Signs in the Sky Image: NASA The space shuttle Discovery and the rising March Sun that will soon cross the celestial equator Precession means advancement and refers to the earlier arrival of the seasons each year as the orbital plane alters. The phenomenon is easily observed in the sky. Since ancient times, stars have functioned as reference points for navigation, time keeping and cultural procedures or beliefs. Early skywatchers noted the changing angles between the stars and the Sun during a lunar eclipse. The Greek astronomer Hipparchus has been credited for the discovery of precession, although the phenomenon was already known to earlier civilizations as well (Kaler). Each year the Sun's path seems to prograde eastward along the horizon and the equinoxes, the two points in the sky where the Sun intersects the celestial equator, move westward along the ecliptic by about 50.26 seconds of arc a year (=0.014°) or 360° over a period of up to 19 000 to 26 000 years. See Fig 2 below and note also how the Earth's equator will shift with regard to the path of the Sun. The equinox which marks the first day of spring in one hemisphere and autumn in the other, arrives earlier each year by about 70 seconds so that approximately 5000 years ago spring would have been in June and autumn in December. The vernal equinox then lay in the constellation of Aries as were all the constellations and stars in a different position and another star served as the northernmost or southernmost guidance star. Some stars that were once seen above the northern hemisphere now lie below the horizon. It has been estimated that is about one half period, the bright star Vega will be North Star and 3000 BC, the Egyptians refered to Thuban in the constellation Draco as the polar star (Kaler). Precession is therefore an important parameter when evaluating astrological coordinates in historical scripts. In the picture below observe how the vernal equinox moves westward with time along the yellow line through the zodiac constellations. Image: Crystallinks.com To see how rhythmic fluctuations of insolation can be determined click forward button below