May 2016 - During most of human history, people told time by the sun. They arranged to meet at dawn, when the sun came up, or at noon, when the sun was highest in the sky, or at sunset, when the sun went down. When you agreed to work for somebody, you measured your working hours by the day, from sunup to sundown. The time from one new moon to another was a month, and the time from one spring to the next was a year.
Around 2000 BC, the Sumerians in West Asia began to develop ways to measure shorter periods of time. Just as the year was divided into twelve months, they decided to divide the day into twelve hours, and the night into twelve hours too. In the summer, when days were longer, the daytime hours were longer too. People used the length of shadows to tell time - when the shadow of a tall pillar was a certain length, it was the second hour.
By about 1200 BC, people in West Asia, Central Asia, and Egypt began to use water clocks and sand hourglasses to keep track of these hours, so you could tell time at night. By about 300 BC, people were using sundials from Greece all the way east to Central Asia. By 500 AD, people were using candle clocks to tell time in China. They marked on a candle how much of the candle would burn in an hour. In this way you could tell time in the dark.
But all through this time, most people just didn't need to know what time it was. They kept right on telling time by dawn, noon, and twilight; that was good enough. Then the new religion of Islam got going in the 600s AD. Islam called for everybody to pray five times a day, at dawn, noon, mid-afternoon, sunset, and dark. It became more important for most people to know what time it was. The muezzin began to call the adhan, the call to prayer, from the top of minarets in each town. This provided an easy way of telling about what time it was, and it gave people the idea that telling time was a public responsibility.
Islamic scientists' interest in time combined with their new understanding of trigonometry in the 1300s to help them figure out how to build sundials that would measure hours that stayed the same length summer and winter, like our modern hours.
But even before that, by the late 1200s Europeans began to build mechanical clocks that used these same standard hours. These clocks worked by winding them up and then letting a weight slowly descend to the ground. The clocks had to be near the top of towers to give the weights room to move down. By 1369 there was a clock on the castle at Vincennes, and the next year there was a clock on the Conciergerie in Paris. These early clocks weren't accurate, and they were expensive, so medieval people also continued to use older methods of telling time, like sundials and candle clocks. Soon after the invention of the clock, in 1335, some Catholic priests in Milan found a cheaper way: they began to ring all the hours on the bells in their church tower (at the church now called San Gottardo). They rang the bell once at 1 am and twice at 2 am and so on until they rang 24 times at midnight. A lot of people thought this was a good idea, and soon most of the church towers in Europe rang out the time every hour, as they still do now.
But all the way through the middle ages, most people really only needed to know morning, noon, and evening, as they always had.