The relevance of longitude is basically to calculate local time of places in the world. A globe is supposedly 360°.
A complete rotation of the earth takes 24 hours or one day. It passes through 15° in one hour or l° in 4 minutes.
Nigeria, for instance, has accepted the meridian of 15°E as her standard time, which is one hour ahead of the Greenwich Mean Time.
Local time is the time at a place on any longitude in relation to the time on Greenwich meridian – i.e. G.M.T Since the earth rotates from West to East, in every 15°, it goes eastwards, the local time is advanced by 1 hour but if it goes westwards, the local time is retarded by 1 hour.
How to Calculate Local Time
To calculate local time, the following hints must be observed:
Workout the difference in Longitude between the two places being considered.
Convert your answer in (a) to a time difference.
Make the time you get in (b) to the direction of movement either east or west.
What is the local time in New York City (Longitude 75°W) when it is 1 10:am in Nigeria (Longitude 1 5°E)
The difference in longitude between New York City and Nigeria is 90°.
If 15° of longitude = 1 hour.
∴ 90° of longitude = 9015 hours = 6hours
The local time in Nigeria is 10:am
The time at New York City will be 6 hours earlier that 10:am So, this is the same as 4am i.e. 10-6 hrs GMT