Time is a fundamental concept that is difficult to define precisely. In physics, time is often defined as a dimension in which events occur in a sequence. Time is considered to be continuous and to flow at a constant rate, although its perception may vary among individuals and in different contexts.

The measurement of time is based on the repetition of a phenomenon with a known frequency, such as the oscillation of a pendulum or the vibration of a quartz crystal. These phenomena can be used to make clocks and other timekeeping devices that allow us to measure time with a high degree of accuracy.

However, the accuracy of time measurement can be affected by various factors, such as gravitational time dilation and relativistic effects. Therefore, in certain contexts, the concept of time can become more complex and nuanced.

Overall, time is a multifaceted concept that is deeply ingrained in our everyday lives, yet it remains an elusive and mysterious phenomenon that continues to fascinate scientists and philosophers alike.

## Measure time?

In the past, people tried to measure time using the position of, for example, the Sun and the stars. The movement of the Sun along the sky gives us already a time, solar time. Around noon, the Sun is at its highest point in the south, and we know that it is then in the middle of the day. You’ve already measured a time then. Now put a stick in the ground, and you get a shadow. Mark the point where the shadow falls at noon with a sign, and you already have a clock. Do this every hour, and you have a real sundial.

## 24 hours?

The Earth rotates around its own axis, and that’s why we can see the stars passing through the sky. Our planet rotates from right to left, and therefore the Sun rises in the east and sets in the west. The Earth rotates around its own axis every 24 hours, and that’s why we have a 24-hour time distribution. But… this isn’t quite right. The Earth orbits around its own axis every 23 hours, 56 minutes and 4.1 seconds. So we’re missing almost four minutes every day! Where are they?

## Leap Year

The Earth orbits the Sun every 365 days, 5 hours, 48 minutes and 45.1814 seconds. So not exactly 365 days! If we don’t care about the difference and complete a year in 365 days, then after four years we are almost a day short. So once every four years there is an extra day, which is the leap day on the 29th of February.

## Time zones

An imaginary line runs across England. This is the line that indicates Greenwich time. The Earth is divided into 24 time zones, all of which have a difference of one hour. They start counting in England. The difference with the Netherlands is 1 hour. If it is 10.00 hours in the Netherlands, then it is 9.00 hours in England. In Beijing (China) it is then 8 hours later than in England, so it is 5 in the afternoon there. In Argentina it is a lot earlier, namely 5 hours. There it is 4:00 in the night. On this website you can see all time zones.

## Jet lag

If you are in a plane, for example to China, and you leave at 12:00 Dutch time, it will take you about 10 hours. So you should arrive at 10 o’clock in the evening, but you go through a number of time zones, and therefore arrive at 3 o’clock in the afternoon! So you miss a number of hours. For you it is already 10 o’clock in the evening, but the Chinese time is 3 o’clock in the afternoon. That is why you are already tired in the afternoon, and want to go to bed. This is called jet lag. If you fly to America now, it’s exactly the other way around.

## Can you travel in time?

Many scientists, including the world-famous Stephen Hawking, don’t think so. But a lot of them think you can. So, what’s going on? I’ll try to explain. Light travels at 300,000 km per second. Imagine if you’re in a rocket that goes faster than the light, then you’re overtaking the light. People then think that your biological clock, the clock that actually is in your body, goes slower compared to the clocks on Earth. Everything in the rocket then goes slower. And because you catch up with the light then, you also catch up with time, and you travel in time. If you have a twin brother or sister who stays behind on Earth, they will be much older than you when you return!