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The Earth and the Solar System - Geography Form 1









Objectives:

By the end of the session, you should be able to;

  • define the solar system
  • describe the composition of the solar system
  • Explain the origin , shape, size and position of the earth
  • Explain the effects of rotation and revolution of the earth
  • describe the structure of the earth




The Earth and the Solar System

Definition

The term solar is a Latin word which means sun. When you look at the sky on a clear day; you will see the sun. The sun is a star that generates its own heat and light.During the night when the sky is clear, we see the moon and the stars. The sun, moon and the stars are generally referred to as heavenly bodies

.

The study of heavenly bodies is known as ASTRONOMY.
Let us learn more about the sun and other heavenly bodies.
The term solar system refers to the sun and its planets, asteroids, meteors,comets and gases that revolve around it.



Point to note
As we learn about the planets, there is still debate on whether Sedna which was detected in 2003 should be considered as one of the planets. Sedna is believed to be a planet with one moon and is bigger than Pluto. It is three times farther away than Pluto from the sun In 2006, some astronomers demoted Pluto from being a planet.



The Nebula cloud theory

This theory states that, there was a whirling mass of hot gases and dust which became the sun.With time some gases and dust broke off from the sun and cooled as they whirled around the sun.

These are what the planets. As the gases condensed to liquid, the denser materials settled at the center while the less dense ones settled around them and solidified to form the earth.


The Passing star theory

According to this theory, it is believed that the earth was born when a star passed very near the sun. The star had a high gravitational pull and hence attracted some materials from the sun.

These were in gaseous form and they spread out in different directions over time, they cooled to form planets and other heavenly bodies such as moons and asteroids


The shape of the Earth

The earth is thought to be spherical in shape. However, it is not a perfect sphere. It is wider around the equator and flat at the poles.

This kind of shape is known as a geiod or an oblate spheroid

Proofs that the Earth is spherical

Satellite photographs taken from very high altitudes prove the earth has a spherical shape.

When a ship is approaching shore from a distance its appearance is gradual. The smoke is seen first.

During an eclipse of the moon a shadow of the earth which is spherical in shape appears on the moon.


Sunrise and sunset happens at different times all over the world since the earth is spherical. If the earth was flat, then sunrise and sunset would be at the same time.

If you travelled across the earth on a straight path without changing direction, you would find yourself back to the same place from where you started. This is called circumnavigation.

 

The earths horizon appears circular or curved when observed from a very high point like a tower.


The earth and the solar system

The earth is the third planet from the sun and the fifth largest in the solar system.The size of the earth is best described by its circumference and diameter.


 

Equatorial circumference 40,085km
Meridional circumference 40,009km
Equatorial diameter 12,762km
Polar diameter 12,722km





Rotation of the Earth

Rotation of the earth is the spinning or movement of the earth on its axis.It rotates from West to East which is an imaginary line through the center of the earth from the north to the South Pole.

The rotation of the earth is causes;

Day and night

High tides and low tides

Time difference between longitudes

Deflection of winds and ocean currents.

Day and Night

A complete rotation of the earth takes approximately 24 hours.As the earth spins or rotates on its axis, the regions facing the sun experience day time while the ones facing away experience night time.

A complete rotation of the earth takes approximately 24 hours.


High and low tides

Tides are the regular rise and fall of the water levels of the seas and the oceans. These tides are caused by the gravitational forces of the sun and moon due to rotation of the earth.

Time difference

The rotation of the earth also causes time difference. Since the Earth takes 24 hours to make a full rotation of 360 degrees.This therefore means that longitudes that are 15 degees apart will have one hour difference.

The world is divided into 24 standard Time zones with 15 degrees differences. Longitudes are imaginary lines that run from North Pole to South Pole. They are also called meridians. Longitude 0 is known as the Greenwich Meridian.It passes through a place in London called Greenwich. It also passes through Accra in Ghana. All other longitudes are measured in terms of degrees east or west of Greenwich Meridian.

All places that are east of Greenwich Meridian are ahead in time while those to the west are behind.Places on the same longitude record the same time. This is known as LOCAL TIME. The local time at Greenwich Meridian is called Greenwich Mean Time (GMT)

Deflection of winds and ocean currents.

Winds and ocean currents are deflected to the right in the northern hemisphere and to the left in the Southern hemisphere


Revolution of the earth

As the earth rotates on its axis every 24 hours, it also moves around the sun once every 365days.The movement of the earth around the sun is known as revolution.

The path followed by the earth as it moves is called the orbit. The path is not completely circular. Some time the earth is nearer to the sun than other times. Therefore the orbit is said to be elliptical. This movement takes a full year and it causes; seasons, changes in altitude of the midday sun at different times of the year and changes in the length of the day and night time at different times of the year.

Seasons


The earths revolution causes the four seasons namely;Winter, Spring,Summer and Autumn

On December 21st the northern half of the earth leans away from the sun. The northern hemisphere therefore experiences winter while the southern hemisphere experiences summer since the sun is in the south apparently overhead tropic of Capricon.

Changes in the altitude of the midday sun

The position of the overhead sun changes as the earth revolves aroud the sun. On 21st March and September 23rd the sun is vertically overhead at the equator. On 21st June the sun is vertically overhead in the Tropic of cancer (23 1/2degrees N).On 22nd December it is vertically overhead at the Tropic of Capricon

 

Changes in length of days and nights

During the summer season in the southern hemisphere, There are longer hours of sunlight and few hours of darkness while the Northern hemisphere is experiencing longer hours of darkness and few hours of daylight.
On March 21st and September 23rd, the sun appears to be overhead the equator. Every part of the earth has an equal number of hours of daylight and darkness. This is called the equinox. The sun is at the equator twice in a year.

Structure of the Earth

The earth is divided into two main divisions namely;

External structure of the earth and Internal structure of the earth

Hydrosphere

Atmosphere


The atmosphere is the air that surrounds the earth. It is made-up of gases, dust particles and water vapor.

External structure of the earth

The external structure of earth comprises of two layers these are:

  • Atmosphere
  • Hydrosphere

Hydrosphere


This comprises all water surfaces. This includes the oceans, seas, Lakes Rivers and underground water

Atmosphere


The atmosphere is the air that surrounds the earth. It is made-up of gases, dust particles and water vapor.

Internal structure of the earth

Internal structure of the earth can be divided into three broad layers. These are the crust, the mantle and the core.


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