In Ancient Egypt (circa 2100 BCE), the astronomers clubbed together 36 groups of stars (decans) and used them as guides for time keeping. These stars rose consecutively on the horizon throughout each earth rotation. Each decan appears (right before the sunrise) for the first time in the eastern sky and the cycle repeats as a new star group appears after every ten days and so on until all the 36 groups appear. This was called dekanoi (tenths in Greek) and was used to calculate the 36 weeks in a year, each with ten days (in total 360 days). The extra five days were added to complete the solar based year of 365 days.
Image – Einsamer Schütze