# Size and Shape of the World

The shape and size of the Earth was a long debated thing but nobody was absolutely sure about its shape. When Astronaut Uri Gagarine moved round the earth on 12th April 1961 and could understand that the earth is round with a little flat in north-south. Moreover, the picture taken by him is also round, but a little swelling to east-west. It means the real shape of the earth is oblate spheroid.

As the shape of the earth is not fully round, so its diameter in equatorial region from east-west is 12,734.5 kilometres and north-south polar diameter is 12,714 kilometres. For the benefit of measurement it is calculated as 12,800 kilometres. According to this measurement the average radius is 6,400 kilometres. Equatorial diameter is 40,009 kilometres but for the benefit of rounding it is taken as 40,000 kilometres.

Latitude, Longitude and other Important Lines

To locate a certain place on earth or to know its location, it is necessary to understand the 'latitude' and 'longitude'. The time of a certain place can be known from the location of longitude. The location from south or north can be known from latitude, in the same way location from east or west can be known from prime meridian.

Latitude

A latitude is the angular distance of a place north or south of the Equator. There are 180° parallels of latitude. Each parallel of latitude is a circle. All the parallels of latitude are not of equal length. The Equator, at 0°, is the most important latitude and the largest circle. It divides the earth into Northern Hemisphere and the Southern Hemisphere. The distance between any two parallels of latitude is always equal. The North Pole and the South Pole are the fixed points and serve as basic points of reference (fig. 2.6).

Determining Latitude

The imaginary line is actually the radius of the earth sphere. The value of the angle thus formed through those two lines is equivalent to the latitude of that particular place. Therefore, the latitude is the angular distance of a place north or south of the equator. If a line is drawn from any place on the earth to its centre then that line will create an angle with the equatorial plane. The value of that angle is the latitude of that place (fig. 2.7).

The latitude of a place situated to the north of the equator is designated as the north latitude and that of the south as the south latitude. The total value of the angle created by the circle at the centre of the earth is 360°. These angles are being divided into degrees minuets () and seconds ('). Thus the value of the equator is 0°. The latitude of the north pole is 90° North and that of the south pole is 90° South.

Some latitudes are very famous. One of them is 23.5° north latitude, it is called the Tropic of Cancer and 23.5° south latitude is called the Tropic of Capricorn. 66.5° north latitude is called Arctic Circle and similarly 66.5° south latitude is called Antarctic Circle (fig. 2.8). The equator is known as the great circle.

There are different methods of determining the latitude of a place. Out of them we are discussing about two methods.

1. Determination of latitude by sextant : Sextant is an instrument by which the altitude of the sun can be measured. The latitude of any place can be determined after finding out the altitude of the noon sun with the help of a sextant. The following formula can be applied to find out the latitude of a place.

The formula is : 90° minus the altitude of the sun at noon± the sun's declination.

So, the latitude= 90°- (altitude of the declination)= 90°- (50° + 12°) = 90° - 38° = 52° south.

If the place under consideration is in the northern hemisphere then it will require to add the declination value of north and to subtract the declination value of south. Similarly if the place is in the southern hemisphere then the declination value of south is to added and that of the north to be subtracted.

2. Determining latitude with the help of a pole star : In the equator, the location of the pole star is in the horizon and its latitude is 0°. From the equator towards north pole for every 1 ° of approach the altitude also increases by 1 °. Ultimately in the pole, the altitude of the pole star rises to 90°. So, the degree of latitude for the equator is 0° and that of the north pole is 90°. In other words, the degree of latitude increases with any approach towards the north pole.

Longitude

Longitude is the angular distance of a place east or west of the Prime Meridian. There are 360° meridians of longitude which run from North Pole to South Pole. All longitudes are half circles. The Prime Meridian is longitude 0°. All meridians are of equal length but they are not parallel. The distance between any two meridian is not equal. They converge from the Equator to the Poles (fig. 2.9).

Determination of longitude

The longitude of a place can be determined by two methods. These are : (1) by the difference oflocal time, (2) by Greenwich mean time.

(1) By the difference of local time: Whenever, the meridian of any place comes in front of the sun i.e. when the sun appears just overheads then it is considered as noon and the local time is taken as is noon. Ultimately other times of the place are determined on the basis of local time. We know that there is a difference of 4 minutes of time for 1 ° of difference of longitude. In other way, we can say that for every 4 minutes, the longitude differs by 1 °. For example, if in any place, the local time is 12 noon, then the local time for any place located 10° east of the previous place will be 12:00 noon+ (10 X 4) minutes or 12 hours 40 minutes. For the place lying 10° west of the former place, the local time would be 12:00 noon - (10 X 4) minutes or 11 hours 20 minutes.

(2) By Greenwich mean time : In order to maintain international uniformity, one uniform time, corresponding to the Prime Meridian, is adopted by all countries. Each place lying on this line of the world has the same time, called Greenwich Mean Time. Every 15° east or west of Greenwich Meridian, the time changes by one hour. East is ahead and west is behind Greenwich Mean Time (GMT). The Greenwich Meridian is 0° and 90° meridian run exactly in the middle of Bangladesh. So, the time difference will be 90 X 4 = 360 minutes of 6 hours. This is the local time of 90° meridian which is considered the standard time of Bangladesh.When it is 12 noon in Bangladesh, London time is 6 in the morning.

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