Calculate time using longitudinal position of the place

lines of longitude

Q. What are longitudinal lines?
Ans. Longitudinal lines are imaginary vertical lines over the Earth's globe.



0° longitudinal line is Greenwich Prime Meridian.
180° longitudinal line is International Date Line.

lines of latitude

Q. What are latitudinal lines?
Ans. Latitudinal lines are imaginary horizontal lines over the Earth's globe.


0° longitudinal line is Equator.




Earth completes one rotation on its axis in 24 hours and in the process turns a complete circle of 360°. This means Earth rotates 360°/24 = 15° in one hour.

Difference of time for 15° longitude = one hour.
Difference of time for 1° longitude = 4 minutes.






Q. Calculate the time at 74° W longitude (New York) when it is 12 a.m. at 82½° E longitude (India)?
Ans. longitude of India = 82½° 
longitude of New York = 74° 
Difference in longitude = 82½° - (-74°) = 156½°
Difference of time for 1° longitude = 4 minutes
Difference of time for 8½° longitude = 4*(156½) = 626 minutes OR 626/60 = 10 hours 26 minutes
As New York is in the west of India, the time at New York will be less than India
Therefore Time of New York = 12 - 10.26 = 01:34 p.m.