- An imaginary line passing through the North and South poles of the earth is called the axis
- Another imaginary line running on the earth divides it into two equal parts. This line is known as the equator.
- The northern half of the earth is known as the Northern Hemisphere and the southern half is known as the Southern Hemisphere.
- All parallel circles from the equator up the poles are called parallels or latitudes. Latitudes are measured in degrees.
- The equator represents the zero degree latitude.
- 90 degrees north latitude marks the North Pole and 90 degrees south latitude marks the South Pole.
- As such, all parallels north of the equator are called ‘north latitudes’ and south are called ‘south latitudes’.
Important Parallels of Latitudes:
Besides the equator (0o), the North Pole (90oN) and the South Pole (90oS), there are four important parallels of latitudes –
- Tropic of Cancer (23oN) in the Northern Hemisphere.
- Tropic of Capricorn (23oS) in the Southern Hemisphere
- Arctic Circle at 66o north of the equator
- Antarctic Circle at 66o south of the equator
Heat Zones of the Earth:
Torrid Zone: the mid-day sun is exactly overhead atleast once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore receives the maximum heat and is called the Torrid Zone
Temperate Zones: The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic circle in the Northern Hemisphere, and the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere, have moderate temperatures. These are therefore, called Temperate Zones.
Frigid Zones: Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold. It is because here the sun does not rise much above the horizon. Therefore, its rays are always slanting and provide less heat. These are, therefore, called Frigid Zones (very cold).
To locate places precisely, one must find out how far east or west these places are from a given line of reference running from the North Pole to the South Pole. These lines of references are called the meridians of longitude, and the distances between them are measured in ‘degrees of longitude’. Each degree is further divided into minutes, and minutes into seconds. They are semi-circles and the distance between them decreases steadily polewards until it becomes zero at the poles, where all the meridians meet.
Unlike parallels of latitude, all meridians are of equal length. The meridian which passes through Greenwich, where the British Royal Observatory is located is called the Prime Meridian. Its value is 0o longitude and from it we count 180o eastward as well as westward. The prime meridian and 180o meridian divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere. The 180o East and 180o West meridians are on the same line.
Why do we have standard time?
- The local time of places which are on different meridians are bound to differ. It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country.
- In India, the longitude of 82o30’ E is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST)
- Some countries have a great longitudinal extent and so they have adopted more than one standard time. For example, in Russia, there are as many as eleven standard times.
- The earth has been divided into 24 time zones of one hour each. Each zone covers 15o of longitude.