox slowly shifts backward through the signs of the zodiac with the forward advancement of time. It takes about 25,700 years to make a complete circuit of the ecliptic, an interval that in ancient times was termed a Great Cycle.

From Earth Under Fire by Paul La Violette

Earth Under Fire by Paul LaViolette