What the decans were
The decans were a set of star groups, or single bright stars, that the ancient Egyptians used to mark the passage of time. Each one was tied to a stretch of the sky near the ecliptic, the band along which the sun, moon and planets travel. Texts and star tables from the Middle Kingdom onward list thirty-six of them, and each is associated with a ten-day interval of the year.
The word “decan” is not Egyptian. It comes from later Greek writing about these stars and is related to the Greek word for ten. The Egyptian names for the individual groups were different and are known from tomb ceilings, coffin lids and temple ceilings.
Why thirty-six, and why ten days
The Egyptian civil calendar had twelve months of thirty days, which makes 360 days. Five extra days were added at the end of the year to reach 365. Each month was divided into three ten-day periods, and thirty-six of those periods fill 360 days. The decan scheme fits that arrangement: one decan for each ten-day period.
The usual explanation is that the Egyptians watched for the heliacal rising of a star, which is its first appearance on the eastern horizon just before sunrise after a period of invisibility. Because a given star rises earlier each night, a new star group begins to appear before dawn about every ten days. The sequence of dawn risings therefore worked like a rolling calendar through the year. Scholars debate details such as which stars were used and how the list changed over time, so it is safest to treat the picture as a general outline.
From star risings to the hours of the night
The same stars were useful during the night. On coffin lids of the Middle Kingdom, painted diagonal star tables list decans in columns and rows. They are often called diagonal star clocks. The tables are generally read as a way to tell the hour by noting which decan was rising, with a new one appearing at roughly regular intervals through the night.
In these tables the night is marked by twelve decans. That is the origin most often given for the Egyptian twelve-hour night. Over the year the decans shifted, so a particular star marked a different hour as the season changed, and the tables were arranged to track that shift.
Daytime was also counted in twelve parts, for example with shadow clocks and water clocks. The Egyptians therefore ended up with twelve hours of night and twelve of day. The hours were not all the same length: a winter night hour was longer than a summer one, because the span between sunset and sunrise was simply divided into twelve.
Why twelve?
It is not certain why twelve became so important. Some writers point to the twelve months of the civil year, and others to the practice of counting on the finger joints of one hand, which gives twelve with the thumb as the pointer. The finger-joint idea is only a suggestion, and it cannot be shown to be the reason behind the Egyptian choice. Twelve-based counting survives in dozens, a legacy discussed under duodecimal counting, and the old method of finger reckoning shows how a hand can serve as a counting device without any written number. For the Egyptians, the twelve decans visible during a night and the twelve months of the year probably reinforced each other.
Becoming a twenty-four hour day
The day of equal twenty-four-hour units was a later development. Egyptian hours stretched and shrank with the seasons. Greek astronomers working in the Hellenistic period, such as Hipparchus, are credited with proposing hours of fixed length, defined by dividing the day-night cycle into twenty-four equal parts. That idea was useful for astronomy and was only slowly adopted in daily life. For many centuries ordinary people in the Mediterranean world still used seasonal hours.
Later still, the hour was divided into sixty minutes and the minute into sixty seconds. That sixty comes from the Mesopotamian sexagesimal system, not from Egypt, so our clock is a blend of Egyptian, Babylonian and Greek contributions.
The decans after the pharaohs
The decans did not disappear with the old kingdom. In the Hellenistic and Roman periods they were folded into astrology, where each sign of the zodiac was divided into three decans of ten degrees. The twelve in astrology are therefore connected to the Egyptian star lists through a long chain of borrowing and reinterpretation, and later astrological writers assigned planets and characters to each decan. Those assignments are beliefs of the tradition rather than anything known about the earlier Egyptian stars.
Egyptian number writing also shows the culture’s interest in counting in tens. The hieroglyphic numerals used signs for 1, 10, 100 and larger powers of ten, which matches the ten-day periods of the decan system.
A note on certainty
Much of the decan story has to be assembled from fragments: ceilings, coffin lids and a few astronomical papyri. Some details, such as the exact identification of decan stars with modern star names, are still discussed. What is well supported is the main thread: a set of thirty-six star groups, a ten-day interval for each, twelve decans marking a night, and a long afterlife of that structure in later timekeeping.