Friday, March 30, 2018

GEOGRAPHY: FORM ONE: Topic 2 - THE SOLAR SYSTEM




TOPIC 2
THE SOLAR SYSTEM

Solar System is the arrangement of sun, planets and other solid objects in the space in relation to the position of the sun. The planets are not arranged in a single line from the sun, they are scattered in the space.

Components of the Solar System

The solar system is made up of the sun, planets, moons, natural satellites, asteroids, meteors, comets, dust, ice, and interplanetary space (it contains interplanetary dust and interplanetary gas). All planets and other bodies revolve around the sun. The sun is the central body of the solar system, and it is the only body that generates its own heat. Bodies that revolve around the sun are kept in their orbits (paths) by the sun’s powerful force of gravity.

There are eight known planets in the solar system. The planets, starting from the one closest to the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The four innermost planets in the solar system, Mercury, Venus, Earth and Mars are also called terrestrial planets. They are called terrestrial because they have a compact, rocky surface like the Earth's.

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores.

Solar System


Importance of the Components of Solar System

Though the solar system has many components, the two most important components of the solar system are the Earth and the sun. The Earth sustains lives of a diversity of living organisms (plants and animals). The Earth contains numerous resources that enable living organisms to thrive in it. The Earth’s atmosphere contains water and air (a mixture of several gases that are used by living organisms for survival). In general, the Earth is the only planet known to sustain life.
1.   Mercury = 57600,000km
2.   Venus = 107,200,000 km
3.   Earth = 148,800,000km
4.   Mars = 227.200,000km
5.   Jupiter = 772,800,000 km
6.    Saturn = 1,417,600,000km
7.   Uranus = 2,854,400,000 km
8.   Neptune = 4,468,800,000 km


A. SUN
Sun is the star. It is one among the millions of stars that one sees at night except that it looks much bigger because it is closer to the earth than other distant stars. The sun is much larger than other distant stars from the earth. In fact much larger than all the planets put together. Its diameter is approximately 14 million kilometers and its mass is approximately 330,000 times greater than that of the earth. The elements that form the material of the sun are also in different proportion from those of the earth: The sun is composed of approximately;
·        75% hydrogen
·        23% helium
·        3% of other elements

The earth is relatively cold body but the sun is so hot that nearly all molecules are broken into their separate atoms and all are mixed together into a single hot gas. Its average surface temperature is about 6000°c, it is much hotter in the interior where it is about 14,000,000°c.  

The sun is the source of all energy that supports life on Earth. The sun is responsible for water cycle and it is responsible for weather conditions and climate. The moon lights the Earth on some days of the month, though it gets its light from the sun.


The Importance of the Sun

1. The sun is very important in the plants’ growth. The sun helps the plants to make their own food by the photosynthesis process.

2. The sun is very necessary for the other living organisms, that is, it provides us with light for the vision and to do our work.

3. The sun provides us with the heat to warm our bodies.

4. The sun provides some animals and humans (that feed on plants) with food as it helps the plants to make its own food by the photosynthesis process.

5. The sun is used in heating water and warming houses by the solar heater which changes the solar energy of the sun into the heat energy. The heat energy is used in warming houses and heating water.

6. The sun is very important in the formation of the clouds, rain, and winds. The sun evaporates the water in the seas, lakes and oceans into water vapour. When the water vapour rises in the sky, it cools and condenses forming the clouds, which eventually forms rain.


B. SOLAR ENERGY

The sun is the source of all energy in the earth. This is called solar energy. Solar energy is the energy produced by the sun

Uses of Solar Energy

1.  In drying clothes, grains, fruits and meat.

2. Growing plants: - when plants grow they use energy from the sun to manufacture their food through the process known as photosynthesis

3. Solar energy captured in solar panels and stored in batteries provides electricity used in generating industrial and home appliances like television, refrigerator, Oven, electric iron and cooker.

