Ancient Egyptian astronomers aligned the pyramids due north
by using two stars that circle the celestial polar point.
Around 2500 BC, each star was about ten degrees from the
celestial pole which lay directly between them. When one star was
precisely above the other in the sky, astronomers could find a line
that pointed due north. But the alignment was only true for a few years.
Before and after that time, the stars deviated from the north-south
line and anyone using the stars to plot a direction would have made
And it is these mistakes that convinced a British
Egyptologist in 2000 that they could be used to estimate very accurately
when the pyramids were built. The theory offered by Kate Spence of the
University of Cambridge suggests that the Great Pyramid at Giza was
constructed within ten years of 2480 BC.
This meant that even the most modern dating theories
for the ancient Egyptians were about a century out when it came to the
4th Dynasty, with Khufu's death usually being placed at 2566 BC. The
theory also contradicted another, far more controversial one, which
dated the alignment of the pyramids to stars that were correctly
positioned in 10,500 BC. A similar date has been offered for the
building of the Sphinx (see There was a Second Sphinx via the
Spence developed her theory while trying to explain
the deviations in the alignment of the bases of many pyramids from
true north. She believes the ancients may have used a pair of fairly
bright stars, which in 2467 BC lay precisely along a straight line that
included the celestial pole. It was known that the ancient Egyptians
were extremely interested in the night sky, particularly the circumpolar
These circle around the North Pole, and as you can
always see them, the Egyptians always referred to them as 'The
Indestructibles'. As a result, they became closely associated with
eternity and the king's afterlife. So that after death, the king would
hope to join the circumpolar stars - and that's why the pyramids were
laid out towards them.
The north-finding stars were Kochab, in the bowl of
the Little Dipper (Ursa Minor), and Mizar, in the middle of the handle
of The Plough or Big Dipper (Ursa Major). An Egyptian astronomer would
have held up a plumb line and waited for the night sky to slowly pivot
around the unmarked pole as the Earth rotated.
When the plumb line precisely intersected both stars,
one about ten degrees above the invisible pole and the other ten degrees
below it, the sight line to the horizon would aim directly north. However,
Earth's axis is unstable and wobbles like a gyroscope over a period of
26,000 years. Modern astronomers now know that the celestial north pole
was exactly aligned between Kochab and Mizar only in the year 2467 BC.
On either side of this date, the ancient astronomers
trying to find true north would lose some accuracy. Writing in the journal
Nature, Kate Spence showed that the orientation errors of earlier and later
pyramids faithfully track the slow drift of Kochab and Mizar with respect
to true north.
And because the error in the Kochab-Mizar alignment can
be readily calculated for any date, the error in each pyramid's orientation
corresponds to a period of several years. Owen Gingerich, of the
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts,
welcomed the findings, stating that 'Spence has come up with an ingenious
solution to a long-standing mystery'.