By the end of the subtopic, learners should be able to:
Describe the shape of the earth.
Outline evidence in proof of the sphericity of the earth.
Describe the internal structure of the earth.
Describe concepts of revolution and rotation and explain their effect on seasons, day and night.
The Shape Of The Earth
The solar system comprises the sun and nine planets that are revolving around it.
It is believed that the stars that we see in the sky are suns of some far away groups of stars (galaxies) and that they might be having planets orbiting around them.
This thinking is one that goes on to suggest that there might be planets of other galaxies where there is life somehow similar to what we have here on earth.
The planet closest to the sun is Mercury followed by Venus (see diagram below).
The Solar System
These planets are unlikely to have any form of life because of the extremely high surface temperatures.
That is why they appear reddish in colour.
The other six planets that come after the earth are too cold to have any life.
The furthest planet in the solar system is Pluto while the biggest is Jupiter.
The Shape Of The Earth
The earth is almost spherical in shape.
It is not a perfect sphere because it is slightly flattened at the poles (North and South poles) and also slightly bulges at the equator as shown in the figure below.
Geoid Shape of the Earth
Earth's Geoidape
This near spherical shape is correctly termed geoid.
Evidence That The Earth Is Spherical
Satellite images show that the earth has spherical shape.
Astronauts who have gone to space give evidence that indeed the earth is spherical.
High flying airplanes and drones bring back pictures of a spherical earth.
The appearance of an oncoming ship which is seen mast first then the hull gives proof that the earth is spherical.
The eclipse of the moon shows the curved nature of the earth's horizon. The shadow of the earth that appears on the shining moon shows that the earth is spherical in shape.
The spherical nature of all other planets, moons and the sun suggests that the earth cannot be an exception.
Earth's Internal Structure
The interior of earth is has four layers going to the middle, which are:
Crust
Mantle
Outer layer
Inner layer
The Crust
The earth's crust is the outermost layer of the earth.
It is a thin layer which ranges from 5—70 km in thickness.
It is the solid layer on which we live together with plants and animals.
The crust contributes to a small percentage of the earth's total mass.
It makes up about 1% of the earth's total volume.
The Mantle
The mantle is the thickest layer of the earth.
It makes up about 84% of the total volume of the earth.
It is found just below the crust as is shown in the diagram above.
There is much heat in the mantle with temperatures of between 500℃ - 4000℃.
The mantle is made up of semi-liquid rock called magma.
The Outer Core
The outer core is a thick liquid layer surrounding the inner core.
It is made of iron and is very dense.
Both the inner and outer cores make up about 15% of the total volume of the earth.
The temperatures are very high reaching up to 5500℃.
The Inner Core
The inner core is in the centre of the earth.
It is the hottest part of the earth.
The inner core is thought to rotate at a different speed than the rest of the Earth and this is thought to contribute to the presence of the Earth's magnetic field.
The inner core is shaped like a ball and is solid in nature.
Day, Night And Seasons
The Equator
The equator is an imaginary line on the earth's surface that divides the earth into the Northern Hemisphere and the Southern Hemisphere.
It is known as zero degrees (0°) latitude, that is, halfway between the North Pole and the South Pole.
Countries found along the equator in Africa include DRC, Congo, Gabon and Equatorial Guinea.
The Tropics
They are imaginary lines 23,5° north and south of the equator.
The Tropic of Cancer is found 23,5° north of the equator.
The Tropic of Capricorn lies 23,5° south of the equator.
The tropics are warm all year round because the sun is almost overhead throughout the year.
Owing to the tilt of the earth the sun is overhead the tropic of Capricorn in December and moves towards Cancer where it is overhead in June.
This apparent motion of the sun gives us two distinct seasons in the tropics, the wet season and the dry season.
The term "Tropical" is at times used to mean a place that is warm-to-hot and moist, often with a sense of lush vegetation.
The North Pole is found on what is known as the Arctic Circle while the South Pole is on the Antarctic Circle
Facts About Arctic And Antarctic Circles
Arctic Circle
The Circle is situated in the North Pole.
It is a sea surrounded by land.
The place has snow and ice thus has icebergs.
Although the Arctic Circle is semi-enclosed ocean, it has shrubs, grasses and some other vegetation.
Antarctic Circle
The Antarctic circle or Antarctica is a region, sometimes referred to as a continent situated in the South Pole.
It is land surrounded by sea.
The place is covered with snow and ice.
Antarctica has almost no vegetation (only two flowering grasses have so far been discovered).
The region is colder and windier than the Arctic Circle.
Rotation
The earth rotates or spins on its axis.
Axis refers to an imaginary line that runs between North Pole and South Pole as shown in the diagram below
A complete rotation takes 24 hours which is a day.
The earth rotates anticlockwise, that is, from the west to east but we see as if the sun "rises" in the east and "sets" in the west.
Earth's Axis and Rotation
It is not the sun's movement that causes days and night, but rather the earth's spin on its axis.
The face of the earth that faces the sun at any particular time experiences day time.
The one that is turned away from the sun experiences night.
Effects of Earth's Rotation
The rotation causes us to see the sun rising in the east and setting in the west.
The rotation of the Earth also contributes to its geoid shape.
It is this spinning that causes the bulge around the equator.
The rotation of the earth contributes to the magnetic field around our planet that protects us against the sun's harmful radiation and solar storms.
Revolution
It is also referred to as the earth's orbital motion.
The earth is going around the sun all the time.
It follows a definite path called an orbit.
It takes days for the earth to make one complete revolution around the sun and come back to the same position.
This is what is called a real year.
The Earth's Revolution
Effects of Earth's Revolution
Earth's revolution bring about the seasons.
Here in Zimbabwe, we have four seasons which are as follows;
Summer
Autumn
Winter
Spring
The reason why we have seasons is that earth is tilted 23,5° on its axis.
When a hemisphere is inclined or tilted towards the sun, its days are longer and sun is directly overhead; meaning sun's rays give out more intense heat.
This is what causes summer.
The Earth's Tilt and Seasons
It means that during summer of northern hemisphere, the North Pole points more toward the sun, thus the whole hemisphere receives more direct solar rays (see diagram above).
In winter of the northern hemisphere, the North Pole is tilted away from the Sun and the rays of the Sun are angled rather than direct.
When it is winter in the northern hemisphere (December) it will be summer in the southern hemisphere.
In reverse, when it is summer in the northern hemisphere (June) it will be winter in the southern hemisphere.
Movement of the sun between tropics
Owing to the tilt of the earth the sun is directly overhead the Tropic of Capricorn around 21st December every year (see diagram above).
It is overhead the Equator around 21st March and 21st September every year.
The sun is directly overhead the tropic of Cancer around 21st June every year.
Around March 21st and September 21st there will be equal sunshine in northern and southern hemispheres and this is called Equinox.
days for the earth to make one complete revolution around the sun and come back to the same position.
