- Longitudes: In order to locate a place precisely, lines of longitudes called meridians are required to determine how far east or west that place are from a given line of reference. Or in simple way we can define longitudes of a place as a angular distance of that place whether east or west from the prime meridian. This angular distance is measured in degrees.
- Meridians of longitudes are imaginary lines running from pole to pole.
- They are semicircle and unlike latitudes gaps between them decreases pole wards and ultimately all meridians meets at a point at the poles.
- Since all longitudes are equal in length, it is very difficult to differentiate one longitude from another. Hence it is very difficult to number them.
- Hence it was decided by all countries that meridian which passes through British Royal Observatory, Greenwhich will be the Prime Meridian whose value will be 0°.
- From Prime Meridian one can count upto 180° east ward and 180° west ward. Here it should be noted that 180° east and 180° west are same meridian.
- Prime meridian (Zero degree meridian) and 180° meridian cuts the Earth into two equal halves, eastern part is called Eastern Hemisphere and western part is called Western Hemisphere. Hence meridians in Eastern Hemisphere are followed by letter E and in Western Hemisphere with letter W.
- There are total 360 meridians with 180 meridians in both the Hemisphere.
- Like latitudes meridians are also measured as degree which is further subdivided into minutes and seconds.
- Great Circle: It is an imaginary circle which partitions the Earth into two equal hemispheres.
- It is the circle which is having largest circumference on the globe.
- Among latitudes only Equator is the great circle as it divides the Earth into two equal parts rest all parallels are small circles.
- However in longitudes all are great circle. We can draw an infinite number of meridians on the globe.
- Uses of Great Circle: It is used to determine shortest distance between two points by drawing arc on the great circle on which these two points lies. This concept is quite useful in aviation and navigation sector.
- Time Zone: There is a natural way for determining local time of a place through shadow cast by the Sun. Shadow cast by the Sun at the noon is the shortest while it is longest at Sunrise and Sunset.
- There were 24 standard time zones in the whole world. These time zones covers 15° in longitude and one hour in time.
- The above calculation was done in this way that Earth rotates by 360° in 24 hours means 1° of latitude in 4 minutes and 15° of latitude in 60 minutes or one hour.
- Earth rotates from west to east due to which Sun rises in the East, hence time in the Eastern Hemisphere is always ahead from the Greenwich time however time in Western Hemisphere are always behind from the Greenwich time.
- We can understand the above fact by simple calculation. Imagine, there is a 12 PM at Greenwich so at 15° Eastern Hemisphere time will be calculated as follows:
- Earth takes time to rotate through 1° of longitude = 4 minutes
- Therefore time taken by Earth to rotate by 15° = 15 X 4 i.e., 60 minutes or 1 hour.
- Since Eastern Hemisphere’s time is ahead from the time of Greenwich, at 15° East longitude time would be 13 PM or 01 PM.
- International Date Line: By understanding above fact it will be easy to understand the International Date Line. Let’s study few of the fact about International Date Line first.
- Meridian of 180° is the International Date Line which was decided by the International Meridian Conference held in 1884 in Washington D.C.
- Here it is decided that one day to be added when crossing 180° meridian from west to east and similarly 1 day to be subtracted when crossing 180° meridian from east to west.
- This odd fact can be understood by the above calculation that 180° east meridian and 180° west meridian are 24 hours apart from each other. It should be kept in mind that 180° east and 180° west meridian are the same line.
- If we see International Date Line on the world map we will find that it is deviated at few places this deviation is done only to avoid continuous land parts otherwise there would be a great confusion on dates.
- Standard Time:
- Every Country is having its own standard time and every one sets there watch according to that time.
- This was done as every longitude is having its own time and there would be a great confusion over time when it comes for railways, airlines, post office etc.
- So to avoid such confusion every country follows the local time of a longitude as its standard time.
- Big countries like Russia have 11 standard times.
- In India 82 ½ ° longitude which passes through Naini near Allahabad is considered as the standard meridian and the local time of this meridian is taken as a Indian Standard Time (IST)
- Indian Standard Time is 5 ½ hour ahead of the Greenwich Mean Time.
- Daylight Saving Time: This is a practice of adjusting clocks forward during start of spring and adjusting clocks backward in autumn as during start of summer evening have more light. This is practiced to save electricity and hence reduce pollution in some countries. In Northern Hemisphere it begins between March-April and ends between September-November and in Southern Hemisphere it starts between September-November and ends between March-April.