Watching the sky for practical reasons
Egyptian astronomy was less a study of heavenly mechanics than a set of practical skills for telling time and keeping order. People looked to the sky to know when to expect the river, when to hold festivals, how to divide the night and how to align sacred buildings. These purposes were tied up with religion: the sun, moon and stars were gods or their manifestations, and the sky itself was the goddess Nut. Much of what we know comes from temple and tomb texts, a few preserved instruments and the buildings themselves, so we see the priestly and official side of sky watching far better than any popular knowledge.
The year, the flood and Sirius
Egypt’s year was shaped by the Nile. The civil calendar had three seasons of four months, named for the flood (akhet), the growing season (peret) and the harvest (shemu). Each month had 30 days, so twelve months made 360 days, and five extra days were added at the end to give 365. That calendar is closely tied to the decans, groups of stars whose dawn risings marked each ten-day period, and which also served as star clocks decans for the hours of the night.
The star Sirius, called Sopdet by the Egyptians (and Sothis by the Greeks), has a special place. For a period each year Sirius is hidden in the sun’s glare, then it reappears just before dawn, an event called its heliacal rising. In Egypt this fell roughly in mid-summer, around the time the Nile began to rise. The Egyptians associated the star with the goddess Isis and the coming of the flood. This coincidence was close enough to be useful, though the exact timing of the river varies from year to year.
There is a catch. The civil calendar had no leap day, so it ran about one day ahead of the solar year every four years. Over a very long time the civil New Year therefore moved through the seasons, and only after something like 1,460 years did it return to its starting point. Later authors called this the Sothic cycle. A Roman writer, Censorinus, tells us that a New Year and the rising of Sirius coincided in 139 CE. Modern scholars have used texts that mention the star’s rising on given civil dates to try to anchor Egyptian chronology, but the method depends on assumptions about where the observation took place and about the age of the text, and it is much argued over. It is not by itself a reliable dating tool, and techniques such as radiocarbon dating are used alongside it.
Egyptians probably also kept a second, lunar calendar, used for religious festivals tied to the moon. Keeping both in step required observation, and a month could begin on a new moon or when the old moon disappeared in the dawn.
Ritual timing and the hour-watchers
Temples needed to know when to perform daily and monthly rites, and when festivals fell. Some priests had the title “hour-watcher” or “keeper of time”, and were responsible for observing the sky and the sun. Their tools included the merkhet, a plumb-line instrument, and a sighting device made from a palm rib, with which they could sight along a line to the north. Water clocks, with markings for the hours, are known from the New Kingdom, and a fragmentary one from the reign of Amenhotep III is among the oldest surviving examples, though no one can say it was the first.
The sun was the central object in Egyptian religion, and its daily journey through the sky and the underworld shaped many rituals. The night was divided into hours, each linked with a decan star. The surviving tables on coffin lids and tomb ceilings record this system, and the same stars set the schedule for night-time priestly duties.
Orienting temples and tombs
Many Egyptian buildings are aligned to a cardinal direction or to a solar or stellar event. Pyramids are aligned very close to the points of the compass, and the Giza pyramid complex is the best-known case. The temple of Amun-Re at Karnak is built along an axis that seems to point toward the sunrise or sunset around the winter solstice, although scholars disagree about which of the two was intended and how much was deliberate. At Abu Simbel, the sun reaches into the inner sanctuary on two days a year, a famous effect, but it is not certain that the days were chosen for a specific royal anniversary, as is sometimes stated.
Texts describe a ceremony known as “stretching the cord”, in which the king and the goddess Seshat marked out the corners of a new temple. In some accounts the king sights a star to fix the line. Whether this was carried out with real astronomical observation every time, or was partly ceremonial, is unclear. The practical result is that careful site orientation was part of building a temple.
Planets, eclipses and prediction
The Egyptians knew the five visible planets and had names for them, often as forms of Horus or other gods, but they did not leave behind the long, numerical records of planetary positions that are known from Babylonian astronomy. There is little sign that they predicted eclipses. This does not mean they were uninterested. It means their written astronomy was aimed at calendar, ritual and religion, and the surviving texts reflect that.
Foreign contact and later developments
In the Ptolemaic and Roman periods, Egyptian traditions mixed with Greek and Babylonian ones, and Alexandria became a leading centre of astronomy. The Egyptian decans became a part of Hellenistic astrology, and the round zodiac at Dendera shows the mixture. A great deal of what is now called Egyptian astronomy is therefore a record of the sky as seen through Egyptian religion, rather than a separate scientific tradition on the Greek or Babylonian model.