Ancient Egyptian chronology and the astronomical orientation of pyramids


The ancient Egyptian pyramids at Giza have never been accurately dated, although we know that they were built approximately around the middle of the third millennium BC. The chronologies of this period have been reconstructed from surviving lists of kings and the lengths of their reigns, but the lists are rare, seldom complete and contain known inconsistencies and errors. As a result, the existing chronologies for that period (the Old Kingdom) can be considered accurate only to about ±100 years, a figure that radiocarbon dating cannot at present improve. Here I use trends in the orientation of Old Kingdom pyramids to demonstrate that the Egyptians aligned them to north by using the simultaneous transit of two circumpolar stars. Modelling the precession of these stars yields a date for the start of construction of the Great Pyramid that is accurate to ±5 yr, thereby providing an anchor for the Old Kingdom chronologies.

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Figure 1: Deviation of pyramid alignments from true north over time.
Figure 2: Modelling the simultaneous transit method for Giza, 2467 BC.
Figure 3: Deviation of alignments from true north resulting from use of the simultaneous transit method of orientation.
Figure 4: Astronomical modelling of the simultaneous transit method of orientation.


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For research funding I acknowledge the British Academy, the Wingate Foundation and the Lady Wallis Budge Fund, Christ's College, Cambridge. I thank all those who discussed this project with me, in particular B. Kemp; J. Dorner for allowing me to use measurements of orientation from his PhD thesis; F. R. Stephenson for astronomical calculations; T. van Albada and A. Egberts for their comments. I also thank N. Gay for technical assistance.

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Correspondence to Kate Spence.

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