The lines of longitude are imaginary lines running through the earth from north to south. These lines are also known as Meridian lines. The prime Meridian is the longitudinal reference line which is at 90°.The lines of longitude are measured east to west of the prime meridian.
The line of longitude which is west to the prime meridian and halfway between the Prime Meridian and the 180° line of longitude is the 90° west Meridian line.
