Within the lifetime of an average person, the stars in the night sky are constant and nearly unchanging. Historical records occasionally reflect rare events such as super novae, or the regular oscillations of variable stars such as Algol in Perseus the Hero. Yet if we were to look back in time as little as 150 years we would detect subtle differences in the positions of some stars. If we go back in time more than 500 years we would notice a much greater difference. This difference of position would be especially noticeable in the stars near the north celestial pole.
In the northern hemisphere, the stars and constellations seem to turn around the north celestial pole as the earth rotates. This means that these stars and constellations neither rise nor set below the horizon. One star in particular seems not to move at all. This is the star Polaris in Ursa Minor. Polaris is less than one degree from the north celestial pole. Only timed photography reveals the motion of Polaris in a tight circle around the pole. This motion is the same today as it was a generation ago. Yet if we could see the northern sky in the past we would find Polaris further from the celestial pole than it is today. This phenomenon is due to a slow motion of the earth called precession.
Earth rotates on its axis, yet just like a top or gyroscope, it does not keep this axis pointed in the same place. Over the course of 26,000 years the rotational axis of the earth slowly traces a huge circle relative to the distant stars. Three thousand years ago, when the Pharaohs built the Great Pyramids of Giza, the north pole of the earth pointed not at Polaris but at the star Thuban in the constellation Draco the Dragon.
Draco is easy to find in April. To begin, locate the Big Dipper high in the northern sky. The “bowl” of the dipper points down toward the horizon, as if dumping out the rains of spring. Using the first two stars of the “bowl” of the dipper follow a line down toward the horizon to locate Polaris. Polaris marks the tip of the handle of the little dipper. Using the top two stars of the “bowl” of the little dipper follow a line to the east. The first 2nd magnitude star you come to is Thuban. Thuban is one of the stars in the long tail of Draco the Dragon.
Precession is a process that is not only evident in the past, it is also something that will be visible in the far flung future. If we could magically turn the clock forward to the year 14,000 AD the nearest bright star to the pole will be the star Vega in the constellation Lyra the Harp.
Precession is a natural motion of our planet, one that has been ticking like a slow clock since the earth was formed. Yet this slow clockwork is only apparent to us across the vast gulf of time, and without historical records of ancient civilizations we might never have known about it. Yet the clock that marks time of human affairs runs much faster. The entire span of human civilization represents the last second of the last minute of the 11th hour before midnight (as reckoned from the time the earth was formed). Is it any wonder then that humans see the stars as constant and unchanging?