4. Many everyday items such as calculators and other low power consuming devices can be powered by solar energy effectively

5. Is used as a source of vitamin D for human being

6. It is used for evaporation of water from water bodies which is necessary for rain formation

Coal energy is the solar energy stored in the bodies of plants grew thousands of years ago which after being barred under the earth for a very long time turned into coal similarly gas and oil formed from died bodies of organic matters which lived thousands of years ago.


How the use of solar energy promotes environmental conservation

1. Solar energy is used by plant during manufacturing its food through the photosynthesis. In this process plants take up carbon dioxide from the atmosphere and releasing oxygen. In dome so carbon dioxide is removed from the atmosphere making the ozone layer safe and more oxygen to be used in troposphere by other living organisms.

2. Moreover, Solar energy is clean and safe type of energy suitable for heating and lighting this is due to the fact that it does not release soot.

3. The use of solar energy in household’s helps to promote and conceive environment in the sense that it would reduce the need for firewood and charcoal, where by both once extracted pollute and destroy forest.

4. The use of solar energy also reduces the use of fuel like kerosene as a result it helps to reduce environmental pollution caused by smoke from burning fuel.

5. Formation of coal and oil: Coal is solar energy stored in the bodies of plants that grew thousands of years ago, and which after being buried under the Earth for a very long period, turned into coal. Similarly, oil was formed from dead bodies of organisms.

6. Formation of rainfall: Evaporation of water which is necessary for cloud and finally rainfall formation also uses solar energy.


Solar energy in relation to the emancipation of women (how solar energy contribute to the emancipation of women)

1. Helps to reduce the time would be consumed used by the women searching firewood from forest.

2. The use of solar energy equipments on cooking makes them to have more time to be involved in money making activities such as business or farming.

3. Through the use of solar energy equipments in cooking and other domestic’s tasks, young girls get time to go to school and private study as it was to the boys.


C. PLANETS

The planets are bodies that revolve around the sun.  Planets revolve around the sun in anticlockwise direction following paths called orbits. As they revolve around the sun, they appear to move among the stars. This is the reason why the Greeks called them the planets, which in their language means “wandering stars”.

All the light and the heat of the planets come from the sun. Hence, the temperatures on the planets depend on their relative distances from the sun. All planets revolve around the sun in the same direction in orbits that are elliptical and nearly in the same plane.

They include:
1.   Mercury
2.   Venus
3.   Earth
4.   Mars
5.   Jupiter
6.   Saturn
7.   Uranus
8.   Neptune

As they revolve around the sun they appear to move around the star. That is why the Greeks called them planets means “Wandering stars”. All planets revolve around the sun in the same orbit that are elliptical and nearly the same plane.

The time taken to complete an orbit round the sun depends on the distance of the planet from the sun. All the light and heat of the planets come from the sun. Hence the temperatures on the planet depend on their relative distances from the sun.  However, Pluto is not a planet because an object to be a planet, it needs to meet these requirements (criteria) defined by the International Astronomical Union (IAU) as follows:
i)             It needs to be in orbit around the sun
ii)            It needs to have enough to pull itself in a spherical shape
iii)          It needs to have “Cleared neighborhood of its orbit”

Note: Any object that doesn’t meet these 3 criteria is considered a dwarf planet. And so, Pluto is a dwarf planet is not a planet.

PLANETS POSITION AND CHARACTERISTICS



D. COMETS
Sometimes at night one sees objects with leading heads and right tail at the sky. These are called comets. At present many scientists believe that comets are composed of ice crystals and fragment matters. Comets revolve around the sun far beyond the limits of Pluto. They can be seen from the earth only when they come close to the sun.

Comet


E. ASTEROIDS
Asteroids are solid heavenly bodies revolving around the sun mostly between the orbits of Mars and Jupiter There are thousands of these, the largest having diameter of less than 800km. These bodies cannot be seen without a telescope because they are very far away.

Asteroid

F. METEORS
Meteors are process of hard matter falling from outer space become visible between 110 and 145 km above the earth’s surface, where as a result of friction with the atmosphere become hot and usually disintegrate. When they do completely disintegrate as they pass through the atmosphere reach the earth’s surface and are known as meteorites.

