LONGITUDES – CBSE Class VI Social Science – Geography by Soma Mukhopadhyay for SuccessCDs Education
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About this Video:
Longitude describes the location of a place on the earth…east or west of a north to south running imaginary line called the prime meridian.
The lines of longitude are lines running north – south through the poles.
Longitude is the number of degrees east or west of the prime meridian (zero degree) which passes through Greenwich (England).
Lines of longitude are measured from the centre of the earth.
These lines divide the world up into segments.
The prime meridian (zero degree) and the 180th meridian(at 180 degree longitude) is a great circle that divides the earth into the EASTERN and the WESTERN hemisphere.
The 180 degree longitude line is also referred to as the international date line.
Longitude is defined in terms of meridians which are half circles running from pole to pole.
The meridians are numbered up to 180 degree to the east and west of the line of 0 (Zero) degree longitude or the prime meridian
The value of a meridian is followed by the letter ‘E’ or ‘W’.
180 degree E and 180 degree W meridians are the same.
There are 360 meridians in total.
The word meridian means the highest position of a star or the sun which reaches its highest position at midday.
A line of longitude is called a meridian of longitude because all places located on it have midday at the same time.
Meridians are not parallel..they come together at the poles.
THE PARALLELS AND MERIDIANS FORM A MESH OR A GRID.
THE GRID IS FOUND ON FLAT MAPS AND GLOBES.
LONGITUDE AND TIME
AS THE EARTH TURNS THE SUN APPEARS TO MOVE ACROSS THE SKY.
THIS MOTION GIVES US DAY AND NIGHT.
OUR GLOBAL TIME SYSTEM IS ALSO ORIENTED TO THE SUN.
EVERYDAY THE SUN RISES IN THE EAST AND SETS IN THE WEST. THIS MOVEMENT IS CAUSED BY THE ROTATION OF EARTH FROM WEST TO EAST. THERFORE PLACES TO THE EAST OF A GIVEN LONGITUDE WILL HAVE SUNRISE FIRST.
The earth takes 24 hours to cover 360 meridians.
This means that it covers 360/24= 15 degree meridians in 1 hour … or one meridian in 4 mins (60 mins/15)
BASED ON THIS CALCULATION THE DIFFERENCE IN TIME OF DIFFERENT PLACES IS CALCULATED.
EARLIER PEOPLE THOUGHT THAT THE WHOLE WORLD SHOULD SET THEIR CLOCK AT THE SAME TIME …………………
BUT THIS WAS NOT POSSIBLE AS THE SUN DOES NOT SHINE ON ALL PARTS OF THE WORLD AT THE SAME TIME.
In 1884 scientist solved this issue by dividing the world into 24 time zones. Each zone is approximately 15 degree longitude wide.
Greenwich England was chosen to be the location for universal time because its located along the prime meridian.
The time in each zone is calculated by this universal time, also known as the GREENWICH MEAN TIME.
For e.g.: CHICAGO is GMT minus 6 hrs….so when its 7 pm in GREENWICH (7-6=1) ,it is 1 pm in CHICAGO.
The local time of a place is fixed on the basis of the apparent movement of the sun.
When the sun shines exactly Overhead it is mid day or 12 noon at that place.
Mid day occurs at different times at different meridians…… so the local time varies from one meridian to another.
It is the time in any of the 24 time zones (each an hour apart) into which the earth is divided.
The Indian standard time is the time observed throughout INDIA or SRILANKA.
The IST is calculated on the basis of 82 ½ degree E longitude which is just WEST of the town MIRZAPUR, near ALLAHABAD (U.P).
The longitudinal difference between Mirzapur and GREENWICH is 5 hr 30 min.
As the standard meridian of India is towards the east of the PRIME MERIDIAN, the standard time of India is 5 hrs 30 mins ahead of GMT.