Have you ever wondered why time differs between one country and another?

You must have seen the globe in your school or at home. A globe is a miniature form of the planet earth where we live. You can rotate it to see all countries and oceans on the earth. As we know, the earth’s axis is slightly tilted. To show this, the needle of the globe goes through the north pole and south pole in a slightly tilted angle. We can see how the earth actually rotates on the axis by rotating the globe from left to right.
There is no real axis on the real earth. It is an imaginary line going through the earth. Another imaginary line cuts the earth into northern and southern hemispheres. This line is called the equator. All the parallel lines that we draw from equator to the two poles are called parallels of latitudes.

The latitudes are measured in degrees. The distance from equator to the poles is one-fourth of a circle and therefore, is measured as one fourth of 360 degrees, which is 90^o.
The 90^o point towards the north of the equator is the North Pole and towards the south is the South Pole. Similarly, all the latitudes north of the equator are “north latitudes” and towards the south are the “south latitudes”.
There are some important latitudes of earth which are very important to understand the climate on earth:
The latitude that exists 23.5 degrees North (23.5^o N) from equator is called Tropic of Cancer.
The latitude that exists 66.5 degrees North (66.5^o N) from equator is called Arctic Circle.
The latitude that exists 23.5 degrees South (23.5^o S) from equator is called Tropic of Capricorn.
The latitude that exists 23.5 degrees South (23.5^o S) from equator is called Antarctic circle.

These latitudes divide the earth into three types of heat zones. The areas between Tropic of Cancer and Tropic of Capricorn receives maximum heat from the Sun. This region is therefore called the Torrid Zone.
As we go from Tropic of Cancer towards the Arctic Circle and from Tropic of Capricorn towards the Antarctic Circle, the temperature keeps decreasing because these regions get slant rays from the sun. The temperature is however, moderate and not very cold. These regions at the north and south are therefore called the Temperate zone.
The area between Arctic Circle and North Pole in the north and the Antarctic circle and South Pole in the south are very cold because the heat from the sun does not reach to surface much. This region is therefore called the Frigid zone.
If you need to describe a place, you cannot just use the latitudes. This is because many places may have the same latitude. You need to know how far east or west they are from some kind of fixed line. These lines need to be drawn from north to south and are called meridians of longitude. The distance between two longitudes is measured in degrees of longitude, which can further be divided into minutes and seconds.

Longitudes are semi circles that run from North Pole to the South Pole. Since all the longitudes are of same length, all countries decided to name the longitude going through the Royal British Observatory in Greenwich as the Prime Meridian with the value of 0^o longitude. It divides the earth into two halves- the Eastern Hemisphere and the Western Hemisphere.
From the Prime Meridian, we count 180^o towards east and 180^o towards west. We represent the longitudes by following them with E for East and W for west.
For example, Agra city in India is located at 77.5^o E
To pinpoint a place, we need to know the information about both the latitude as well as the longitude. Knowing only one won’t help us to locate the exact place we want. We write the information about the latitude first, followed by the longitude.
For example, if we want to locate Vidisha city on the map of India, we can use the latitude and longitude information as 23.5^o N, 78^o E.
Do you know that we can use longitudes to find out the time for any place in the world?

We can do this because of the sun. When it’s noon at one place on the earth, all the places located on that longitude would also see noon time. Since there are 360 degrees of longitudes in total, every 15 degrees, we can see a time difference of 4 minutes.
(The earth rotates 360 degrees in 24 hours. This means, 15 degrees in 1 hour or 1 degree in 4 minutes. This also means that 15 degrees west of you will have time that is 1 hour behind you.)
As the longitude changes, time of a place is bound to change. This would cause a lot of difficulties managing the time of important things such as train schedule, school timings etc. This will also mean that different parts of a country would have different timings at the same moment.

To avoid these confusions in timing, different countries use a single time for the entire country. This time is followed in the country, without the need for change due to the longitudes. The time at 0 degrees longitude at Greenwich is called Greenwich Mean time or GMT.
India also follows a single time which belongs to the longitude going at 82.5^o E, which is 5 hours and 30 minutes ahead of GMT. This is called Indian Standard Time (IST).
Some countries have very wide longitude range. They use more than one standard times for their countries. They use many standard times for their convenience.
For example, Russia uses 11 standard times.
The earth is divided into 24 time zones with 15 degrees difference between them and 1-hour time difference.
So, here we saw that imaginary lines cover the earth. These are called Latitudes and Longitudes. Latitudes are horizontal lines running parallel to the equator of the earth. They divide the earth into three temperature zones. Longitudes are vertical semi-circles running from North pole to the South Pole and are important to determine the time difference between two places on the earth. Both latitudes and longitudes are measured in degrees and both information are necessary to pinpoint a place on the earth. Longitudes also help in setting standard times and time zones on the earth.
And just like that, you have finished your revisions for the chapter- Globe: Latitudes and Longitudes from your Class 6 NCERT Geography textbook.

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