Earth's surface is divided into imaginary lines for locating positions or points. These imaginary lines are (i) Latitudes ( Parallels ), which are 180 in
number and (ii) Longitudes ( Meridians ), which are 360 in number. The shape of the Earth is Distorted sphere or Oblate spheroid. But on globe, we ignore the
these distortions and the relief of the earth's surface. Rotation of the earth on its own axis results in day and night and Revolution of earth around Sun
results in seasons. Due to inclined axis of the Earth around which it rotates, the day and night durations are not equal. They vary from one place to the
Locating points on Earth's Surface
- Latitudes are parallels and are at equidistant to each other. The spacing between latitudes is equivalent to 110 km. Pole is
represented just by a point. Meridians connect the poles ( converge at the poles ). The Spacing between two adjacent Meridians decreases from
Equator to poles. At Equator, they are at distance of 111 km.
- On a given Latitude, all the Longitudes are equally spaced. All the Meridians and Parallels intersect at 900 at any point on earth. This
arrangement of network of lines representing Latitudes and Longitudes is known as "Graticule".
- Any point or position on the Earth can be found out using the longitudes and latitudes taken as coordinates and given in degrees and minutes.
- International Date Line is taken roughly around the 1800 Meridian line. It is selected because it passes mostly through water and
not through countries. The date line is somewhat deviated at some places so as to cover the political boundaries of different countries.
Longitude and Time Calculation
- Meridians control the time as Earth rotates from West and East.
- 3600 rotation of earth is equivalent to 24 hours. So, 1 degree equals to 4 minutes and 15 degrees is equal to 1 hour. As one
Meridian represents 1 degree, each Meridian is separated by the next Meridian by 4 minutes.
- Indian Standard Time ( IST ) is based on the Meridian passing through 82 1/20 East. It is Passing through Mirzapur and
Kakinada. The Standard Meridian is passing through Greenwich near London and it is taken as the Standard for Time, called Greenwich Mean Time ( GMT ).
In 1884, at International Meridian Conference in Washington, Greenwich Meridian was taken as international standard for zero degree. IST of India is 5 1/2
hours ahead of GMT.
- Canada, US and Russia have different time zones. For each 150 Meridian, they have one time zone.
Equinox - It is the moment where the center of the Sun is right above the Equator. This situation arises twice in a year, on 21st March and on 23rd
Solstice - It is the moment where the Sun appears to be at maximum deviation either northerly or southerly from the Equator. This situation arises twice
in a year, 21st June and 22nd December.