CBSE Class 9 Answered

Longitude is a geographic coordinate which specify the East or West position of a place on the surface of the Earth.  Distance between the longitudes are measured in 'degrees of longitude'. There are 360 meridians of longitude because a circle has 360 degrees. Each degree is divided into minutes and minutes into seconds. The Earth takes 24 hours to complete one rotation or to cover 360° of longitudes. This means that the Earth covers 15° of longitudes every hour. One degree of longitude takes 4 minutes (1 hour = 60 minutes, divided by 15° per hour = 4 minutes per longitude). 

As the earth rotates from west to east, those places that lie east of Greenwich are ahead of the places that lie to the west of Greenwich. The earth rotates 1⁰ in four minute.

Example: When it is 08:00 am in Mumbai, calculate the time at Wellington.  Wellington is located at 175⁰E longitude and Mumbai is located at 73⁰E longitude. Now difference in longitude between Mumbai and Wellington is 175⁰-73⁰= 102⁰ (since both the places are in the eastern hemisphere, we will subtract one longitude from the other).The difference in time between the two places is 102⁰X4 minutes=408 minutes (we multiply it by 4 because one degree of each longitude takes 4 minutes to travel to the other longitude). Now 408 min= 6 hrs and 48 min. Since Wellington is at the east of Mumbai, its local time is ahead of Mumbai. We will add the difference i.e. 6 hrs and 48 minutes to the time in Mumbai.

So, 8:00am+6 hrs and 48=14 hrs and 48 min= 2:48pm.