Meteor and Meteorite

Meteorites are usually made of nickel, iron or silica fragments of disintegrated comets. There are two known meteorites in Tanzania one is found in Mbozi District and the other fell at Malampaka in Kwimba District in 1930. Sometimes meteors reach the earth’s surface with such force, hence they make large holes or craters. An example of such craters in the world is the great meteor crater in Arizona desert in United States of America which is 150 meters deep and about 1 kilometer width.

G. SATELLITES
Satellites are moons of the planets. The number of satellites depends on the nature and size of planet up to the moment space researches has proved that only seven planets have satellites.

H. THE EARTH
The Earth is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and supports life. Earth is the fifth largest of the planets in the solar system, smaller than the four gas giants namely Jupiter, Saturn, Uranus and Neptune, but larger than the three other rocky planets, Mercury, Mars and Venus. We live on the planet earth. The Earth is made of the;
·        Atmosphere (air)
·        Hydrosphere (water bodies)
·        The crust solid
·        Molten material
·        Biosphere (living things)

About 3/4 of the earth’s surface is covered by water. In fact no other planet in the solar system is known to have water bodies, the shape of the earth is a flattened sphere This flattening is very slight as indicated by measurements in diameters through poles and at the equator. The diameter through the poles is 12,713 km while at the equator it is 12,757 km. 


The Shape of the Earth and its Evidence

The Earth is not perfectly round. Its shape is an oblate spheroid which is a flattened sphere. The flattening is very slight at the poles than at the equator. There are some several evidences, which are used to prove that the earth is sphere like structure, some of them are shown in the following: 

(i) SUNRISE AND SUNSET
The sunrise and sunset at different places of the earth, people in the east see the sun earlier than the people in the west due to earth’s rotation from west to east. If the earth was flat the whole world would have sunrise and sunset at the same time. 
Spherical earth and Flat earth


(ii) CIRCUMNAVIGATION OF THE EARTH
If traveling from a certain point of the earth and you go straight around the earth you will come to the point of origin. The first traveler around the world named Magellan in 1519 - 1522 proved this; image did not encounter abrupt edge over the world in his voyage. 

Circumnavigation of the Earth 


(iii) AERIAL PHOTOGRAPHS
Photographs taken by satellites or aeroplane from the air show that the earth has a curved or round shape

Aerial Photographs 

(iv) SHIP’S VISIBILITY
If you are in the coast viewing a ship which is very far you will see the soot, then the pipe and eventually the whole ship gradually appears. If the earth were flat the ship would have been seen all at once 

Spherical Earth and Flat Earth

(v) LUNAR ECLIPSE
The shadow of the earth thrown to the moon during lunar eclipse is always round. Only a spherical object can give a circular shadow.  
 
lunar eclipse

THE MOON
The moon is a natural satellite of the earth It has a solid spherical body with a diameter of 3456 kilometers. The distance from the earth to the moon is a 384,403 kilometers. The moon takes 129 ½ days to make a complete revolution around the earth.

The moon appears to rise in the east and set in the west because the earth spins from west to east.  Among all planets only the earth sustains life due to its position from the sun. It is not very close or very far from the sun. Other body such as Pluto which is very far from the sun does not sustain life because it is very cold. Likewise Mercury which is very close to the sun does not sustain life because it is very hot.


EARTH’S MOVEMENT
The earth is in motion all the time. One does not feel this motion because one moves with it, like all other planets, the earth has two motions known as: -
(a) Rotation
(b) Revolution

ROTATION
Rotation refers to the spinning of a body on its axis. The Earth rotates or spins on its axis in an anti-clockwise direction, from West to East through 360 degrees. It makes one complete rotation in 24 hours. Thus, for every 15 degrees of rotation, the Earth takes one hour which is the same as four minutes for every 1 degree.

An axis is an imaginary line joining the N (North) and S (South) poles through the centre of the Earth.

