Geography

Physical and human environments make up the two major geographical phenomena. The term phenomena refer to facts or circumstances observed, or observable within nature. Therefore, a geographical phenomenon, in this case, is an occurrence or fact in the geographical science. Geographical phenomena can be artificial (e.g. buildings), natural (e.g. rivers) or mixed type (e.g. smog).

The Meaning of the Term Geography

Define the term geography

The term "geography" comes from the ancient Greeks, who needed a word to describe the writings and maps that were helping them make sense of the world in which they lived. It is a combination of two Greek words: "geo" and "graphia". In Greek, "geo" means “Earth” and "graphia" means “to write, draw or describe”.

These two words together form geography, which means to draw, write about or describe the Earth. These meanings led to the development of the early definition of geography which referred to description of the Earth by words, maps and statistics and included both the physical Earth and everything found on it such as plants, animals and people. Therefore, geography is the study of the distribution and interrelationship of phenomena in relation to the Earth’s surface. Alternatively, geography can be described as the study of the Earth and its environment.

Geographers explore both the physical properties of Earth’s surface and the human societies spread across it. They also examine how human culture interacts with the natural environment and the way locations and places can have an impact on people. Geography seeks to understand where things are found, why they are there, and how they develop and change over time.

There are two branches of geography, namely physical geography, and human and economic geography:

1. Physical geography - mainly concerned with the study of Earth’s seasons, weather, climate, soil, streams, landforms, and oceans. All of these features form the physical environment which includes all natural features found on the Earth’s surface such as water bodies, mountains, rivers, plains, natural forests, etc.
2. Human and economic geography - involves the study of human activities on the Earth's surface. Human activities include farming, trade, mining, transportation, settlement, tourism, etc.

Inter-relationship between different geographical phenomena

There exist interrelationships between different geographical phenomena. Physical environment interacts with living organisms in a number of ways. For example, land resources provide soil that supports plant growth.

Sun rays generate heat which leads to the evaporation of water; then water vapour forms clouds and eventually rain. The rainfall feeds plants, i.e., it supports plant growth. Plants are food for herbivores and omnivores such as human beings.

Climate determines the types of plant and animal species that can survive in a particular geographical area and influences human population distribution. Climate also determines human activities like farming, tourism, and settlement. On the other hand, human activities can lead to modification of physical environments, for example, soil degradation, land reclamation, and forest conservation

The Importance of Studying Geography

Geography answers questions about the natural and human worlds. The following are some of the reasons for studying geography:

1. To develop knowledge of places and environments throughout the world. This will help in solving issues about the environment and sustainable development.
2. Geography serves as an important link between the natural and social sciences. As you study geography, you encounter different societies and cultures. This helps you realize how nations rely on each other.
3. To help us understand basic physical systems that affect everyday life. For instance, how water cycles and ocean currents work are all explained with geography. These are important systems to monitor and predict in order to help lessen the impact of disasters.
4. To learn the location of places and the physical and cultural characteristics of those places in order to function more effectively in our increasingly interdependent world.
5. To enable us explore the methods and strategies used by other nations for economic developments and how Tanzania can borrow and employ the same for a similar purpose.
6. To be able to make sensible judgement about spatial distribution of human settlements in relation to physical environment.
7. To gain knowledge about the available, finite resources that the Earth has been endowed with and how to manage and use them sustainably.
8. To help us take care of the world around us by understanding others better and knowing the limitations of the Earth. This enable us make our planet a more liveable one.
9. To understand various types of natural environments and how to harness them for equitable use by the present and future generations.
10. To gain positive attitudes and values which enable one become a responsible and successful member of the society.
11. To serve as the basis for further studies in specialized fields such as cartography (the science of map making), land survey, meteorology (the study of weather and weather forecasting), climatology (study of climate), seismography (the scientific measuring and recording of the shock and vibrations of Earthquakes), teaching, aviation and research.

