1. GREENWHICH MARIDIAN

2. Figure of the Earth
Since the Earth is flattened at the poles and bulges at the equator, geodesy represents the shape of the earth with an oblate spheroid.
The oblate spheroid, or oblate ellipsoid, is an ellipsoid of revolution obtained by rotating an ellipse about its shorter axis.

3. Degree of rotation of earth
earth rotates once in 23 hours 56 minutes. Aproximately.
But since earth is moving one degree per day around Sun for solar day we need 4 minutes more.
degree of rotation of earth is degrees.

4. GREENWICH TIME
Greenwich Mean Time or GMT is the clock time at the Royal Observatory in Greenwich, London.
When the sun is at its highest point exactly above the Prime Meridian.
GMT is still widely used as the standard time against which all the other time zones in the world are referenced.
It is the same all year round and is not affected by Summer Time or Daylight Saving Time.

5. TIME ZONES Standard time
It is the synchronization of clocks within a geographical area or region to a single time standard, rather than using solar time or a locally chosen meridian (longitude) to establish a local mean time standard.
Indian Standard Time (IST)
The time observed throughout India and Sri Lanka,Indian Standard Time is calculated on the basis of 82.30' E longitude.
The International Date Line
Established in 1884, passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. It is located halfway round the world from the prime meridian—the zero degrees longitude established in Greenwich, England, in 1852.

6. To calculate time
The earth revolves itself in an orbit.To take one revolution for earth(360 degree) itself earth takes 24hours.
1 hour =60 minute
24 hours =(24hours*60minutes)=1440 minutes.
To rotate one degree earth takes (1440/360)minutes=4 minutes.

7. problem
1) what will be the time in new york(74 degree west) when it is 12 noon at greenwich?
The longitudinal difference between new york and greenwich =74 degree
The time difference for 1 degree longitude=4 minutes
The time difference for 74 degree longitude=4*74
=296 minutes
=4 hours 56 minutes

8. problem
As new york is located to the west of greenwich,the time in New york will be 4 Hours and 56 minutes behind Greenwich Mean Time.
Therefore the time at new York= Time at greenwich – Time difference
=12 noon -4 hours 56
minutes
=7.04 am.