Globe: Longitudes
- Use of latitudes and longitudes
- Calculation of time in different time zones
- Indian Standard Time
The imaginary lines which run from one pole to another are called longitudes. These are also called meridians of longitude. Unlike latitudes, all meridians of longitude are of equal length. Hence, it was difficult to decide the zero degree longitude.
Greenwich
So, all countries decided that the line passing through Greenwich should be made the zero degree longitude. This happened because the British Royal Observatory is located at Greenwich. The zero degree longitude is also called the Prime Meridian. Furthermore, the earth has been divided into 1800 longitudes towards the east and 1800 longitudes towards the west. The Prime Meridian and 1800 longitude together divide the earth into two equal halves. Both 1800 east and 1800 west are on the same line.
Use of Latitudes and Longitudes
Latitudes and longitudes help us in finding the exact location of a place on the earth. When we say that Delhi is 280N; we are just saying that Delhi is in the Northern Hemisphere on 280 latitude. But this does not show Delhi’s position with respect to the Prime Meridian. So, we also need information about longitude to know the exact position of Delhi on the earth. Delhi is approximately at 770 E longitude. So, to give exact position of Delhi; we need to say that it is 280N and 770E.
To understand this in a better way, let us draw a set of vertical parallel lines. Let us draw another set of horizontal parallel lines over the earlier set. This will give us a grid. Let us now mark the equator and prime meridian; as shown in the figure. Now, you can easily tell the exact locations of points A, B, C and D by mentioning their longitudes and latitudes.
Interesting Fact
The GPS (Global Positioning System) uses the grid system of latitudes and longitudes to find exact location of a place on the Earth.
Longitude and Time
We know that the earth takes about 24 hours to make one rotation on its axis. Thus, the sunrise and the sunset easily tell us about morning and evening times. When the sun is directly overhead, it is mid-day and you can match your watch to 12 noon at this time. The sun casts the smallest shadow when it is directly overhead. But the relative position of the sun in sky keeps on changing as we move across the globe either from east to west or vice-versa. So, if it is mid-day in India, it can be another time in England.
People felt a need for some standard system for keeping time. Time in different countries is fixed according to its relative position to Greenwich because the Prime Meridian passes through Greenwich.
We know that the earth rotates from west to east and hence, the sun’s apparent position changes from east to west. So, Australia will witness an earlier sunrise compared to India. Similarly, the sun will rise later in England than in India. Hence, the time in Australia would always be ahead of time in India. Similarly, the time in England would always be behind the time in India.
Calculations for Time in different time zones
Total number of degrees = 360
Number of hours in a day = 24
So, number of degrees per hour = 3600 ÷ 24 = 150
Since there are 60 minutes in an hour
So, 150 = 60 min
Hence, number of minutes per degree = 60 ÷ 15 = 4 min
This means that when we move 1 degree longitude towards east, the time advances by 4 minute. On the other hand, the time goes behind by 4 minute when we move 1 degree towards west.
Indian Standard Time
India is a big country and its longitudinal span is quite large. So, there is a difference of about 1 hour 45 minute in sunrise times between Dibrugarh (Assam) and Dwarka (Gujarat). But we need a standard time throughout the country for the ease of maintaining time schedules. The Indian Standard Time (IST) has been fixed on 82.50 E (820 30’ min) meridian.
82.5 × 4 = 330 min = 5 hr 30 min
The above calculation shows that the Indian Standard Time is ahead of the Greenwich Meantime by 5 hours 30 minutes.
Some countries are so large that only one time zone is not enough for them. For example; there are 11 time zones in Russia. There are 9 time zones in the USA. The whole earth has been divided into 24 time zones. Each time zone covers 150 longitudes.