What is time? A look at some of the ways we measure this unstoppable force of nature and its intimate connection with space
Time is something we all take for granted, yet it can be very difficult to grasp when we think hard about it. As long ago as 400 CE, Saint Augustine remarked that he knew exactly what time was, except when he tried to explain it. Many of us might say the same thing today, and the world’s philosophers are still arguing with about the exact nature of time and how we perceive it. Fortunately, there are some aspects of the subject that we’re now able to pin down much more precisely, thanks to super-accurate timepieces and sophisticated mathematical theories. The next few pages are a guided tour of some of the most fascinating highlights. We’ll start by looking at the way our measurement of time has evolved over the centuries, from ancient sundials and megalithic calendars like Stonehenge, through the earliest mechanical timepieces to the ubiquitous quartz-driven electronic clocks and watches of today. Then we’ll take a dip into Einstein’s mind-bending theory of relativity, which shows how space and time are intimately interconnected, and how different observers can see time passing at different rates. Finally, we’ll come bang up to date with the latest generation of high-tech atomic clocks, some of which are so accurate that they wouldn’t lose so much as a second in the entire lifetime of the universe.
For prehistoric people, keeping track of the time of year was much more important than knowing the time of day. As a result, their most sophisticated attempts at timekeeping were not so much clocks as calendars. They watched the Sun and Moon changing position in the sky in order to keep track of the seasons, and several of their most striking monuments are built around solar or lunar alignments for this purpose. The summer solstice alignment of Stonehenge is the most famous of these, although Newgrange in Ireland – which is aligned with the winter solstice – is equally impressive and even older.
As for the time of day, the oldest and simplest method of working it out also used the Sun. This is the sundial, which does have the disadvantage that it won’t work at night or on a cloudy day. A more reliable alternative, which emerged in ancient Egypt, is the water clock. This may be less familiar today than the hourglass, a somewhat later invention, but it works on the same basic principle, just using flowing water instead of sand grains.
The first mechanical clocks appeared in the latter part of the 13th century. A Europea
