Lines of Longitude

When we look at at a globe or a map of the world, we may see some lines running from west to east and other lines running from north to south. Can you see these lines on the map below? The lines which run from north to south are known as lines of longitude.

What are lines of longitude?

Lines of longitude are imaginary lines which run from north to south on a map or globe. They are also called meridians. Lines of longitude do not go all the way around the globe. Each line of longitude runs from the North Pole to the South Pole. Lines of longitude are not parallel to each other. They meet at the poles and are farthest apart at the Equator. The diagram below shows some of the world’s lines of longitude.

Notice how each line of longitude runs from the north pole to the south pole. (Photo by Pearson Scott Forman and used under a creative commons license)

Some important lines of longitude are:

The Prime Meridian: This line of longitude is the zero degree (0°) line of longitude. It passes through Greenwich in London. For this reason it is also called the Greenwich Meridian. The Prime Meridian divides the earth into the eastern hemisphere and western hemisphere. Other lines of longitude are measured in degrees east or west of the prime meridian.
The International Date Line: This is the 180° line of longitude. By international agreement, this line marks the point where each new calendar day begins. Because if this, places on the eastern side of the International Date Line are one calendar day earlier than places on the western side of it.

Some lines of longitude are used to divide the world into time zones. There are twenty-four time zones. Each one has its own local time. Take a look at the map below which shows the time zones of the world.

How do time zones work?

St. Lucia is located in one time zone. Jamaica is in the next time zone to the west of St. Lucia. Because of this, it is always one hour earlier in Jamaica than it is in St. Lucia. For example, when it is 9am in St. Lucia, it is 8am in Jamaica.

However, if you cross the International Date Line, things are not that simple. If you cross it heading east, it will be one day earlier. If you cross it going west, it will be one day later. For example, when it is Wednesday just to the west of the International Date Line it is still Tuesday just to the east of it.