The rotation of the Earth is very rapid although it is difficult to feel its motion. At the equator, every point of the Earth's surface is travelling eastwards at about 1600 km per hour. At latitude 40 degrees, the speed is about 1280 km per hr. The Earth’s axis makes an angle of 66 ½ degrees with the plane of its orbit. In other words, the axis is tilted 23 ½ degrees from the perpendicular.

Rotation of the Earth

It makes one complete rotation after every twenty four hours or one day.

The following observations illustrates the earth’s rotation from west to east.

1. When traveling in a fast moving vehicle we notice trees and other objects on both sides of the road moving in the opposite direction. This observation is similar to the movement of the earth’s rotation to the sun.

2. At night most of the stars appear to move across the sky from east to west. This shows that the earth is moving from west to east.

3. Sunrise and sunset: the sun rises over the eastern horizon in the morning and sets over the western horizon in the evening. But since the sun is in the centre of the solar system and the fact that it does not move, this shows that the point of observation (the Earth) is moving by rotation from West to East.

4. Day and Night: During the Earth's rotations some regions face the sun while others do not face it. The regions facing the sun experience day time whereas the regions which are not facing the sun are in darkness (night). If the Earth was not rotating, one half of the Earth would be having daylight while the other half would be in total darkness forever. The occurrence of day and night proves that the Earth is rotating.

5. Photographs of the Earth taken from the satellite at different times of the day show that different parts of the Earth experience daylight at different times. If the Earth was not rotating, different photographs taken at any time of the day would all look alike.

Day and night

Significances of Earth's Rotation

The rotation of the Earth is very important because it causes the following phenomena:

1. Alternation of day and night: Rotation of the Earth causes the side of the Earth which face the sun to experience daylight which is the day, whereas the side that is not facing the sun at that time will be in darkness (night). This ensures that, at any time of the day, one half of the Earth is in darkness and the other is in daylight. If the Earth did not rotate then its one half would be in daylight while the other half would be in darkness all the year round.

2. Occurrence of tides: Tides are the periodic rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun on the rotating Earth.

3. Deflection of winds and ocean currents: As the Earth rotates on its axis from West to East, winds and ocean currents flowing over the Earth’s surface are deflected. The anticlockwise rotation of the Earth deflects prevailing winds to the right in the northern hemisphere and to the left in the southern hemisphere.

4. Time difference between longitudes: The rotation is responsible for the difference in time between different places on Earth. It causes the difference of one hour in every 15 degree interval between longitudes, which is equivalent to 4 minutes for each degree of longitude.

Deflection of winds and ocean currents

REVOLUTION
Is the movement of one body around another.
Earth’s revolution:-
·        Is the movement of the earth around the sun
·        The earth takes 365 1/4 days for a complete revolution
·       When the earth takes 366 days to accomplish one revolution is called a Leap year
·        The earth revolution revolve around the sun in an elliptical
·       Due to the shape of the earth’s orbit, the earth is very closer to the sun one point of the year than at another


APHELION AND PERIHELION

Aphelion is the furthest position of the earth’s orbit from the sun
- The earth is at Aphelion each year on 4th July when it is in 152 million kilometers from the sun.

Perihelion is the nearest position of the earth from the sun
- The earth is at perihelion each rear on 3rd January when it is 147.3 million kilometers from the sun. Therefore the speed of revolution is about 29.66 km per second. 

                         Earth Revolution


The Effects of Earth’s Revolution

The revolution of the Earth around the sun and the inclination (tilting) of its axis results in the following:

1. The four seasons of the year, namely summer, autumn, winter and spring. A season is one of the distinct periods into which the year may be divided.

2. Change in the altitude of the midday sun at different times of the year at any place.

3. Varying lengths of the day and night at different times of the year. The axis of the Earth is inclined to its elliptical plane at an angle of 66.5 degrees. If the axis of the Earth was vertical, the sun rays would always be overhead at the Equator, thus all places on the Earth would always experience 12 hours of daylight (day) and 12 hours of darkness (night).

4. The eclipses (eclipse of the sun and eclipse of the moon).


SEASONS

Season is one of the four periods of the year separated from each other by different temperature conditions The seasons are summer, autumn, winter and spring They are more pronounced between 23°c and 66°c of latitudes At the equator the year is divided between hot and wet seasons while at the poles is very cold all the year around and the season cannot be identified easily.

