Some important facts so that we understand how we locate various points on the earth:

1. Direction of earth rotation – Our earth moves rotates from west to east around its axis
2. Equator – An imaginary line that divides the globe in 2 equal parts vertically – The upper part which is closer to the North Pole is called as “Northern Hemisphere” and the southern part which is closer to the south pole is called as “Southern Hemisphere”. They are both equal halves.

Latitudes

• General
  • All parallel circles from the equator up to the poles are called parallels of latitudes.
  • Latitudes are measured in degrees.

    

  • The equator represents the zero degree latitude
  • Since the distance of the equator from either of the poles is 1/4 of the circumference of earth, i.e. 1/4 of 360 degree. Hence, the latitudes can be measured from 0 degree to 90 degree only.
  • Along with degrees, we also use the letter “N” or “S” to indicate whether its the northern latitude or the southern latitude
• Some important parallels of latitudes:
  • Tropic of Cancer – 23.5 degrees N
  • Tropic of Capricorn – 23.5 degrees S
  • Arctic Circle – 66.5 degrees N
  • Antarctic Circle 66.5 degrees S

Longitudes

• Meridians of Longitude – To measure how far East or West a place is on a latitude, we use another set of imaginary lines, which are called as Meridians of Longitudes.
• Each Meridian of Longitude is separated by 1 degree from the other longitude
• Longitudes extend from North Pole vertically until South Pole.
• Hence, all the meridians meet at the North Pole and the South Pole, which implies the distance between these meridians goes on decreasing as we move towards the poles.
• Each degree is further divided into minutes and seconds.

  
  Image taken from Lewis-clark.org

• Prime Meridian
  • Unlike parallels of latitude, all meridians are of equal length. Thus, it was difficult to number the meridians. Hence, all countries decided that the count should begin from the meridian which passed through Greenwich, where the British Royal Observatory is located. This meridian is called the Prime Meridian. This is 0 degree longitude.
  • Hence, we can count 180 degree east or 180 degree west from the Prime Meridian
  • Thus, the Prime Meridian divides the earth in 2 equal halves – Eastern Hemisphere and the Western Hemisphere.
  • We use letters “E” and “W” to denote the hemisphere along with the degrees.

Heat Zones of the Earth

1. Torrid Zone

   The area between Tropic of Cancer (in northern hemisphere) and Tropic of Capricorn (in southern hemisphere) is termed as the Torrid Zone. As it receives the maximum heat from the sun due to the fact the mid day sun is exactly overhead in each of the latitudes lying in this zone, at least once a year.

2. Temperate Zone

   The area between the Arctic Circle and the Tropic of Cancer in Northern Hemisphere. And the area between the Antarctic Circle and the Tropic of Capricorn in the Southern Hemisphere is termed as the “Temperate Zone”.
   The sun’s rays are never straight over any of the latitudes lying in this area. As the angle of Sun’s rays goes on decreasing with increasing latitudes. Thus these areas have moderate temperatures.

3. Frigid Zone

   The area covered between North Pole and Arctic Circle in Northern Hemisphere and the area bound by South Pole and the Antarctic Circle in the Southern Hemisphere are called as “Frigid Zones”.
   As the sun’s rays are always slanting here and is not able to provide much heat here, they are very cold all the time.

How is time related to Longitude?

• Usually local time can be found out by shadow of an object casted by the Sun, its longest at sunrise and sunset, while its shortest at noon.
• When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.
• As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it
• The rate of difference can be calculated as follows:
  • The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes.
  • Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 p.m.
  • But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m.
  • Similarly, at 180°, it will be midnight when it is 12 noon at Greenwich.

What is the need to have Standard Time?

• The local time of places which are on different meridians of longitudes are bound to differ.
• For example, it will be difficult to prepare a time-table for trains which cross several longitudes. In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.
• 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 82.5 degrees E (i.e. .82 degrees 30 minutes East) is taken as the Standard Meridian of India. Thus, its called 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 twenty-four time zones of one hour each. Each zone thus covers 15° of longitude.

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