What is time, really?
We glance at the clock all day long. We rush to catch the bus, count down to the weekend and wish our holidays lasted a little longer. But have you ever stopped to wonder what the clock is actually measuring? Here is a short journey through the history of time, from sundials and stars to atoms and time zones.
A way to order events
Time is hard to grasp. We cannot see it, hear it or hold it in our hands. Yet we notice it constantly, because things change. The sun rises and sets, coffee goes cold, flowers bloom and children grow out of their shoes. Without time, everything would happen at once. Physicists often describe time as the thing that puts events in order: first one thing happens, then another. We remember the past but not the future, and that gives time a clear direction forward. To talk about change, make plans and work together, people divided time into pieces we can count. That is really all a clock does. It counts something that repeats regularly and shows us the result.
The sky was the first clock
The very first clocks were in the sky. The sun's path across the sky gave us the day, the phases of the moon gave us the month, and the changing seasons gave us the year. Thousands of years ago people built monuments that marked when the sun stood highest or lowest. Stonehenge in England is a famous example. For farmers it was vital to know when to sow and when to harvest, which is why the calendar is one of the oldest inventions we have. But the sky is a rather rough clock. It tells you roughly what time of day it is, but not whether you are ten minutes late for a meeting. For that, something more precise was needed.
Why 24 hours and 60 minutes?
We can thank the ancient Egyptians for the 24-hour day. They divided the night into twelve parts using stars that rose in the sky, and the day into twelve parts using sundials. Twelve was a handy number because it can be divided in many ways: by two, three, four and six. Minutes and seconds come from the Babylonians, who counted in sixties. Their number system survives both in our clocks and in how we measure angles, where a circle has 360 degrees. That is why an hour has 60 minutes and a minute has 60 seconds. Nature did not decide these numbers. They are old habits that worked so well we never replaced them.
From sundials to pendulum clocks
Sundials only work when the sun is shining, so people spent a long time looking for other ways to measure time. Water clocks let water flow at a steady pace from one vessel to another. Hourglasses did the same with sand, and marked candles showed how many hours had burned away. In the 1300s the first mechanical clocks appeared in church towers across Europe. They were big, noisy and not very accurate, sometimes drifting by fifteen minutes a day. The big breakthrough came in 1656, when the Dutch scientist Christiaan Huygens built a pendulum clock. A pendulum swings at almost exactly the same rate every time. Suddenly clocks were only off by a few seconds a day, and the minute hand became truly meaningful for the first time.
From the sun to the atom
For a long time a second was defined by Earth's rotation, as a tiny fraction of a day. The problem is that Earth does not spin perfectly evenly. Tides, earthquakes and movements deep inside the planet make the length of a day vary slightly. For modern technology such as GPS, the internet and mobile networks, that is not good enough. Since 1967 a second has therefore been defined using the caesium atom. The atom flips between two energy levels, and one second is exactly 9,192,631,770 of those oscillations. Atomic clocks are so incredibly precise that they drift by only one second over millions of years. The official world time, UTC, is based on hundreds of atomic clocks around the globe. Occasionally a leap second is added so atomic time does not drift away from Earth's rotation.
Why are there different time zones?
Earth turns once in roughly 24 hours. That means the sun is always shining on a different part of the planet. When it is lunchtime in Stockholm, it is the middle of the night in Sydney and early morning in New York. Towns used to keep their own time, based on when the sun stood highest. Gothenburg and Stockholm actually had slightly different times. That worked perfectly well as long as people travelled by horse and carriage. But when railways arrived in the 1800s, timetables turned into chaos, since every station followed its own clock. Sweden introduced a shared railway time as early as 1879. In 1884 countries from around the world met in Washington and agreed that Greenwich in London would be the starting point for the world's time.
How time zones work
The globe is 360 degrees around, and if you divide that by 24 hours, each time zone is about 15 degrees wide. In theory the zones are straight bands from pole to pole, but in reality they often follow national borders. Nobody wants the time to differ on either side of a street. That is why the time zone map looks like a patchwork quilt. China is almost as wide as five time zones but uses a single time for the whole country. India is offset by half an hour, and Nepal even has a time zone that differs by a quarter of an hour. Sweden follows Central European Time, CET, which is one hour ahead of UTC in winter. In summer we move the clocks forward and use Central European Summer Time, CEST, two hours ahead.
Summer time and winter time
Daylight saving time was introduced so we could make better use of daylight. By moving the clock forward one hour in spring, we get brighter evenings, and the original idea was to save energy. Sweden tried summer time as early as 1916 but only made it permanent in 1980. Today the clocks change on the last Sunday of March and the last Sunday of October, at the same time across the EU. The change is debated. Many people love the bright summer evenings, while others feel the lost hour in spring disrupts their sleep. The EU has discussed stopping the clock changes, but no decision has been put into practice yet. Until then, we keep turning the clocks back and forth twice a year.
Time is relative
Perhaps the most surprising thing about time is that it does not pass at the same speed everywhere. In the early 1900s Albert Einstein showed that time runs slower the faster you move and the stronger the gravity around you. It sounds like science fiction, but it actually affects your everyday life. The clocks on GPS satellites tick slightly faster than clocks on the ground, because gravity is weaker up there. If nobody corrected for this, the location on your phone would be several kilometres off after just one day. So time is not just a way to schedule meetings. It is part of the very fabric of the universe.
Why does time fly when you are having fun?
The clock ticks at the same pace all the time, but our brains do not. When we are having fun, we are so absorbed in the moment that we stop noticing time, and suddenly three hours have passed. When we wait in a queue, we do the opposite: we check the clock again and again, and every minute feels long. Researchers also think this is why the years seem to speed up as we get older. As children we experience many things for the first time, and the brain stores lots of new memories. As adults, everyday life becomes more alike, and when we look back there are fewer memories marking the time. One trick to make life feel longer is to do new things: travel, learn something new or take a different route to work.
Animals and plants keep time too
Humans are not alone in keeping track of time. Almost every living thing has an internal clock, a so-called circadian rhythm, that controls when we feel tired, hungry and alert. In us it sits in a small part of the brain that responds to light. That is why jet lag feels so rough: your body thinks it is night while the sun is shining outside. Birds use their internal clocks to know when it is time to fly south, and some flowers open and close at almost the same time every day. The Swedish botanist Carl Linnaeus even designed a flower clock, where you could tell the time by seeing which flowers were open.
So next time you look at the clock
The numbers you see are the result of thousands of years of curiosity and invention. The Egyptians' twelve hours, the Babylonians' sixties, Huygens' pendulum, railway timetables and the physicists' caesium atoms are all hidden in that small display. Time moves on as relentlessly as ever, but we have become very good at counting it. Here on klockan.info you can always see exactly what time it is in Sweden, compare it with other time zones and keep track of when it is time to change the clocks next.