The Northern hemisphere summer months are May, June and July. Autumn months are August, September and October; Winter Months are November, December and January while spring months are February, March and April. In the Southern Hemisphere summer months are November, December and January, autumn months are February, March and April Winter has May, June and July while spring months are August, September and October.

CAUSE OF SEASONS
Seasons are caused by inclination of the earth’s axis and the earth’s revolution around the sun The earth’s axis is tilted at an angle of 660 to the earth’s orbital plane and it is always pointing to the same direction in space. In its revolution around the sun one of the hemispheres is inclined towards the sun to one period of the year and away from it at another period of the year.
Seasons of the year

ECLIPSE

This is the movement of one heavily body between the two others, such that it casts shadow over the other. The eclipse involves three heavily bodies namely; the Sun, the Earth and the Moon. So long as the sun is the central both- of the solar system, it never moves, only the earth and the moon are in the motion all the time.

An eclipse is said to be total eclipse when the whole body is obscured i.e completely blocked from the sunlight and it is described as a partial eclipse when the only part of the body becomes obscured. At any place an eclipse will last short time, hardly seven minutes because both the earth and the moon are in motion.

Types of Eclipse
There are two types of eclipse i.e Lunar eclipse and solar eclipse

1. LUNAR ECLIPSE (ECLIPSE OF THE MOON)
This occurs when the earth moves between the sun and the moon, this casting its shadow over the moon. 

Lunar Eclipse


2. THE SOLAR ECLIPSE
The solar eclipse is also known as the eclipse of the sun. This occurs when the moon passes between the earth and the sun casting its shadow over the earth.

Solar Eclipse

- Umbra or total eclipse is when the whole body is obscured i.e completely blocked from the sun’s light.
- Penumbra or partial eclipse is when only part of the body becomes obscured.


THE APPARENT MOVEMENT OF THE OVERHEAD SUN

The apparent movement of the overhead sun is related to the different positions of the earth on its movement as it revolves around the sun. The overhead sun appears to move northwards and south wards in an osculating (swinging) manner.

However the overhead sun’s northward limit is latitude 23°N People beyond this latitude never see the sun vertically above their head. The latitudes 23°N is known as Tropical of cancer. Similarly the overhead sun ends 23°S in its apparent Southward movement. This latitude is known as the Tropic of Capricorn on 2l June the sun is vertically overhead on the Tropical of cancer. This is known as the summer solstice in the Northern hemisphere.

On 22nd December the sun is vertically overhead on the Tropic of Capricorn this is the winter solstice in the Northern Hemisphere.

Solstice means equal night or is when the sun on these days appears to stand still between its northward and southward journeys.                  


THE SOLSTICES
The sun is overhead twice a year at the equators 21st March and 23rd September. 21st March is known as the spring equinox and 23rd September is known as the autumn equinox in the Northern Hemisphere.  Equinox means equal nights, at equinox the length of day and night is equal over all places on the earth’s surface. Viewed from the Southern Hemisphere the solstices and the equinoxes are reversed.

LENGTH OF DAY AND SIGHT
Places along the equator experience equal day and night all the year, but northwards or south wards towards the poles. The length of day and night varies with latitudes. For instance in Northern Summer when the Northern Hemisphere is inclined towards the sun, days are longer than nights. However at latitude 66°N known as the Arctic. Cycle and beyond the sun appears around the sky without setting in the North Pole daylight is experienced for six months before the sunset.

Then this region remains in darkness for the next six months; Latitude 66° south is known as the Antarctic cycle. The Polar Regions south of the Antarctic cycle experience the long six months night from March to September and the six months day light from October to February.


