The grid and latitude

The earth is nearly a sphere: its rotation makes it bulge at the equator, so its shape is an oblate spheroid. A round surface has no natural starting point for measuring positions, so we draw a network of imaginary lines, the geographical grid.

• The earth spins west to east on its axis, which gives two natural reference points: the North and South Poles.
• The grid has parallels of latitude (running east-west) and meridians of longitude (running north-south). Together, latitude and longitude are called geographical coordinates: they fix the location, distance and direction of places.
• They are angular distances, measured in degrees (°); each degree has 60 minutes (') and each minute 60 seconds (").

Parallels of latitude:

• Latitude is a place's distance north or south of the equator, measured as an angle from the earth's centre along the place's meridian. Lines joining places of equal latitude are parallels.
• The equator (0°) lies midway between the poles. It is the largest circle and divides the globe into two equal halves: a great circle. All other parallels get smaller towards the poles and are small circles.
• The poles are 90° N and 90° S. Write N or S with every latitude.
• At 1° intervals there are 89 parallels in each hemisphere, so 179 including the equator.
• On a perfect sphere a degree of latitude would be 111 km everywhere. In fact it is about 110.6 km at the equator and 111.7 km at the poles.
• A place's latitude can be found from the altitude of the sun or of the Pole Star.
• Latitudes mark the temperature zones: torrid (0° to 23½° N and S), temperate (23½° to 66½°) and frigid (66½° to 90°).

Drawing a parallel: draw a circle and a horizontal line through its centre for the equator. Put a protractor with its 0°-180° line on the equator, mark the chosen angle on both sides (above the line for north, below for south), and join the two points where the angle's arms cut the circle with a line parallel to the equator.

Example (new): Chennai is at about 13° N. Its distance from the equator is roughly 13 × 111 = about 1,440 km, measured straight north along its meridian.