made possible before the invention of the balance spring. Before these improvements, the equation of time, the difference between apparent and mean solar time, was not even noticed. During the 17th and 18th centuries, private ownership of clocks and watches became more common, as their improved manufacture made them available for purchase by at least the bourgeoisie of the cities. Their proliferation had many social and even religious consequences for those who could afford and use them. Before time became standardized, clock masters used "True Time". The day work began and ended with the sun. This time period was divided into 12 equal hours. This meant that these hours would vary with the seasons, as the length of daylight changed. Each town would have their own variance of this "True Time". Eventually, cities adopted "Mean Time", which is how we think of time nowadays. Astronomers used the Earth's rotation and the stars to calculate the time, and divided the day into 24 uniform and equal hours. Geneva was the first city to adopt mean time in 1780, followed by London in 1792, Berlin in 1810, Paris in 1816, and Vienna in 1823. Religious consequences Religious texts of the period make many more references to the irreversible passage of time, and artistic themes appeared at this time such as Vanitas, a reminder of death in the form of a still life, which always included a watch, clock, or some other timepiece. The relentless ticking of a clock or watch, and the slow but certain movement of its hands, functioned as a visible and audible memento mori. Clocks and sundials would be decorated with mottos such as ultima forsan ("perhaps the last" [hour]) or vulnerant omnes, ultima necat ("they all wound, and the last kills"). Even today, clocks often carry the motto tempus fugit, "time flies." Mary, Queen of Scots was said to have owned a large watch made in the shape of a silver skull. Economic consequences Economically, their impact was even greater; an awareness that time is money, a limited commodity not to be wasted, also appears during this period. Because Protestantism was at this time chiefly a religion of literate city dwellers, the so-called "Protestant work ethic" came to be associated with this newly fashioned time discipline. Production of clocks and watches during this period shifted from Italy and Bavaria to Protestant areas such as Geneva, the Netherlands, and