PARALLELS AND MERIDIANS

Parallels are more commonly known as latitudes. Latitude refers to the angular distance North or South of the equator calibrated in degrees, minutes and seconds, measured from the centre of the Earth. The equator is given a value of 0º. It is an imaginary line which divides the Earth into two hemispheres (northern hemisphere and southern hemisphere). The Northern hemisphere has latitude of 90ºN and the Southern hemisphere has latitude of 90ºS. All other latitudes are drawn north or south, parallel to the equator. A particular latitude, say 60ºN joins all points on the surface of the Earth which make an angle of 60º from the centre of the Earth (the equator). Any circle drawn around the Earth, parallel to the equator, is a parallel of latitude. Table 1.3 shows important parallels and figure 1.18 shows the location of these parallels on the Earth’s surface.

Latitude Parallel
Meridians are commonly known as longitudes. A longitude is an imaginary line drawn on the map from the North Pole to the South Pole. Meridians are numbered in degrees East or West of longitude 0°, called Greenwich Meridian (because it passes through a town in England called Greenwich). It is also known as the Prime Meridian because it is the line of reference from which all other meridians are established.

A longitude, therefore, refers to angular distance measured in degrees East and West of the Greenwich Meridian. The Prime Meridian runs through the poles and the Greenwich observatory near London. All lines of longitude are in equal length and divide the Earth into two equal semi circles. There are 360° in a circle, with 180° lying east of the Greenwich Meridian and the other 180° west of Greenwich.

The Greenwich line has been chosen by convention (meaning that any other line could have served the same purpose).

Longitude/Meridian: Is angular distance measured in degrees East or west of the prime meridian, the run from north to south. 

Longitudes and Latitudes 




The importance of parallel and meridians

Longitudes and latitudes are very important to any geographer or map reader. The importance of longitudes and latitudes include the following:

1. They are used by pilots and sailors to guide their paths as they steer the planes and ships.

2. When used together, longitude and latitude define a specific location through geographical coordinates (Fig 1.23).Each location on Earth has its unique latitude and longitude. For example, the location of a point shown on figure 1.23 below is 40°N,60°W.

3. Longitudes enable geographers to calculate the local time of a place, X, given the local time and longitude of place Y, as the point of reference.. 

4. Latitudes are used as a guide to explain the variation in climate on the surface of the Earth. It is generally known that places along the equatorial belt experience a hot and wet climate for most of the year. As you move north or south of the equator, the climate progressively become cold. Places at the north and south poles are extremely cold and are covered by ice and snow throughout the year.


LATITUDE AND LONGITUDE

Latitude:  Is the angular distance north or south of the equator measured in degrees, minutes and seconds. Also can be defined as the lines drawn on a map from east to west.
- The equator is latitude of
- The equator divides the earth into two equal parts (Hemisphere)
 The hemisphere north of the equator is northern hemisphere
 The hemisphere south of the equator is known as Southern hemisphere
- Latitude are also known as a parallel lines of equator because they never meet.
- The lines of latitude are measured from 0° (the equator) to 90° north and south.

Lines of Latitude are;
·        The equator 0°
·        The tropical of cancer 231/2°N
·        The Tropical of Capricorn 231/2°S
·        Arctic cycle 661/2°N
·        Antarctic cycle 661/2°S

PARALLEL OF LATITUDE 30°N 


Longitude
Is an angular distance measured in degrees east or west of the prime meridian, they run from north to south.

Greenwich is a longitude 0°. It is also known as prime meridian. The prime meridian is the line running through the poles (North and South) and is known as Greenwich 0°. Longitudes are measured from 0° to 180° East or West of Greenwich, all meridians are passing through the North and south poles. 



LONGTUDE  AND TIME

The earth rotates on its own axis from west to east once every twenty-four hours (one day). This means that the earth turns through 360° in twenty-four hours. All places along a given meridian will experience midday along the same meridian, it is known as Local Mean, on the Greenwich meridian is known as Local Mean Time (LMT).

