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 12^{th} 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.

✔ Learn about The Universe and Our Earth

✔ Learn about Size and Shape of the World

✔ Learn about Important Lines Described in Geography

✔ Learn about The Movement of the Earth

✔ Learn about The Change of Season on Earth