Latitude, Longitude and Time

1 26 Practical Work in GeographyChapter 3 Latitude, Longitude andTimeTHE EARTH is nearly a sphere. It is because of the fact that theequatorial radius and the polar radius of the earth is not the rotation of the earth over its axis produces bulging at the , the actual shape resembles that of an oblate spheroid. Theshape of the earth presents some difficulties in positioning its surfacefeatures, as there is no point of reference from which to measure therelative positions of other points. Hence, a network of imaginary linesis drawn on a globe or a map to locate various places. Let us find outwhat are these lines and how are they spinning of the earth on its axis from west to east providestwo natural points of reference, North and South Poles. They formthe basis for the geographical grid. A network of intersecting lines isdrawn for the purpose of fixing the locations of different grid consists of two sets of horizontal and vertical lines, whichare called parallels of latitudes and the meridians of lines are drawn parallel to each other in east-westdirection.

2 The line drawn midway between the North Pole and theSouth Pole is called the equator. It is the largest circle and dividesthe globe into two equal halves. It is also called a great circle. All theother parallels get smaller in size, in proportion to their distancefrom the equator towards the poles and divide the earth into twounequal halves, also referred to as the small circles. These imaginarylines running east-west are commonly known as the parallels vertical lines running north-south, join the two poles. Theyare called the meridians of Longitude . They are spaced farthest apartat the equator and converge at a point at each , Longitude and TimeGlossaryParallels of Latitude : The parallels of latitude refer to the angulardistance, in degrees, minutes and seconds of a point north or southof the Equator. Lines of latitude are often referred to as of Longitude : The meridians of Longitude refer to theangular distance, in degrees, minutes, and seconds, of a point eastor west of the Prime (Greenwich) Meridian.

3 Lines of Longitude areoften referred to as latitudes and longitudes are commonly referred to as geographicalcoordinates as they provide systematic network of lines upon which theposition of various surface features of the earth, can be represented. Withthe help of these coordinates, location, distance and direction of variouspoints can be easily an infinite number of parallels and meridians may be drawnon a globe, only a selected number of them are usually drawn on a and longitudes are measured in degrees ( ) because theyrepresent angular distances. Each degree is further divided into 60minutes ( ) and each minute into 60 seconds ( ).PARALLELS OF LATITUDESThe latitude of a place on the earth s surface is its distance north orsouth of the equator, measured along the meridian of that place as anangle from the centre of the earth. Lines joining places with the samelatitudes are called parallels.

4 The value of equator is 0 and the latitudeof the poles are 90 N and 90 S (Fig. on the next page). If parallels oflatitude are drawn at an interval of one degree, there will be 89 parallelsin the northern and the southern hemispheres each. The total numberof parallels thus drawn, including the equator, will be 179. Dependingupon the location of a feature or a place north or south of the equator,the letter N or S is written along with the value of the the earth were a perfect sphere, the length of 10 of latitude (a onedegree arc of a meridian) would be a constant value, 111 kmeverywhere on the earth. This length is almost the same as that of a2021-2228 Practical Work in GeographyFigure Parallels of LatitudesFigure Drawing of Parallels ofLatitudesdegree of Longitude at the equator. But to be precise,a degree of latitude changes slightly in length fromthe equator to the poles.

5 While at the equator, it km at the poles, it is km. Latitude of aplace may be determined with the help of the altitudeof the sun or the Pole THE PARALLELS OF LATITUDESHow to draw the parallels of latitudes? Draw a circleand divide it into two equal halves by drawing ahorizontal line in the centre. This represents theequator. Place a protractor on this circle in a waythat 0 and 180 line on the protractor coincide withthe equator on the paper. Now to draw 20 S, marktwo points at an angle of 20 from the equator, eastand west in the lower half of the circle, as shown inFig. The arms of the angle cut the circle at twopoints. Join these two points by a line parallel to theequator. It will be OF LONGITUDEU nlike the parallels of latitude which are circles, themeridians of Longitude are semi-circles that convergeat the poles. If opposite meridians are taken together,they complete a circle, but, they are valued separatelyas two meridians intersect the equator at right the parallels of latitude, they are all equal inlength.

