The Earth as
a Globe
Locating
places on the earth
Have you seen globe in your school? It is a model of
the Earth on a small scale. How do we locate places on the Earth has no edges
pr corners which we can take as reference points or lines for measurement. If
you observe the surface of the globe, you will find a number of lines runs
drawn on it. These lines are meant for locating places on the earth. Note that
one set of line runs east to west and the other set runs north to south. These
two set of lines are at right to one another.
How do you locate points on a graph paper? You need te
X and Y co-ordinate measures the horizontal distance, while the Y co-ordinate
indicates the vertical distance. When the cp=ordicates of a point P are given
as 3-4, it means the point is 3 units of horizontal distances are measured from
the Y-axis and the vertical distances are measured from the X-axis. Thus in
order to locate a point on a graph paper, you need two reference lines from
which distance could be measured.
Therefore, we need two reference lines to locate
places precisely on the Earth. These reference lines are imaginary lines
adopted on a uniform basis by all countries in order to avoid confusion.
Distances from these reference lines are measurement, such as feet, meters,
etc. Different countries adopt different units of linear measurement location.
Each degree is divided into 60 minutes and each minute is divided into 60
seconds.
Latitude
The Earth rotates on a imaginary axis, the two ends,
which appear as points on the Earth's surface. These are called the poles of
the earth and the pole, which points towards the pole star, is called the North
Pole while the opposite pole is the South Pole. The Equator is the imaginary
line, which lies midday between the two poles. The plane of the equator intersects
the Earth's axis at right angles.
The equator is the reference line used for measuring
distances to the north and south. The equator divides the Earth into two
hemispheres. The North of the Hemisphere lies to the north and the southern
Hemisphere lies to the south.
The latitude of a point A is the angular distance of
that point north of south of equator measured in degrees. Point A is located
40o to the North of the equator. The latitude of A is expressed as 40o N south
of the equator. The latitude of B is 60o S.
The equator is taken as 0o latitude. Latitudes are
measured from 0o to 90o N and 0o to 90oS. The distance from the equator to 90oN
covers a distance of ¼ of the circumference of the earth of 10,000 Km. One
degree of latitude is equivalent to length of about 111km.
Lines of
latitude
A line of latitude is an imaginary line which joints
all places having the same value of latitude north or south of the equator is
equal to the circumference of the Earth. Other lines of latitude are of shorter
length. The lengths of the lines of latitude decrease with distance from the
equator. For example, 60o latitude is only half the length of the equator.
All lines of latitude are circles parallel to the
equator. Therefore, lines of latitude are called parallels. The two poles are
however points.
Beside the equator and two poles, the other important
lines of latitude are the Tropic of cancer (231/2oN), the Tropic of Capricorn
(231/2oS), the Arctic Circle (661/2oN) and the Antarctic Circle (661/2oS). The
Tropic of cancer and Capricorn mark the limits of the Tropical zone. In this
zone, the sun's rays are overhead at least on one day in a year. The Artic
Circle marks the limit of the North Polar zone. The between the Tropic of
cancer and the Artic Circle is called the North Temperate Zone. The zone
between the Tropic of Capricorn and the Antarctic Circle is called the South
Temperate Zone.
Longitude
The Greenwich is the reference line measuring
east-west distances. The Greenwich Meridian converges on the two poles and passes
through the Observatory at Greenwich near London. This is also called the prime
Meridian. The longitude of a point is the angular distance of 60o east of
Greenwich Meridian the longitude.
The Greenwich Meridian is numbered 0o 180oe
and 0o to 180o W.
Lines of
longitude
Lines of longitude are lines which join all places
having the same angular distance east or west of the prime Meridian. All lines
of longitudes are semi-circles of equal length. Lines of longitude are also
called Meridians because all places along a lone of longitude experience
mid-day at the same time. The same line of longitude is numbered both as 180o
W. The zone between the prime Meridian and 180oE longitude is called
the Eastern Hemisphere the opposite zone is called the western Hemisphere. 360
lines of longitude intersect the equator. The space between two consecutive
longitude lines decreases gradually with distance from the equator. This is
because the meridians of longitude converge at the two poles. Whereas, the
distance between two consecutive meridians decrease equally, the distance
between two consecutive latitude remains constant. This because lines of
latitude are parallel lines.
Longitude
and time
Owing to the spherical shape of the Earth, the sun
illuminates only one half of the Earth at any given time. The other half is
dark and experiences night. As the Earth rotates from west to east, the zone of
illumination also moves slowly towards the east. This accounts for variation in
the times of sunrise and sunset at different places of the earth. The Earth
takes 24 hours to complete one rotation. This means 360 degrees of longitude
are covered in a period of 24 hours. This gives rise to a time difference o
[24x60/360] of 4 minutes for 1o of longitude. The time difference is to be added
in the case of places to the west, the time difference is to be subtracted. It
is possible to calculate the time at a given place A from the time recorded at
B and the longitude difference between stations A and B. A few worked examples
are below.
Worked
examples
(1) When it is 12 noon on Monday at Greenwich, what is the
local time at Tokyo (140oE)? Time at Greenwich (0o) = 12
noon Difference in longitude between Greenwich and Tokyo is 140o.
Difference in time should be added because Tokyo is to the east of Greenwich.
The local time at Tokyo is 9.20 p.m.
