[William Cloud Hicklin]

When mechanical clocks were first introduced in the West in the 14th century (in monasteries, to ring the bells of the canonical hours-- faces and hands came later), they caused no end of confusion because they could only tell hours of a fixed length- which didn't work at all with Roman and early medieval practice of dividing the solar day into hours whose length varied with the seasons. Prime came at dawn and vespers at sundown, and the length of the hours in between were much longer in summer than in winter. Sundials didn't have this problem.

(NB: The Gospels, written in the 1st C. AD, reference the sixth and ninth hours (noon and midafternoon).