Components of the Solar System

Name the Components of the Solar System

When we look at the sky at night we see thousands of bright bodies. These are stars and planets. Our Earth is one of the planets. Sometimes we can see a group of stars which form patterns called constellations. In some occasions we can see bright, moving objects. These are called meteorites.

The arrangement of the planets in relation to the position of the sun is called the solar system. The name is derived from a Latin word sol which means sun. 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.

You grew up learning that the solar system consists of nine planets, including Pluto as one as those planets. However, the International Astronomical Union (IAU) stripped Pluto of this status in 2006.

When Pluto was discovered in 1930, it was hailed as the ninth planet in the solar system based on an overestimation of its size. But it looked out of place among the larger planets following the discovery of swarms of ‘ice dwarfs’—icy rocks in the Kuiper Belt, at the very edge of the solar system billions of miles from the sun. This prompted some astronomers to suggest that Pluto could be just another Kuiper Belt Object (KBO), disappointing generations of schoolchildren who grew up learning that Pluto was the smallest planet in the solar system.

On the other hand, this was also a question of meaning or definition of the term ‘planet’. Pluto is clearly big enough for gravity to give it a round shape like any planet (unlike KBOs that tend to be misshapen). It revolves around the sun like other planets. It has an atmosphere and seasons, too.

When the IAU demoted Pluto to a “dwarf planet”, it spelt out three conditions that a celestial body must meet to qualify as a planet: 1) it must be round; 2) it must orbit the sun; and 3) it must have “cleared the neighbourhood” of its orbit (a planet’s gravity sweeps and clears the space around it of other objects). Pluto follows the first two rules, but it hasn’t cleared the neighbourhood of its orbit—which put it in the category of “dwarf planets”. (Of the five known dwarf planets, Pluto, Makemake, Haumea and Eris are located beyond Neptune. The fifth, Ceres, is located in the asteroid belt between Mars and Jupiter.).

Importance of the Components of Solar System

Describe the 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.

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 following is an outline of some 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.

The Sun

The sun is a medium star. It is one of the millions of stars that you see at night in the sky. It looks much bigger because it is closer to the Earth than other stars. The sun is much larger than the Earth, in fact much larger than all the other components of the solar system put together. The sun is the main source of energy that the planets receive. It is composed of approximately 75% hydrogen, 23% helium, and 2% for all other elements. The Earth is relatively cold 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.

Dimension of the Sun in Relation to Other Space Bodies

State the dimension of the sun in relation to other space bodies

The diameter of the sun is 1.392 million kilometres. Its mass is approximately 330,000 times greater than that of the Earth. The sun shines brightly because of its very high temperatures. The average surface temperature is 6,000 degrees Centigrade. It is much hotter in the interior where it is about 14 million degrees Centigrade.

Characteristics of the Sun

Describe the characteristics of the sun

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%.

Solar Energy

The Term Solar Energy

Define the term solar energy

The sun is the source of all energy on Earth. This sun’s energy is what we call solar energy. Solar energy can be defined as the radiant (light and heat) energy produced by the sun.

Different Uses of Solar Energy

Suggest different uses of solar energy

Solar energy has been used for thousands of years in many different ways by people all over the world. The following are some of different uses of solar energy in everyday life:

(i) Generation of electricity: It is used to generate electricity by use of solar panels. Solar energy is captured by solar panels and stored in batteries. The electricity generated by this means can be used in powering various machines and devices where other power supplies are absent, such as in remote places.

A solar panel charges a rechargeable battery that in turn charges your cell phone. This means you can charge your phone even when there is no sunlight, at night for example, so long as you've charged your battery during the day.

(ii) Heating, cooking and drying: It is used directly in heating, cooking, and drying clothes, meat, fish, fruits and grains, among other traditional uses. A solar cooker which traps heat from the sun can be used for cooking food.

(iii) Photosynthesis: It is used by growing plants for making their own food through the processes of photosynthesis.

(iv) 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.

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

(vi) Production of vitamin D in the skin: The most well-known source of vitamin D is via synthesis in the skin induced by sun exposure.

