geography

                                                            LONGITUDES

Longitude is an angular distance of a point on the earth's surface measured in degrees from the center of the earth.The starting point of longitude is 0 (observe in the below figure).

ANALYSIS FOR UNDERSTANDING:

 we have drawn all the lines which exactly passes through north and south poles with unit angular distance. Then we kept the longitude which is passing through ROYAL laboratory ENGLAND as base starting from 0 degrees.

Total longitudes are 360 and earth is taking 24 hrs to  pass through all of these longitudes, this implies that in one hour earth will passes trough 15 degrees(or longitudes). In half-an-hour it will passes through 7.5 degrees. So each country has taken the longitude which is multiple of 7.5 to follow the time in the entire country.