When it is 12:00 noon; on the Greenwich meridian it will be 1:00 pm at a place of 15°E or 11:00 at a place of 15°W. To find time for example for Musoma in Tanzania (34°) when it is 12:00 in Kinshasa Zaire
1.   Note the 1onitude position of Kinshasa 15°30°E and Musoma 34°
2.   Find the difference in degree of longitude between Kinshasa and Zaire 34°­ 15°30 = l81/2
3.   Find the difference in time between Kinshasa and Zaire


4.   Since Musoma is to the east of Kinshasa,  Musoma time will be ahead of that of Kinshasa by 1 hour and 14 minutes therefore time for Musoma will be 12:00 + 1:14 = 13:14 pm Or 1:14 pm.

In the other hand given the time difference between two places and the longitude of one of them, one can calculate the longitude position of the second places Kinshasa 15° 30° and 1 hour and 14 minutes. Behind the time of Musoma.

Find the longitude position of Musoma
Difference in time between Musoma and Kinshansa 1 hour and 14 minutes.
Difference in degrees of longitude between Musoma and Kinshansa is lhr and 14 minutes = 74/4 or 18°30°

Since the time of Kinshasa is behind that of Musoma, Musoina must be east of Kinshansa. Therefore the longitudinal position of Musoma will be 15°30´ + 18°30´ = 34°E.

GREAT CIRCLE
The intersection of the surface of a sphere and a plane through its centers for example meridians of longitude and equator e great circle in the earth’s surface. Therefore there is no limit to the number of great circle that can be drawn. 
Great Circle

Great circle also is a circle drawn on a globe (or other sphere) with a center that includes the center of the globe. Thus a great circle divides the globe into two equal halves.

IMPORTANCE OF GREAT CIRCLE
1. The great circles are important for aero planes which use them as route ways to guide their path
2. Great circles are important for ship to follow routes along great circles


TIME
Refers to a period that is used for a event or activity. It is measured in seconds, minutes, hours, days, months or years.

TIME ZONES
Is the region having the same standard time, Standard time is common on time for all countries belonging to the same time zone for example; Tanzania, Kenya, Uganda, Ethiopia, Djibouti and Somalia use the same standard time. This is commonly referred to the East African Standard time.

There would be problems of telling time if every place had its own time set according to local mean time. For example, there would be great confusion in railway and airway timetable or in radio programs if they had to show difference time each one place within a small area. To avoid this problem, different stretches of land take their time from great Meridian. The time adopted is known as STANDARD TIME.

In East Africa, standard time is taken from meridian of 45ºE when a whole stretch of land keeps to the same standard time that stretches from a time zone. Therefore time zone refers to a stretch of land where standard time is accepted through out a longitudinal zone 15° width. Countries with large stretches of land have several standard time zones. There are 24 time zones in the world. The Greenwich Meridian is the starting point for dividing the globe into 24 time zones, the standard time for Greenwich is known as the 

Greenwich Mean Time (GMT)
Essence of Time Zone
In a certain place there could be a place on the surface using its’ own local time. This would brought a lot of confusion example every radio station would have to announce different times for every region within the same country: Local time of Bukoba would be different from that of Dodoma.

- The above confusion was avoid when it was internationally agreed to split the world into 24 time zone according to Longitudes

- The longitudinal division across the earth with an approximates with of 15 of longitude which is regular across the oceans.

- Each time zone has a standard time which is the time of the longitude (meridian near the center of time zone. In the same way, all countries belonging to the same time zone have common time.

Note: Large countries like Canada, USA and Russia have different standard times for different regions within them because they are crossed by many time zone.

Exercise 5

1.  If it is 9:30 am at Kasse 33°15°E what time is in Zanzibar 45°15°E?

2.   Find the time for the Youncle 30°w if it is 12.00 noon London 



3.   When it is 3.30 pm at Nairobi (25°E) what is the time for Comoro 120°E?

INTERNATIONAL DATE LINE.
The line where date is changed or where the calendar day begins. 


One travels eastwards and cross the date line, one will gain a day, if one travels westwards and cross the date line, one will lose a day, if Greenwich it is noon on Tuesday a place 90°W would be 10 am on Tuesday, at a place 180°W it would be midnight Monday. On the other hand a place 90°E would be 6.00pm on Tuesday and at a place 180°E would be midnight on Tuesday. 

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