(vii) Production of salt from sea water: The process of harvesting salt from sea water involves evaporation of sea water in evaporation ponds or tanks. As water evaporates, it leaves the salt in tanks or ponds, from which it is collected and processed further before use.

How the Use of Solar Energy Promotes Environmental Conservation

Explain how the use of solar energy promotes environmental conservation

Solar energy is one of the clean sources of energy in the world due to the fact that it does not produce pollution to the environment during its use. It is a clean source of energy and environmental friendly. The following outline explains how solar energy promotes environmental conservation to our communities:

1. Solar energy does not release carbon dioxide gases into the atmosphere (our aerial environment).
2. The use of solar energy reduces the demand for charcoal and firewood as a source of energy in our households, an act which reduces the cutting down of trees, which could otherwise lead to environmental degradation.
3. The use of solar energy encourages the preservation, conservation and sustainable utilization of forest resources.
4. Reduced use of firewood means retaining more trees that could be cut down for firewood. Trees, in turn, absorb excess carbon dioxide from the air, thus helping to purify the air and balance the gases in the atmosphere. Similarly, solar energy from sunlight is used by plants during photosynthesis to produce glucose and release oxygen into the atmosphere.
5. Solar energy reduces dependency on kerosene and spirit which burn to produces smoke that pollutes the environment.
6. Unlike oil (used in generators to produce electricity), coal, charcoal, and firewood, solar energy is renewable source of energy. It is obtained daily during sunshine and it is permanent and reliable.

How Solar Energy May Contribute to Emancipation of Women

Explain how solar energy may contribute to emancipation of women

In African societies, the task of fetching firewood, cooking, washing, and taking care of children falls mainly under women and girls. The use of solar energy for cooking, heating and performing other household duties, means allowing more free time for women and girls as they are relieved from the burden of fetching and carrying firewood. As a result, women will have ample time to participate in other income-generating activities such as trade and farming. On the other hand, girls will attend to schools and perform as better as boys do.

Better still, the use of solar energy in place of firewood will improve the women’s health because they will not be exposed to soot, ashes and smoke generated by firewood when burnt. Solar energy is clean energy. It does not pollute the environment or affect women while cooking. Therefore, the use of solar energy in performing different household duties will make life of the women easy.

The Planets

Planets in the Solar System

Locate the planets in the solar system

Revolving around the sun are bodies called planets (mentioned in the previous section). 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.

Relative Distance of Planets from the Earth

Show relative distance of planets from the earth

Each planet has its own orbit and takes a different length of time to complete one revolution around the sun. Being close to the sun, Mercury takes the shortest period of time to complete one revolution. On the contrary, Neptune which is furthest from the sun takes the longest time. The table below shows the relative distances of planets from the Earth.

Planet	Distance from the sun (million kilometres)	Average temperature (degree Centigrade)	Period per orbit (or revolution)
Mercury	58	340 **	88 days
Venus	108	-40	225 days
Earth	150	-	365 ¼ days
Mars	228	-100	1 year and 322 days
Jupiter	777	-138	11 years and 315 days
Saturn	1426	-247	29 years and 167 days
Uranus	2869	-200	84 years and 6 days
Neptune	4495	-265	164 years and 288 days

** +340 degrees Centigrade is the average temperature for sunlit side. The dark side has a temperature of -253 degrees Centigrade.

Other bodies in the Solar System

Apart from planets, there are other heavenly bodies that are found in the solar system. Most of these bodies are much smaller than the planets and some of them revolve around the planets. They include comets, asteroids, meteors, and natural satellites.

Characteristics of Comets, Asteroids, Meteors and Satellites

Describe the characteristics of comets, asteroids, meteors and satellites

Comets

Comets are objects with leading heads and bright tails in the sky. Sometimes they can be seen at night. They are composed mainly of rock, ice, dust, and frozen gases. A combination of all of these components gives the comet a shiny appearance. As the comet passes near the sun, it leaves a tail of dust. Comets orbit around the sun far beyond the limits of Pluto (Fig 1.4). They can be seen from the Earth only when their orbits overlap that of the Earth. This is because other times their orbits are very far away.

