Time Zones

What Are They & Why Do We Need Them?

We measure the day by saying that the sun is at it’s highest point at 12 o’clock, midday. Because the earth spins, as it travels around the sun, one half of the earth is in darkness and one half in light, all the time. This brings us to a problem. If it is midday where you are, on the opposite side of the earth, it is dark, so what time is it on the opposite side of the world?

Day and Night

Before time keeping and clocks were invented, people used to know approximately what time it was by watching the sun. If you look out of the window, at different times of the day, you will see that the sun is in a different place each time you look. This is the same everywhere in the world. Once clocks were invented, they simply had to look at a clock to know what time it was.

The earth spins on an imaginary line known as its axis, this movement is known as rotation. While it is rotating on its axis, it is also traveling around the sun, this movement is known as revolution. Although the earth is revolving around the sun, the sunlight only falls on the part of the earth facing the sun. So, the part facing the sun has day and the part away from the sun has night. As the earth rotates, different parts are exposed to the sun, and put in the shade.

    • Look at the two marks on the globe.
    • Notice that the one enters and leaves the sunlight before the other one does.

Check where the sun is shining right now.

The Problem

Remember the problem: if it is midday where you are it must be dark on the opposite side of the earth, it obviously can’t be midday, but what time is it? The earth is divided into time zones. The time is the same everywhere within one time zone, but different to all the other time zones.

How Time Zones Work

Because everyone wants to measure their day with the sun being at its highest point at midday, scientists came up with the idea to divide the earth into different time zones. There are 24 hours in a day, so there are 24 time zones. The time is the same everywhere within one time zone, but different to all the other time zones. When you move from one time zone to the next one, you change your watch by one hour. If you are traveling in an easterly direction you move your watch one hour forwards, if you are moving in a westerly direction you move it one hour backwards.



  • New York is west of London, and they are separated by 5 time zones.
  • This means that when it is midday in New York it is 5 pm in London.
  • When it is midday in London it is 7 am in New York.


You can try this yourself, first read the instructions:


  • Note the time on the clock. This is the time in London.
  • In the right-hand box, select USA/Eastern USA.
  • Watch how the bar moves to the left and the red spot shows -GMT 05:00
  • Notice how the time has changed on the clock. It shows 5 hours earlier.


Now try it!

Extra Activity

There are 360 degrees of longitude on the earth, so what the scientists did was divide these by 24, that means that there are 15 degrees of longitude in every time zone. If you travel more than that, you have to adjust your watch. You may be wondering why time zones are measured in degrees of longitude and not miles or kilometers. See if you can figure it out for yourself, then check your answer.

More about time on this site