The phases of the moon became man's first calendar. The full moon is waning, becomes a narrow sickle, eventually disappears altogether and then grows back into a full one, circular disk. The people discovered, that the cycle of the year approximately 12 full moons or 360 lasted days. This was the first measurement of the length of a year.
The relationship between the return of the seasons and the position of the sun and stars was the next observation to determine the passage of time.. Then the Egyptians observed, that the sun and the bright fixed star Sirius were only in a certain position in relation to each other once a year: When the sun went down in the west, Sirius appeared on the eastern horizon. You counted the days and found, that this process all 365 days repeated. Already at 4000 v. Chr. The Egyptians used it to determine the length of the year.