Asteroids

Asteroids are solid heavenly bodies revolving around the sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered between the orbits of Mars and Jupiter. The largest asteroid has a diameter of just less than 800 kilometres. These bodies can only be seen with a telescope because they are very far away from the Earth.

Asteroids are found in a portion of the solar system called asteroid belt. As the name suggests, this is a belt where asteroids are most likely to occur. The section is found between Mars and Jupiter. The largest of the existing asteroids is called Ceres, with a diameter of approximately 930 kilometres.

Meteors

Meteors are pieces of hard matter falling from outer space. They can be seen when they come close to the Earth, at about 110-145 kilometres above the Earth’s surface, whereas as a result of friction with the atmosphere, they become hot and usually disintegrate. But when they don’t disintegrate completely in passing through the atmosphere, they teach the Earth's surface as large boulders known as meteorites. These bodies are usually made of nickel, iron and silica. Sometimes meteors reach the Earth’s surface with such a force that they make large holes or craters called meteor craters.

Natural satellites

A natural satellite or moon (in the most common usage) is an astronomical body that orbits a planet or minor planet (or sometimes another small solar system body).

The largest known satellite is the moon. Other planets also have large moons, e.g., Jupiter’s Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn’s moon (Titan) and Neptune’s moon (Triton).

Four dwarf planets are also known to have natural satellites: Pluto (Charon, Hydra, Nix, Kerberos, and Styx); Haumea (Hiʻiaka and Namaka), Makemake [S/2015 (136472) 1]; and Eris (Dysnomia). As of January 2012, over 200 minor-planet moons have been discovered.

Most of the 178 known natural satellites are irregular moons, while only 19 are large enough to be round. Ganymede, followed by Titan, Callisto, Io and Earth's Moon are the largest natural satellites in the solar system.

Local Incidents Linked to Meteorites

Narrate local incidents linked to meteorites

There are two known meteorites in Tanzania. One is found in Mbozi district in Mbeya region and another is in Malampaka in Kwimba district of Mwanza region. These meteorites fell from space and reached the Earth in 1930 and they formed large holes on the Earth’s surface.

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. About three quarters (¾) of the Earth’s surface is covered by water.

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.

The Shape of the Earth and its Evidence

Describe 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 many ways to prove that the Earth is spherical. The following are some of them:

1. Circumnavigation of the Earth: The first voyage around the world by Ferdinand Magellan and his crew, from 1519 to 1522, proved beyond doubt that the Earth is spherical. No traveller going round the world by land or sea has ever encountered an abrupt edge, over which he would fall. Modern air routes and ocean navigation are based on the assumption that the Earth is round. If you travel in an aeroplane in a straight path, flying non-stop, you will eventually come back where you started your journey. This is what is called circumnavigation.
2. The circular horizon: The distant horizon viewed from the deck of a ship at sea, or from a cliff on land is always and everywhere circular in shape. This circular horizon widens with increasing altitude and could only be seen on a spherical body.
3. Ship’s visibility: When a ship appears over the distant horizon, the top of the mast is seen first before the hull. In the same way, when it leaves harbour, its disappearance over the curved surface is equally gradual. If the Earth were flat, the entire ship would be seen or obscured all at once.
4. Sunrise and sunset: The sun rises and sets at different times in different places. As the Earth rotates from west to east, places in the east see the sun earlier than those in the west. If the Earth were flat, the whole world would have sunrise and sunset at the same time. But we know this is not so.
5. The lunar eclipse: The shadow cast by the Earth on the moon during a lunar eclipse is always circular. It takes the outline of an arc of a circle. Only a sphere can cast such a circular shadow.
6. Planetary bodies are spherical: All observations from telescopes reveal that the planetary bodies, the sun, moon, satellites and stars have circular outlines from whichever angle you see them. They are strictly spheres. Earth, by analogy, cannot be the only exception.
7. Driving poles on level ground on the Earth: Engineers when driving poles of equal length at regular intervals on the ground have found they do not give a perfect horizontal level. The centre pole normally projects slightly above the poles at either end because of the curvature of the Earth. Surveyors and field engineers therefore have to make certain corrections for this inevitable curvature, i.e. 12.6 cm to 1 km.
8. Space photographs: Pictures taken from high altitudes by rockets and satellites show clearly the curved edge of the Earth. This is perhaps the most convincing and the most up-to-date proof of the Earth's sphericity.

