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Strand IV: Simulated observations in 2500 BC
Imagine that you are an Egyptian priest standing on the roof
of a mortuary temple, ca. 2500 BC. Ra is setting in the west
and it is your duty to observe the stars so that you can call
out the passage of every quarter hour. How would you do it?
Which stars would you observe? What instruments would
you use?
Suppose that it is the time of the winter solstice, 2500 BC
January 11. The Sun has just set and you are facing the western
horizon. Earlier you reviewed the list of decans to be observed
during the night. Now, right on schedule, you are able to
identify the star Deneb with an altitude of 24°. You set your
Ptah-sceptre on a ramp so that Deneb is just visible above the
top disk. A string with knots marking lengths from 26 to 36
fingers hangs from the top of the staff. You hold the knot for
26 fingers close to your cheek and stand back till the string
is taut. As time passes, Deneb appears to sink behind the top
disk and then reappear between the first and second disks. At
that moment you would call out “star 1, disk 1” or some such
signal to indicate that the first quarter of the first hour of the
night had passed. Fellow priests in the temple sanctuary would
hear your call and chant the first set of spells to assist Ra and
the ba of a deceased pharaoh to meet the first challenge in the
underworld that night.
After another quarter hour, Deneb would sink to a position
between the second and third disks and you would call out
“star 1, disk 2,” and so on. At the end of 69 minutes, Deneb
would appear below the bottom disk and you would call out
“end of hour 1.” Then the Ptah-sceptre is moved and set so
that Alpheratz (the next decan on the list for the night) appears
just above the top disk (its altitude would be approximately
31°). Alpheratz would then be monitored until it appeared
below the bottom disk. And so on for four more decans. After
the first six hours of the night had elapsed, Ra and the ba of
the pharaoh would have passed the low point in their nightly
journey and would begin rising towards the eastern horizon.
Accordingly, attention would shift to the east. Facing towards
the eastern horizon, decans would appear to rise from below
the bottom disk to above the top disk. Spica would be the
seventh decan observed, with a beginning altitude of 23°; then
Menkent with an altitude of 16°. If all went well, just as the
12th star on the decan list appeared over the top disk of the
djed pillar, dawn would be breaking in the eastern sky.
In this manner, the Egyptian priests could have measured the
passage of the hours during any night of the year.
A Summary of the Model Components
Taken together, the 6 model components provide an explanation
for why the ancient Egyptians divided the night into 12
equal hours, and how they could have accomplished that task
even as the length of the night varied during the year. The
model also provides a reasonable explanation for the purpose,
design, and use of the djed pillar and Ptah-sceptre.
For clarification, the concepts that have been proposed in this
paper are in italics.
1. The ancient Egyptian priests marked the passage of the
night-time hours so that they could chant the appropriate
spells and thus aid Ra and the ba of a deceased pharaoh
through the underworld each night.
2. A table of 37 select stars (decans) was developed to mark
the passage of nighttime hours over an entire year. Twelve
stars were observed each night, six in the west after sunset,
six more in the east before sunrise. Usually, after each decand
(10 days) one star was dropped from each list and the next
decans were added.
3. Using simulations of the Egyptian sky in 2500 BC, a list
of pseudo-decans can be constructed that should be similar to
the content and sequence of the ancient Egyptian list of decans.
4. The djed pillar symbol represents a physical device that was one
cubit tall. The top third of the device consisted of four evenly
stacked disks. The stack had a total height of seven fingers, and
a diameter of seven fingers.
5. The djed pillar, mounted as a Ptah-sceptre, was an
astronomical instrument used to observe the vertical motions
of decans near the western and eastern horizon so that the
nighttime could be divided into 12 equal parts.
6. The ancient Egyptians were able to adjust the length of
their hours to match 1/12th of the night during all
seasons of the year. They did this by varying the distance of
the djed pillar from the eye of the observer.
Concluding Comments
The ancient Egyptians were not astronomers, but priests who
observed and worshipped the Sun, Moon, planets, and stars
as gods moving through ethereal realms. By studying celestial
cycles, they were seeking ways to appease, gain favour, and
assist the gods.
There is scant direct historical evidence describing the
astronomical techniques that were used in ancient Egypt
(Wells, 1996). Many tomb decorations provide some clues and
there is one section in the Carlsberg Papyri Ia with a short
description of the use of decans. From the available evidence, it
is apparent that the Egyptians did measure the seasonal hours
of the night, and they did make use of a set of 37 specific stars
now called decans.
Interpreting the djed pillar as an astronomical instrument is
a key feature of this paper. A djed pillar, with a design similar
to that shown in Figure 5, could have been used to observe
the motion of a sequence of decans near the western and/or
eastern horizons. Such a conjecture meshes with the available
October / octobre 2011 JRASC | Promoting Astronomy in Canada
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