How was the time for the first Adhan known in ancient times, before the existence of clocks, given that the time for the second Adhan was known by specific signs?

The time for the first Adhan of Fajr prayer begins at the last sixth of the night according to the Malikis. Experts in timekeeping define this by dividing the night, from sunset, into twelve hours. It is forbidden to make the Adhan before this time.

A sixth of the night is two hours, based on the premise that the night is always twelve hours long, regardless of the varying duration of the hour itself.