6 For convenience of numbering, the meridianof Longitude passing through the Greenwichobservatory (near London) has been adopted as thePrime Meridian by an international agreement and hasbeen given the value of 0 .The Longitude of a place is its angular distanceeast or west of the Prime Meridian. It is also measuredin degrees. The longitudes vary from 0 to 180 2021-2229 Latitude, Longitude and TimeFigure Meridians of LongitudeFigure Meridians of 00 and 1800join at the North PoleFigure Drawing of Meridians ofLongitudeeastward and westward of the Prime Meridian ( ). The part of the earth east of the PrimeMeridian is called the eastern hemisphere and inits west referred to as the western the Meridians of LongitudeHow to draw the lines of Longitude ? Draw a circlewhose centre represents the North Pole. Thecircumference will represent the equator. Drawa vertical line through the centre of the circle, crossing the North Pole.

7 This represents the0 and 180 meridians, which meet at the NorthPole (Fig. ).When you look at a map, the east is towardsyour right and the west is towards your , to draw a Longitude , imagine that youare on the North Pole, at the centre of thecircle as shown in Fig. Observe now thatthe relative directions of east and west wouldreverse in this case and east would be towardsyour left while west would be towards your , draw 45 E and W as shown in Fig. this, place your protractor along the verticalline, coinciding with the 0 and 180 meridiansand then measure 45 on both the sides, whichwill denote 45 E meridian and 45 W meridianon your left and right, respectively. The diagramwill represent the appearance of the earth if welook at it from directly above the North AND TIMEWe all know that the earth rotates from west toeast over its axis. It makes the sun rise in theeast and set in the west.

8 The rotation of the earthover its axis takes 24 hours to complete one circleor 360 of longitudes. As 180 of longitudes fallboth east and west of the Prime Meridian, thesun, thus takes 12 hours time to traverse the2021-2230 Practical Work in GeographyTable A Comparison between the Parallels of Latitudes and the Meridians of LongitudesS. of LatitudeMeridians of is the angularLongitude is the angulardistance of a point north ordistance along thesouth of the equator asequator measured inmeasured in It is measuredeast or west ofGreenwich (0 ), from 0 to 180 . latitudes are parallelAll meridians ofto the converge atthe a globe, parallels ofAll meridians oflatitudes appear as appear ascircles running throughthe distance between twoThe distance betweenlatitudes is approximatelytwo longitudes is111 at theequator ( km) andminimum at the poles (0km). Midway, at 450 oflatitude, it is 79 00 latitude is referred toThere are 360 ofas the equator and the 90 Longitude , 180 each inas the poles.

9 The east and west of thePrime latitudes from theThe longitudes are usedequator to the poles areto determine the localused to demarcatetime with reference totemperature zones, 0 tothe time at Prime23 north and south as zone, 23 to 66 as the temperate zone and66 to 90 as the frigid2021-2231 Latitude, Longitude and Timeeastern and western hemispheres. In other words, the sun traverses 150 oflongitudes per hour or one degree of Longitude in every four minutes oftime. It may further be noted that the time decreases when we move fromwest to east and increases with our westward rate of the time at which the sun traverses over certain degrees oflongitudes is used to determine the local time of an area with respect tothe time at the Prime Meridian (0 Longitude ). Let us try to understandthe question of the determination of time with respect to the Prime Meridianwith the following set of examples :Example 1 : Determine the local time of Thimpu (Bhutan) located at 90 east Longitude when the time at Greenwich (0 ) is : The time increases at a rate of 4 minutes per one degree oflongitude, east of the Prime :Difference between Greenwich and Thimpu =90 of longitudes Total Time difference=90 4 = 360 minutes=360/60 hours= 6 hours\Local time of Thimpu is 6 hours more than that at Greenwich, 2 : Determine the local time of New Orleans (the place, whichwas worst affected by Katrina Hurricane in October 2005), located at 900 West Longitude when the time at Greenwich (00) is : The time decrease, at a rate of 4 minutes per one degree oflongitude, west of the prime.

10 Difference between Greenwich and New Orleans= 90 of longitudes Total Time difference= 90 4 = 360 minutes= 360/60 hours= 6 hours\Local time of New Orleans is 6 hours less than that atGreenwich, a. the same way, the time may be determined for any place in theworld. However, in order to maintain uniformity of time as far as possiblewithin the territorial limits of a country, the time at the central meridian ofthe country is taken as the Standard Meridian and its local time is takenas the standard time for the whole country. The Standard Meridian isselected in a manner that it is divisible by 150 or 7 30 so that the difference2021-2232 Practical Work in Geographybetween its standard time and the Greenwich Mean Time may beexpressed as multiples of an hour or half an Indian Standard Time is calculated from 82 30 E meridian passingthrough Mirzapur.