Evolution of the Earth - and of life.  Internal structure, continental drift, day length, seasons, oceans, climate, weather and El Nino.
 

Outline:

Earth as a beautiful blue world, with an abundance of water, water vapour,  ice - and life. Seismic waves reveal its internal structure, and the cause of its magnetic field.  Magma upwelling from below produces mid-ocean ridges, continental drift, and volcanoes.  Life may have begun in deep-sea ocean vents.  Green plants transform Earth's early poisonous atmosphere into breathable air.

Earth's rotation is responsible for the apparent motion of the Sun - and stars - across the sky, and for day and night.  Earth's tilted axis and orbit around the Sun cause the length of the day to vary and gives rise to seasonal changes.

The Sun drives the circulation of air between the equator and poles, and powers the winds. Earth's spin causes winds to slew east and west instead of blowing north and south.  The interaction between the oceans and atmosphere.  The great ocean currents drive Earth's weather systems.  The global consequences of El Nino, a climatic upheaval in the Pacific Ocean.
 

Sub-chapters:

Blue Planet

*  Earth and its uniqueness among the planets.  The importance of water to the existence of life.
 

Inside Earth

*  From its early violent beginnings, Earth differentiates into core, mantle and crust.

*  Seismic waves reveal Earth's internal structure.

*  The solid inner core moving inside the molten outer core cause the magnetic field.  The magnetic axis and wandering of the magnetic poles.
 

Continents and Atmosphere

*  Movement of plates in the Earth's crust creating continental drift.  The locations of mid-ocean ridges, volcanoes - and deep ocean vents, where life is thought to have begun.

*  Role of green plants in converting Earth's early toxic atmosphere into breathable air - containing oxygen.
 

Day, Night and Seasons

*  The effect of Earth spinning on its axis - day and night, and the daily east-to-west motion of the Sun across the sky.

*  The 23.5-degree tilt of Earth's axis of rotation.  How this causes the seasons and varying length of day and night during the year.

*  Graphics showing sunsets at different times of the year; solstices and equinoxes.
 

Winds and Ocean Currents

*  The influence of hot and cold ocean currents on Earth's climate.

*  Three great engines of air circulate between equator and poles - driven by the Sun.

*  Earth's pattern of winds - how our planet's spin slews wind direction to the east or to the west.

*  The Gulf Stream as a bringer of mild climatic conditions to northern Europe.
 

Climatic Tantrums

*  Generation of tropical storms - the power of a hurricane.

*  El Nino - a climatic tantrum in the Pacific Ocean, caused by a change in the pattern of trade winds.  How El Nino can influence weather patterns around the world.
 
 

Background:

Orbit of the Earth

The Earth's orbital path around the Sun is not quite circular, but follows a path called an ellipse.  Earth takes exactly one year to travel once round its elliptical orbit of the Sun.  On average, Earth moves along its orbit at 30 kilometres per second, this being almost 2.6 million kilometres every day.  As it orbits, Earth's distance from the Sun varies slightly, between 147 million and 152 million kilometres.  Earth is closest to the Sun - a position known as perihelion - in early January.  Earth is farthest from the Sun - a position known as aphelion - in early July.

The plane of the Earth's orbit around the Sun is called the ecliptic.  If Earth's axis of rotation were perpendicular to this plane, all points on Earth would experience equal day and night throughout the year.  Earth's rotational axis, however, is tilted by 23.5 degrees to the ecliptic.  This gives rise to the seasons, and variation in the length of day and night.
 

Sundials and Solar Time

During a day on Earth, the Sun rises in the east and sets in the west.  This movement is caused by Earth's spin on its axis.  The same effect occurs at night, when the stars appear to rise in the east and set in the west.  The apparent motion of the Sun across the sky in the course of a day can be studied with a shadow stick or sundial.  Time as indicated by the Sun using a shadow stick or sundial is called apparent solar time.  The Sun, however, does not always move at the same speed across the sky, but sometimes travels slower or faster than "average".  This is because of Earth's elliptical orbit around the Sun, and the tilt of its axis of rotation to the plane of its orbit.

Time, as shown by a clock, is based on the movement of a fictitious "mean Sun", which travels at a constant rate across the sky whatever the time of year.  This is called mean solar time.  The difference between mean solar time (or clock time) and apparent solar time (or sundial time) is called the equation of time.  Usually this difference amounts to several minutes.  In mid-February sundials lag behind the mean Sun by roughly 14 minutes, but in early November sundials are ahead of the mean Sun by approximately 16 minutes.  Sundials and clocks agree only four times a year - on April 16, June 15, September 1 and December 25.
 

Earth's Seasons

The tilt of Earth's axis results in a given point of its surface being heated unequally by the Sun over the course of a year.  During summer months in the northern hemisphere, Earth's north pole is tilted towards the Sun.  Sunlight falls on the northern hemisphere at its highest angle, creating the greatest heating effect, earlier sunrises and later sunsets, and hence longer hours of daylight.  Six months later, during winter in the northern hemisphere, Earth's south pole is tilted towards the Sun.  Sunlight now falls on the northern hemisphere at its lowest angle, reducing the heating effect - the Sun is lower in the sky, and provides fewer hours of daylight than in the summer.  Seasons in the southern hemisphere are always the opposite of those in the northern hemisphere.
 

Solstices and Equinoxes

The positions along Earth's orbit, when its axis is tilted exactly towards the Sun's direction, are called solstices.  Currently, they occur on or around December 21 and June 21.  On December 21, the Sun is directly overhead at local noon for places on the Tropic of Capricorn (latitude 23.5 degrees south), and on June 21 at places on the Tropic of Cancer (latitude 23.5 degrees north).  In between these times, there are two occasions when the Sun is overhead at local noon for places on the Earth's equator.  These are known as the equinoxes, currently occurring on or around March 21 and September 22.  On these two dates, day and night are equal the world over.
 
 

Links for Further Information:

A page with various links to topics on the environment.
http://www.cnie.org/

A site with shots of Earth and climatic changes.
http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents

A list of links for references.
http://www.usgs.gov/education/othered.html

Provides other links to different aspects of planet Earth.
http://www.arts.ouc.bc.ca/geog/links/environment.html
 
 

Questions and Activities for the Curious:

1.  Describe the various ways in which Earth is unique among the planets of our Solar System.

2.  Research the reasons why the sky on Earth is blue, and why the Sun appears red when low in the sky, near sunrise or sunset.

3.  Describe the internal structure of the Earth.

4.  Identify the major tectonic plates on Earth and indicate regions of volcanic activity associated with them.  Name the highest mountain range on Earth and identify the two tectonic plates that caused its formation.

5.   As a result of continental drift, Africa and South America are currently moving apart at a rate of approximately three centimetres per year.  Assuming this rate has been constant, calculate when these two continents must have been in contact.

6.  Use a piece of wood, such as a long pencil or piece of dowel, placed vertically in a container (eg. a plant pot) of soil or sand, to make a shadow stick.  On a sunny day, mark the point where the end of the stick's shadow falls on a piece of white card placed under the container.  Do this every ten minutes from 10am to 2pm.  Work out the time when the length of the shadow was shortest.  This is the time of local noon.

7.  Find out the exact dates of the equinoxes and solstices for the year.  What are the sunrise and sunset times on these days?  Calculate for how many hours the Sun is up.

8.  In winter, the fjords in Norway remain largely free from ice, even though they are farther north than the St. Lawrence river in Canada which freezes over.  Why is this so?