What is time?
Clock masters aren’t the only people who find time fascinating. Advanced physicists and philosophers study it too—and even they don’t understand how time works! What is time, anyway? People have spent lots of long words trying to answer that question. They have also used simple words like these: “Time is what a clock measures,” or, “Time is what keeps everything from happening at once.” We can’t see time. But the changing world around us lets us know it exists. So do ticking clocks.
But what is a clock?
That’s easy! Right? A clock is a round thing with hands on its face. It’s also the digital numbers on a stove or phone. But before digital numbers and clock faces existed, a clock was a sundial. A sundial is basically a pointer on a plate. Sunlight causes the pointer’s shadow to fall on the plate at a different spot each hour. (Imagine checking your sundial on a cloudy day in ancient Egypt. Forget showing up on time for your dentist appointment!) Ancient people also used water clocks—containers that water flowed into or out of at a steady rate. In a water clock, time is measured by the amount of water remaining in the container. Really, many other things can work as clocks too. Anything that does something over and over again at the exact same speed can tell you about passing time. Just count the number of times the action repeats.
Why isn’t it the same time everywhere?
When you hop into bed for the night, kids on the other side of the world are hopping out. Where they live, the Sun is just rising. Their side of the Earth has rotated to face the Sun. Is it the same time where you live and where they live? It’s not—even though you’re all living at the same moment in time. Now that’s confusing!
Scientists helped with the confusion in the late 1800s by creating time zones. They divided Earth into 24 zones. Each zone is 15 degrees of longitude wide. They drew an imaginary line running north and south across the face of the Earth and called it the prime meridian. In each time zone to the west of the prime meridian, the time is an hour earlier than the one before. In each zone to the east, the time is an hour later than the one before. That way, times match the way the Earth and Sun have moved.