Earth's Movements

The Earth and all other planets revolve around the sun.

Types of Earth's Movements

Describe the types of earth's movements

The Earth is in motion all the time. People cannot feel this motion because, like all other planets, they move with it. There are two types of movements of the Earth, namely:

1. The rotation of the Earth on its own axis.
2. The revolution of the Earth around the sun.

The Term Rotation

Describe the term 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.

Evidence to Prove that the Earth Rotates

Give evidence to prove that the earth rotates

The following observations illustrate that the Earth rotates from West to East:

1. When travelling in a fast moving vehicle, you notice that trees and other objects on both sides of the road are moving fast in the opposite direction. This observation is similar to the movement of the Earth in relation to the sun.
2. At night most of the stars appear to move across the sky from West to East. This observation shows that the point of observation (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.

Significances of Earth's Rotation

Explain the 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. The 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.

The Term Revolution

Define the term revolution

In Geography and Astronomy, the word rotation is defined as the motion of one body around another. The Earth revolves around the sun and the moon revolves around the Earth.

The Process of Revolution

Explain the process of revolution

The Earth is at aphelion each year on 4th July, when it is at the maximum distance of 152 million kilometres form the sun. The Earth is at perihelion each year on 3rd January when it is at the minimum distance of 147 million kilometres.

The Earth's revolution around the sun takes a year (365¼ days). Therefore, the average speed of revolution is about 29.6 kilometres per second. A normal year has only 365 days. The remaining fraction of ¼ day is added once in four years to make a leap year of 366 days.

The Result of the Earth's Revolution Around the Sun

Describe the result of the earth's revolution around the sun

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).

Four seasons

Because of the inclination of the Earth’s axis to the orbital plane, the angle at which the sun rays shrike the Earth’s surface varies. This leads to seasonal changes which are mainly experienced in the high and mid latitude regions. This results in four distinct seasons namely, spring, summer, autumn and winter.

Seasons are not experienced in the equatorial regions (for example most parts of Africa, Tanzania inclusive) because the sun is almost overhead at all places and the lengths of days and nights are almost equal throughout the year.

Change in altitude of the midday sun

Because of the inclination of the Earths’ axis, the midday sun is directly overhead at the Tropic of Cancer on 21st June; and at the Tropic of Capricorn on 22ndDecember. This is called the solstice. On 21st June, it is winter solstice in the southern hemisphere and summer solstice in the northern hemisphere. On 22nd December, it is winter solstice in the northern hemisphere and summer solstice in the southern hemisphere. On 21st march and 23rd September, the midday sun is directly overhead at the Equator. These are the only two days in the year when all places on Earth have almost equal hours of day and night. This is known as the equinox.

Table 1.2 shows dates on which the sun is vertically overhead at the Tropic of Cancer, Tropic of Capricorn and the Equator. The resulting seasons are also indicated.

Latitude	Date of overhead sun	Hemisphere	Name
Equator	21stMarch	Northern	Spring equinox
Southern	Autumnal equinox
23rdSeptember	Northern	Autumnal equinox
Southern	Spring equinox
Tropic of Cancer	21st June	Northern	Summer solstice
Southern	Winter solstice
Tropic of Capricorn	22ndDecember	Northern	Winter solstice
Southern	Summer solstice

Varying lengths of day and night

Not all places across the Earth experience the same lengths of day and night. Some places receive long hours of daylight than darkness while others receive long hours of darkness than daylight. This is because the Earth’s axis is inclined at 66 ½ degrees to the orbital plane (23 ½ degrees to the perpendicular). The Earth remains permanently inclined at this angle as it revolves around the sun.

