A new study indicates that ancient Egyptians used the fall equinox for the alignment of the Great Pyramid of Giza to the four cardinal points, north-south-east-west, with great accuracy. The method uses a vertical rod to track the movement of the sun on the equinox. The fall equinox occurs when the Earth’s tilt is such that leads to an approximately equal length of day and night.

The Great Pyramid of Giza, constructed about 4,500 years ago following the orders of pharaoh Khufu is the largest of the three pyramids on the Giza Plateau. Researcher Glen Dash, who is the founder of the Glen Dash Foundation for Archaeological Research  and conducts work on the Giza Plateau with Ancient Egypt Research Associates and has conducted radar work in the Valley of the Kings, tried to identify what method ancient Egyptians used to build the Great Pyramid in alignment to the cardinal points with extreme accuracy. In a paper published in The Journal of Ancient Egyptian Architecture he supports the view that they might have used the fall equinox or equinoctial solar gnomon method to achieve the above accuracy in alignment.

Dash conducted an experiment in Pomfret, Connecticut, on the day of the fall equinox, placing a rod -or gnomon- on a wooden platform and marking the location of its shadow throughout the day. On the day of the fall equinox the tip of the shadow runs in a straight line and nearly perfectly east-west, Dash wrote in the paper. The degree of error, slightly counterclockwise is similar to the one found in the Great Pyramid, Khafre Pyramid (also in Giza) and the Red Pyramid (at the site of  Dahshur).

The technique would work the same in Giza and ancient Egyptians would calculate the fall equinox counting 91 days after the summer solstice. However, Dash cannot say with certainty that this is the method they actually used, since other methods would have worked just as effectively and no evidence has been found to provide information on the matter. Therefore it is possible that more than one methods were used for the alignment of the Pyramids, perhaps using the position of the sun or stars. The advantage of the fall equinox method is that it is quite simple and time-efficient.