c. 2500 BCE

Egyptian Astronomy

The great pyramids of Giza are a monument to the technological (and labor management) prowess of ancient Egyptian civilization. They are also testaments to the designers’ astronomical skill, which figured prominently in Egyptian society and religion 4,500 years ago.

Because the earth’s spin axis slowly precesses, or wobbles like a spinning top, back in 2500 BCE Polaris was not the North Star. Indeed, much like the skies near our south celestial pole today, there was no bright star near the north celestial pole in those days. To the pharaohs, astrologers, and commoners, the sky at night appeared to spin around a vortex-like dark hole, thought to be a gateway to the heavens. In ancient Egypt, this gateway was located about 30 degrees above the northern horizon, and so the pyramids were carefully aligned to the north, with a small shaft leading from the pharaoh’s main burial chamber to the outside, pointing directly into the center of the gateway. If the plan was to join the gods in the afterlife, why not go in through the main door?

Egyptian astrologers also played an important role in developing a rather sophisticated calendar system that was already well established by the time the pyramids were being built. A new year was defined by the first sighting of the brightest star in the sky, Sirius (Sopdet to the Egyptians), just before sunrise in midsummer. The year was divided into 12 months of 30 days each, with 5 extra days of worship or parties tacked onto the end for a 365-day year. They also knew from carefully observing and recording star positions on different dates that they needed to add an extra day every fourth year—what we call a leap day—to keep their calendar synced to the motions of the sky. The predawn rising times of a number of bright stars were tracked in order to determine times for major religious festivals, as well as to plan for the annual floods of the Nile.

The pyramid shape itself may even represent a facet of ancient Egyptian cosmology, as some myths claim that the god of creation, Atum, lived within a pyramid that, along with the land, had emerged from the primordial ocean.

The great pyramids of Giza, burial places of the pharaohs and astronomical pointers to the presumed gateway to the heavens at the north celestial pole. These were the largest human-made structures in the world for nearly 4,000 years.