If the Earth’s axis was perpendicular to its orbital plane, all places on the Earth’s surface would have equal days of daylight and darkness throughout the year.

In December, it is winter in the southern hemisphere and the hours of darkness increase steadily. The further away a place is from the equator, the longer the nights. Beyond the Arctic Circle (66 ½ºN) towards the North Pole, the number of days of complete darkness increase. The North Pole is in complete darkness for half a year.

Eclipses

Eclipse refers to the partial or complete obscuring of one celestial (heavenly) body by another. An eclipse occurs when one celestial body moves in between another heavenly body and its source of light (the sun). Eclipses normally occur when the sun or moon is obscured from the view for a short period. This means that an eclipse will only occur when the sun, moon and Earth are in straight or nearly straight line.

Now, how are eclipses related to revolution of the Earth? The answer is simple: as the Earth revolves around the sun and the moon revolves around the Earth, there comes a time when the moon and the Earth are in a straight line. As a result, an eclipse of the moon (lunar eclipse) or the sun (solar eclipse) occurs, depending on which body (the Earth or the moon) causes an obstruction:

Lunar eclipse

This eclipse occurs when the Earth passes between the moon and the sun and the Earth’s shadow falls on the moon.

Solar eclipse

Solar eclipse occurs when the Moon passes between the Earth and the Sun, thus casting its shadow onto the Earth. In this type of eclipse the Moon fully or partially blocks the Sun, leading to total or partial eclipse respectively.

The Importance of the Parallels and Meridians

The Parallels and Meridians

Define the 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.

Parallel	Latitude
North Pole	90°N
Arctic Circle	66 ½ °N
Tropic of Cancer	23 ½ °N
Equator	0°
Tropic of Capricorn	23 ½ °S
Antarctic Circle	66 ½ °S
South Pole	90°S

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).

How Latitudes and Longitudes are Determined

Describe how latitudes and longitudes are determined

Determining latitudes

All latitudes are determined in reference to the equator (0°). The angle of latitude is determined by measuring angles from the centre of the Earth and from the equatorial plane towards the North or South Pole. It is stated in degrees north or south of the equator, e.g., 30°S, 15°N, 45°N, etc.

Determining longitudes

The angle of longitude is determined by measuring the angle from the centre of the Earth, along the equatorial plane towards the East or West of Prime Meridian. The value of a latitude is expressed as the angle between the Prime Meridian and the point on any latitude East or West of the Prime Meridian, e.g. 45°E, 30°W, etc.

Importance of a Great Circle

Explain the importance of a great circle

A Great Circle is any circle that circumnavigates the Earth and passes through the centre of the Earth. A great circle always divides the Earth into halves. Thus, the Equator is the only latitude that is a great circle since it divides the Earth into two equal halves; and all lines of longitude are great circles.

Countless great circles can be drawn around the Earth so long as they pass through the centre of the Earth and that they divide the Earth into two equal halves.

Importance of great circles

The shortest distance between any two points on the Earth lies along a great circle passing through the two points. For this reason, great circles are used in plotting routes for ships crossing large stretches of ocean water and aircraft flying great distances in space. Captains and pilots of ships and aircraft, respectively, travel by following great circles in order to save fuel and time because by following the great circle they travel the shortest possible distance to reach their destinations.

Importance of Parallels and Meridians

Discuss the importance of parallels 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.

Calculating Local Time

Calculate local time

The Earth rotates on its own axis once every 24 hours (1 day). Since the Earth turns 360° in 24 hours, it turns 1° in 4 minutes (24×60/360 = 4 minutes). All places along a given meridian will experie

...