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International Date Line (IDL)

The International Date Line is an imaginary line that has been drawn from the eastern
part of longitude. These longitudes meet each other at 180°. It is called International
Date Line. It changes the day and date. Time from International Date Line towards
the eastern hemisphere is 1 day faster than towards the western
hemisphere. It is also called a Calendar Line because it demarcates
one calendar day towards eastern and western hemisphere, i.e. the
date changes from this line. The time after 12 noon is called PM
and after 12 midnight is called AM.

The International Date Line is zigzag, irregular, curved and bending
in several places to maintain the same date through which the
imaginary line would pass. It is an imaginary line passing through
the Pacific Ocean roughly corresponding to 180° longitude to the east of which by an
international agreement the calendar date is one day earlier than to the west. It is set
according to the International Meridian Conference in 1884 AD. Longitudes help us
in finding the local time of any place around the world and latitudes tell us about the
possible climate condition of a place. While calculating time of a place with the help of
longitudes, the following points should remember:
The direction of rotation is west to east, therefore, the sun seems to rise in the
east.
1° longitude has a time difference of 4 minutes as 360° is covered in 24 hours.
The earth rotates once on its axis in 24 hours.
The Greenwich Meridian is the reference point for time calculation.

Ways to Calculate Time

Example - I
What will be the local time of Kathmandu located in 85°E longitude when it
is 2PM at Moscow (Russia) 40°E longitude?

Solution:

The difference in terms of longitude between Kathmandu and Moscow
= 85°- 40° = 45°
Different in time = 45° × 4 = 180m
180
Changing minutes into hours = = 3 hours.
60
Since Kathmandu lies to the east of Moscow, the time is ahead.
So 2:00 + 3:00 = 5:00 pm
∴ The time in Kathmandu is 5:00 pm while it is 2:00 pm in Moscow.

189 Dynamic Social Studies - Book IX