stone slabs or pillars, were also used for astronomical pur-
poses. Modern archaeologists are oft en amazed at the pre-
cision with which ancient builders sited their buildings; the
four sides of the Great Pyramid at Giza (Egypt), for example,
are oriented precisely north, south, east, and west.
Early astronomy was not an exact science, primarily be-
cause trigonometry had not yet been discovered. Th e ancient
Greeks developed trigonometry, which deals with the rela-
tionships between angles and distances. Only then were early
astronomers able to make increasingly accurate observations
of the stars and predictions of their movements. In time, early
astronomers began to compile star catalogues, mapping the
heavens in the same way they tried to map the earth.
AFRICA
BY TOM STREISSGUTH
Th e astronomical lore and knowledge of ancient Africa are
still largely a mystery to archaeologists. Several sites of as-
tronomical signifi cance have been excavated, but their true
purpose is still unknown. With no written records to consult,
historians must carefully examine each site—with its align-
ments of standing stones and markings—and speculate on
how these “observatories” may have been used.
In ancient Africa the brightest objects of the night sky
were important for the purposes of navigation and time-
keeping. Th ese objects include the moon, the Milky Way,
the constellation Orion, the planet Venus, the stars Sirius
and Canopus, and the star cluster known as the Pleiades.
Observation of the sky may have also served more utilitar-
ian purposes. On the frontier between the Congo and Ugan-
da, a lunar calendar dating to 6500 b.c.e. has been found
in the form of the Ishango bone. Th e bone has a series of
notches carved into it that kept track of the 28-day cycle
of the moon’s phases. Th e Batammaliba people in Togo and
Benin built their houses so that they were aligned with the
course of the sun at the equinoxes—the days of spring and
fall when the lengths of day and night are equal. In the king-
dom of Kush, established in about 1000 b.c.e., pyramids
raised near the capital of Meroë have their fronts facing the
eastern rising of Sirius, a star that was also important to the
astronomers of ancient Egypt.
Observatories in the form of stone megaliths (groups of
enormous stones that form a structure) arranged to indicate
the positions of the stars and other bright objects have been
discovered in Zimbabwe, Togo, Kenya, Sudan, and Benin. Th e
oldest such site in Africa lies in the Nabta Basin of southern
Egypt. Here stone megaliths weighing up to one and a half
tons were raised in a circle 12 feet in diameter. Archaeologists
have dated the circle to the fi ft h millennium b.c.e., a date that
makes it the world’s oldest astronomical observatory.
Nabta was once the site of a shallow lake and a settle-
ment of herders who migrated through the region when it
was much more temperate and fertile than it is now. On the
western edge of the settlement, the inhabitants shaped and
raised the large stones, aligning two groups of stones along a
north-south axis while pointing another to the rising of the
sun at the summer solstice, the day on which the sun reached
its northernmost point in the sky. In this arid region the sol-
stice signifi ed the return of annual rains and the life-giving
fl ooding of the Nile River.
A similar group of stones known as the Namoratunga
II circle was found in a remote region of northwestern Ke-
nya. Th is arrangement of 19 basalt stones was used as a calen-
dar by the Borana, a people of northern Kenya and southern
Ethiopia who reckoned their days and months using seven
prominent stars and their positions relative to the moon. At
Great Zimbabwe, a city raised by the Karanga people, a series
of pillars and other structures are enclosed within a thick,
high wall. Patterns in the wall indicate the position of the sun
at the solstices.
A vast system of more than 1,000 stone circles and mon-
uments also exists in Gambia and Senegal, covering an area
of 15,000 square miles. Each circle has as many as 24 stones,
reaching up to seven feet in height. Archaeologists date the
circles to as early as the fi ft h century c.e. and believe them to
be burial sites. However, the careful alignment of the stones
also suggests that they may have been used as astronomical
observatories. Another mysterious set of more than 70 stand-
ing stones was excavated in the 1960s near Bouar, a town
of the Central African Republic. Th ese stones were raised
around 500 b.c.e., according to radiocarbon dating of arti-
facts found nearby.
With scarce physical evidence, historians must also draw
on contemporary traditions for clues to the astronomical
knowledge of ancient Africa. Along the coasts of Africa, sail-
ors and fi sherman orient themselves using prominent stars,
such as Sirius and the Orion constellation. Th e Bozo people of
Mali carefully observe the Pleiades star cluster and time the
season of fi shing in the Niger River when the Pleiades disap-
pear from the night sky.
Another people of Mali, the Dogon, have developed re-
markable insight into Sirius, the brightest star of the sky. In
Dogon lore Sirius has a companion star, known as po tolo,
which is associated with a small white grain known as fonio.
Th is companion star is small, white, and very dense, with a
mass that holds all other stars of the sky in their place. Po tolo
circles Sirius every 50 years, and the Dogon hold a regular
ceremony to celebrate this event. In fact, the nature of Siri-
us as a binary star with a dense, “white dwarf ” companion
known as Sirius B, which is invisible to the naked eye, was
not established by modern astronomers until the 20th cen-
tury and the era of the modern telescope.EGYPT
BY LEO DEPUYDT
Th e term astronomy implies a level of complexity that
Egyptian astronomy does not exhibit. Because Egyptian
astronomy was relatively unsophisticated, the designation124 astronomy: Africa