through Greenwich. It is marked as 0°. Any meridian of a place is measured east or west of the Prime Meridian. Meridians are marked at a gap of 1° each. The Prime Meridian divides the Earth into two hemispheres, namely, the Eastern Hemisphere and the Western Hemisphere. The Earth has 180 meridians each, towards the east and the west of the Prime Meridian. The longitude of 180° E and 180° W is the same line. If we travel halfway around the world from the Prime Meridian, we get the 180° longitude. Thus, there are 360 lines of longitude in total. The International Date Line is marked as 180°.

A grid

You may locate a place on the globe by knowing the value of its parallel and meridian. The point where both intersect will give you the location of the place. This intersection point is known as a grid.

Calculating time

Meridians help us calculate time. The time difference between two longitudes is 4 minutes. Let us understand how it is calculated. We know that the Earth takes 24 hours to complete one rotation. Also, 1 hour = 60 min. Thus, the Earth takes 24 × 60 = 1440 minutes for one complete rotation.

Again, we know that there is a total of 360 meridians. So, if we divide the total number of minutes by the total number of meridians, we will get the time difference between two longitudes. That is, 1440 ÷ 360 = 4

Furthermore, the places on the Eastern Hemisphere are a day ahead of those that lie in the Western Hemisphere. The International Date Line splits the Earth to give us two different dates on a calendar. For instance, India, a country on the Eastern Hemisphere, is 12 hours ahead of USA, a country in the Western Hemisphere.