Meridians of longitude

Meridians of longitude:

• Meridians are semicircles running north-south that meet at the poles. Two opposite meridians make a full circle, but each is counted separately.
• They cross the equator at right angles and, unlike parallels, are all equal in length.
• By international agreement, the meridian through the Greenwich observatory (near London) is the Prime Meridian, 0°.
• Longitude is a place's angular distance east or west of the Prime Meridian, from 0° to 180° each way: 360° in all. East of it is the eastern hemisphere; west of it, the western hemisphere.

Latitude and longitude compared:

• Parallels are all parallel to the equator; meridians converge at the poles.
• On a globe, parallels are circles; meridians are circles through the poles.
• The gap between two parallels is about 111 km everywhere. The gap between two meridians is greatest at the equator (111.3 km), about 79 km at 45° latitude, and 0 km at the poles.
• Latitudes mark temperature zones; longitudes are used to find local time relative to the Prime Meridian.

Drawing meridians: draw a circle whose centre is the North Pole and whose edge is the equator. A vertical line through the centre gives the 0° and 180° meridians. Imagine standing at the North Pole looking down: east and west now swap sides, so east is on your left and west on your right. Place the protractor on the vertical line and measure the angle on both sides for the east and west meridians.

Example (new): two pairs of ships are each 1° of longitude apart, one pair on the equator and one pair near the Arctic Circle. The pair on the equator are about 111 km apart; the northern pair are only about 44 km apart, because the meridians have drawn close together.