# Time and Its Measure: Historical and Social Implications
## 3 Evolution of Clocks  
Time measurement systems have always reflected the social activities of the community. Among primitive peoples, the alternation of day and night, the changes in the moon and the seasons formed the basis of their systems. To record the course of the seasons, they considered the budding, flowering, falling of leaves and bearing fruit of vegetation and the mutation, migration and mating of animals and birds. In the same division of the day, many tribes recognized four diurnal periods: the rising of the sun, the setting of the sun, noon and midnight. Social considerations entered the judgments of time on which the relative values of temporal durations then depended: quantitatively equal periods of time were made \"socially\" unequal, and, vice versa, unequal periods were socially equal. In modern times, the need to measure relative time, as required by technology and social organization, is increasingly widespread in social classes, so much so that, today, every individual is able, either with watches or through digital means, to know exactly both the absolute and relative time. Clocks, in their historical evolution, were initially limited, especially in the first centuries, to the classic sundials, which, in public places, allowed one to \"follow\" the trajectories of the Sun giving \"spannometric\" indications about time (ten minutes of uncertainty was typical). Sundials found competitors in the more precise hourglasses (water or sand), but these were less suitable for providing absolute data. It was only later (15 th century) that water clocks (Figure 9), based on the laws of hydrodynamics, were developed, of which some admirable monumental applications remain. Even if they could not guarantee small-sized models, they found applications in the definitions of well-defined time intervals, such as \"the times of love\" available for customers, and they accounted for thoroughly perforated basins filled with water [17]. In a second moment, around the XXVIII century, mechanical watches were established. Always based on pendulum-like phenomenology, they gradually became less bulky thanks to the making of the first stackfreed, a dispositive able to equalize the varying force of the mainspring in the course of its unwinding (early clocks and watches slowed down drastically during the clock's running period as the mainspring lost force, causing inaccurate timekeeping) [18]. The first pendulum clock, regulated by a mechanism with a \"natural\" period of oscillation, was made by [PERSON] in 1656. [PERSON] pendulum clock had an error of less than 1 min per day, and its subsequent refinements have reduced the errors of his watch to less than 10 s a day. However, no device could keep accurate time at sea until [PERSON], a carpenter and toolmaker, perfected the techniques for temperature compensation and found new ways to reduce friction. In 1761, he built a marine chronometer with a spring escapement and balance wheel that kept the time very accurate. With the final version of his chronometer, which looked like a large pocket watch, he was able to determine the longitude to within half a degree. His fame was overshadowed in 1921 with the invention of the first Shortt-Synchronome named after its inventor, British railway engineer [PERSON] [19]. The basic idea was to eliminate the interference factors on the pendulum, making it in an invar alloy to reduce thermal expansion and leaving it free to oscillate in a copper tank in which a vacuum was created (about 30 mmHg) to reduce aerodynamic resistance. The further disturbance caused by the link to the clock's mechanism (the main cause of error in precision clocks of the early 20 th century) was eliminated by decoupling the two mechanisms and using an electromagnetic system to supply power pulses to the two pendulums (Figure 10). Since the 1950s, wristwatches have increased their reliability thanks to the automatic winding obtained by means of a flywheel rotated by wrist movements.