(2) When the local time is 6 a.m. on Monday at Alexandria
(30oE) in Egypt, what will be the local time at New Oreans (90oW)
in the United States? Time at Alexanderia (30oE) in Egypt, what will be the
local time at New Orleans (90oW) in the United States? Time at Alexandria
(30oE) = 6 a.m. on Monday. Difference in longitude = 30 + 90 or 20 degrees.
Difference in time = 120x4 or 480 minutes = 8 hours
(3)
A cricket match
commences at Delhi (77oE) at 10 a.m. on Sunday. What will be the
local time at Sydney (150oE) and New York (74oW) when the radio
commentary is received? Show your working clearly. Time at Delhi ….. 10a.m. on
Sunday. Longitude of Sydney …… 150oE. Difference in longitude
between Delhi (77oE) and Sydney (150oE) = 150 – 77 = 73
degrees. Difference in time = 73x4 minutes = 292 minutes or 4 hours 52 minutes.
Difference in time has to be added as Sydney is in east. Local time at Sydney =
10 + 4h 52m or 14h 52m = 2.52p.m.
ii. Longitude
of New York ….. 74oW. Difference in longitude between Delhi and New
York (77+74) = 151 degrees. Difference in time = 151x4 minutes. = 604 minutes
of 10h 4m As New
York is located to the west of Delhi, the difference in time is to be
subtracted from the time at Delhi. Local time at New York will be 10 a.m. – 10h4m
of 4 minutes before midnight on Sunday.
iii. The longitude of a station can be calculated when
the difference in time between two stations is know and the longitudes of one
station is given. The difference in time can be converted to difference in
longitudes. If the time difference is ahead, the station lies to the east. If
the time difference id behind the station lies to the west of the known
station. The following worked examples will from a station whose longitude is
known.
(1) A radio news report broadcast from Tokyo (140oE)
at p.m. on Friday was received by a ship at 7 p.m. the same day. Find out the
longitude of the ship. Time difference between Tokyo and the ship = 2hours =
120 minutes. Time difference of 4 minutes = 1o of longitude. Time difference of
120 minutes = 120/4 longitude. As the local time of the ship is ahead of that
of Tokyo, the ship must be 30o to the east of Tokyo. The longitude
of the ship = 140o+30 of 170oE.
(2) A ship's chronometer shows the Greenwich Mean Time as
14.30 hours when the local time is 12 noon at the ship. Find out the longitude
of the ship. Greenwich Mean Time is based on 0o longitude.
The difference between the ship and Greenwich Mean
Time = (147.30-12.0) hours
=
2 hours and 30 minutes
=
150 minutes.
Time difference of 4 minutes – 1o of longitude. Time
difference of 150/4 of 37o of longitude. As the local time of the ship is
behind that of Greenwich, the ship lies to the west of Greenwich.
(3) A team of scientist exploring for minerals set up a
camp in Siberia. The radio broadcast at 08.00 hours from Moscow was heard
campsite at 14.30 hours local time. The longitude of Moscow is 37o 30'E. Find
out the longitude of the campsite in Siberia.
Difference in time between Moscow and the campsite.
=
(14.30-08.00) hours
= (6
hours and 30 minutes)
= 390
minutes
Time difference of 390 minutes = 1o longitude. Time
difference of 390 minutes = 390/4 or 97o 30' longitude.
As the local time at the camp site is ahead of the
time at Moscow, the difference in longitude of the camp site in Siberia.
-
37o30'+97o30'
-
135oE
Local time
Local time at a place is fixed with reference to the
path of the sun in sky. It is 12 noon local time when the angle of inclination
of the sun is a maximum. This can be found easily by studying the length of the
shadow of a flagstaff of the shadow is shortest, it is noon. At that time the
sun is crossing the longitude at that point. All points on a given longitude
have the same local time. Local time differs from place to place because of
variation in longitudes. You know that a longitude variation of 1o causes a
time difference of 4 minutes.
Standard
time
As local time varies from place to place, it will
create considerable confusion if each place is to follow its own local time.
The preparation of timetables for railways, buses etc. will be difficult.
Moreover, people travailing from one place to another will have to adjust their
watches according to the local time of each place. In order to avoid such
confusion, many countries follow uniform time throughout the country. Such
uniform time is based on the central meridian of the country or the meridian on
which the most important city is located. Such a central meridian is called the
standard meridian is selected in such a way that it is divisible by at least
71/2o, so that the standard time by multiples of half an four. This uniform
time that followed throughout a country is called the standard time.
In our vast country, the local time differs by nearly
two hours between the easternmost point in Arunachal Pradesh and the
westernmost point in Gujarat. The country as a whole follows the time based on
the central meridian for India. This is called the standard Meridian passes
through Allahabad and quite close to Madras. The time based on the standard
Meridian is called the India standard Time (I.S.T) the India standard Time is 5
hours and 30 minutes ahead of the Greenwich Mean Time.
Greenwich Mean Time (G.M.T.) serves as the standard
Time for Britain and countries of Western Europe and West Africa. The Greenwich
Mean Time is based on 0o longitude. Greenwich Mean Time is adopted uniformly by
all countries for international travel. International airlines maintain their
time schedules in term of Greenwich Mean Time. As aircraft's pass through
several time zones in a short time, it will create confusion it they were to
change their time whenever they cross a time zone. Therefore, they follow the
Greenwich Mean Time uniformly. Ships also keep their chronometers showing
Greenwich Mean Time so that they can find out their longitudinal position with
reference to the time difference between Local time and Greenwich Mean Time.
Time zones
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