A caution about “oldest”
The title promises the oldest known observations, but no one can say that Egyptian observers were the first people anywhere to record the sky. Prehistoric peoples watched the sun, moon and stars long before writing, and Mesopotamian scribes were also keeping systematic records. What can be said is narrower and more interesting: Egypt has some of the earliest surviving charts and tables of the stars, and they show how the Egyptians organised the night sky for practical and religious purposes. This article looks at those documents and at what is known of earlier practice.
Before the charts
The oldest evidence for Egyptian interest in the sky is indirect. The Pyramid Texts, carved inside royal pyramids in the later Old Kingdom (around the twenty-fourth to twenty-second centuries BCE), describe the dead king rising to join the stars, especially the circumpolar stars that never set, called the “imperishable ones”. Sirius, called Sopdet, and the constellation we call Orion, called Sah, also have a place in these texts. Orientation is another clue. The careful alignment of the pyramids with the cardinal directions suggests that builders could find north by observing the stars, although how precisely the pyramid alignment was achieved is still discussed.
Further back, some researchers have argued that stone settings at Nabta Playa in the Western Desert, dated to several thousand years before the pharaohs, were aligned to astronomical events. This is debated, and it is not clear that the people who built them were ancestors of the Egyptians in any direct sense.
Decans and the star clock
The key idea in Egyptian star charts is the decan. A decan was a star or small group of stars that rose just before dawn at a particular time of year. The Egyptians chose 36 of them, and each was visible rising at dawn for about ten days, which is where the term “decan” (from a Greek word relating to ten) comes from. Thirty-six groups of ten days account for 360 days, and five extra days at the end of the year made the 365-day civil calendar.
The same stars could be used at night. Because the stars rise earlier each night, a given decan marks a different hour as the year proceeds. The Egyptians divided the night into twelve hours, each tied to the rising of a decan, so a table listing the decans for each ten-day period worked as a star clock.
Diagonal star tables on coffins
The earliest surviving decan tables are found on the lids of wooden coffins of the Middle Kingdom, from sites such as Asyut, and are dated by specialists to roughly the end of the third millennium into the early second millennium BCE. They are called diagonal star tables because the names are arranged in a grid that slants across the lid: each column is a ten-day period and each row an hour. The decans seem to step down the grid as the year advances.
It may seem odd to put an astronomical table inside a coffin. The reason is religious, tied to Egyptian afterlife beliefs. The inside of a coffin lid could represent the sky goddess Nut, who arches over the earth, and the stars were thought to help the dead person travel through the night in the same way they marked the passage of time for the living.
The ceiling in Senenmut’s tomb
The next major source is the astronomical ceiling in the burial chamber of Senenmut, an important official under Hatshepsut, at Deir el-Bahari near Thebes (a tomb catalogued as TT353). It dates to the early fifteenth century BCE, during the Hatshepsut reign, when Senenmut held several offices. The ceiling is often described as the earliest surviving example of a large, fully developed astronomical ceiling in a private tomb, though earlier painted ceilings may simply not have survived.
Its design has two parts. One half is a calendar-like set of twelve circles, one for each month of the lunar calendar, with the days of the month marked. The other half shows the northern sky, including the figure of the foreleg (our Big Dipper), alongside the decan stars, the planets and the deities connected with them. The two panels give a view of how Egyptians divided the sky: stars of the north, the moving planets, and the hour-marking decans of the south.
Later star clocks and zodiacs
The tradition continued into the New Kingdom. The ceiling in the cenotaph of Seti I at Abydos (about 1290 BCE) shows the sky in the form of the Book of Nut, and the tombs of Ramesses VI, VII and IX in the Valley of the Kings contain star clocks from the twelfth century BCE. Their method is unusual. A seated figure represents the observer, and the position of a star is given relative to body parts, such as the left shoulder or right ear, so two observers sitting opposite each other could mark when a decan crossed a given position.
The famous round zodiac in the temple at Dendera is much later, around the first century BCE, and shows Mesopotamian and Greek influence. The zodiac of twelve signs was not an Egyptian invention. Babylonian astronomy had a major role in shaping later astronomical thinking, and its records, which run from earlier centuries, are in some respects more detailed than the Egyptian ones.
What this evidence shows
Egyptian sky watching was primarily descriptive and calendrical. There is little sign of the mathematical predictions of planets or eclipses found in Babylonian tablets. Instead, the Egyptians created organised lists of stars, used them for timekeeping and ritual, and left them in tombs and temples. These documents are among the earliest surviving star tables from anywhere, which is a more defensible way of saying “oldest”.