The Earth is nearly a sphere, but because its equatorial radius and polar radius are not the same, its actual shape resembles an oblate spheroid. The rotation of the earth over its axis produces a bulging at the equator. To position surface features and locate places, a network of imaginary lines is drawn on a globe or map, forming a geographical grid.
The spinning of the earth on its axis from west to east provides two natural points of reference: the North Pole and the South Pole. The grid consists of two sets of horizontal and vertical lines, called parallels of latitudes and meridians of longitudes.
The latitude of a place is its angular distance north or south of the equator, measured from the centre of the earth. Lines joining places with the same latitudes are called parallels.
Unlike parallels, meridians of longitude are semi-circles that converge at the poles. Opposite meridians together form a complete circle. They intersect the equator at right angles and are all equal in length.
The following table summarizes the key differences between parallels of latitudes and meridians of longitudes:
| S. No. | Parallels of Latitude | Meridians of Longitude |
|---|---|---|
| 1. | Angular distance north or south of the equator. | Angular distance east or west of the Prime Meridian. |
| 2. | All latitudes are parallel to the equator. | All meridians converge at the poles. |
| 3. | They appear as circles on a globe. | They appear as semi-circles running through the poles. |
| 4. | The distance between two latitudes is constant (approx. 111 km). | The distance between longitudes is maximum at the equator (111.3 km) and $0$ km at the poles. |
| 5. | There are 179 parallels. | There are 360 meridians. |
| 6. | Used to demarcate temperature zones (Torrid, Temperate, Frigid). | Used to determine local time with reference to Greenwich. |
The earth rotates from west to east, completing a $360^\circ$ rotation in 24 hours. This relationship allows us to calculate time based on longitude:
Time increases (gains) as we move east of the Prime Meridian and decreases (loses) as we move west.
To maintain uniformity of time within a country, the local time of a central meridian is adopted as the Standard Time. The selected meridian is called the Standard Meridian, which is ideally divisible by $15^\circ$ or $7^\circ30'$.
The International Date Line roughly follows the $180^\circ$ meridian. Crossing this line changes the calendar day: