Innovation No. 14 of 77 in the book
The 24-Hour Day
Day and night divided into 12 hours each
- When
- c. 2000 BCE
- Dynasty
- 11th Dynasty
- Era
- Middle Kingdom
Why 24 hours? Because Egypt split day and night into 12 parts each, and the habit stuck. The system is already visible on coffin lids from around 2000 BCE.
By night they told time by 36 star groups, the decans, that rose in sequence; roughly twelve passed between dusk and dawn. By day, shadow clocks marked twelve more.
The hours weren’t even equal — daylight hours stretched in summer and shrank in winter — but the count of twelve-and-twelve never changed.
Every 24-hour day, on every clock and phone, is keeping Egyptian time.
The earliest evidence is the diagonal star tables painted inside wooden coffin lids of the 11th and 12th Dynasties, roughly 2100 to 1800 BCE, which map the dawn risings of 36 star groups at ten-day intervals and cut the night into twelve parts. The oldest surviving shadow clock is a green schist device from the reign of Thutmose III, around 1450 BCE, now in the Neues Museum in Berlin.
Frequently Asked Questions
Were Egyptian hours the same length as ours?
No. They were seasonal. Daylight was always divided into twelve parts, so a summer daytime hour ran longer than a winter one, and night hours stretched and shrank the other way. Only the count of twelve stayed fixed.
How did the shadow clock give twelve daytime hours?
The T-shaped bar was marked to split the sun's path into six parts. The device was reversed at noon and the six read again — two of them covering morning and evening twilight.