Time intervals have their root in astronomical observations.
The sun, the moon, and the five planets which were observed during antiquity may have led to the establishment of the week. The concept of a 24 hour day dates back to the ancient Egyptians who divided their sky into 24 nonequal and changing parts. The meridian is a great circle which intersects the horizon at the observer's south and north points, while also intersecting the zenith. The sun and the reference star would soon be out of synchronization. Time based on the successive transits of a star across the meridian would soon find the star and the sun out of phase.