Understanding time zones

It takes 24 hours for the Earth to rotate once on its axis. We split the globe into time zones using imaginary lines called meridians. They run from the North Pole to the South Pole, crossing lines of latitude. There are 24 time zones.

There is an imaginary line running through the UK called the Prime Meridian. It runs through a place in London called Greenwich.

The Prime Meridian splits the world into eastern and western hemispheres.

Time in countries to the east of the Prime Meridian is always in front of that in the UK.

Time in countries to the west of the Prime Meridian is always behind that of the UK.

The International Date Line (IDL) is an imaginary line on the Earth's surface defining the boundary between one day and the next. Located at about 180° east (or west), it is halfway around the world from the Greenwich Meridian (0° longitude). When you cross the date line travelling east, you subtract a day, and if you cross the line travelling west, you add a day.

As the Earth rotates on its axis, the Sun only shines on the side of the Earth that it is facing. This means:

• It is daytime for the parts of the Earth that have the Sun shining on them
• It is night-time for places that are on the opposite side of the Earth and are in the shade

Time zones are not always in straight lines on the longitudes on Earth. This is because they may need to curve around country borders. You can see this in the map below.

Different places in the world have different times.

This is why the world is divided into 24 different time zones. One for each hour in a day.

Very large countries such as Australia or the USA are spread out across many time zones.

Most smaller countries keep to the same time zone even if part of them falls outside a meridian line.