PLANET EARTH



 



When you look out to sea, the horizon (or skyline) seems curved. This is because our planet is shaped like a ball; it measures about 13,000 km across. The land and water that you see are only a small part of the Earth’s surface. Seen from space, Earth is almost perfectly round and appears to have a smooth surface. The highest mountain is 9 km high, and the deepest ocean is about 11 km deep. These distances are very small compared to the size of the Earth.



 



The Earth is one of many planets in our solar system. But it is the only planet which has the right conditions for human life. Other planets are too cold or too hot for us to live there, or their atmospheres are too poisonous.



Our planet Earth is made up of the atmosphere the land and the oceans.



 




THE EARTH’S ORIGIN



 



The Earth began over 4,600 million years ago. We believe that the Earth and other planets were formed from a flat gas cloud around the Sun. This cloud formed into small, cold particles which attracted one another, collided, and formed larger particles. This took place over a few million years. As the larger particles collided, they became hot, and melted. Iron from these formed the central core of the Earth, and other substances surrounded it.



The molten outer layer of the Earth cooled to form a thin shell. Sometimes molten rock escaped from under the surface in volcanic eruptions, as it still does today. Gases escaped from inside the Earth to form an ‘atmosphere’.



 



 



 





 



Structure of the Earth



The outer layer of the Earth is a thin, solid skin, called the ‘crust’. Below it is a region called the ‘mantle’? The outer layer of the mantle is made of molten rock, called ‘magma’. Below the mantle is a region of molten rock under great pressure. The central region of the Earth is a solid core.



Scientists predict that the temperature in the Earth’s core is about  6,000 degree C. They have studied temperature changes at different depths beneath the Earth’s surface and also believe that the melting point of iron – found near the Earth’s central core – is a good indication.



 



 


Continue reading "THE EARTH’S ORIGIN"

MOVING CONTINENTS

The Earth’s crust is relatively thin, and is formed of large, flat pieces, called ‘plates’. Each crustal plate may be thousands of kilometres across. These plates are moved very slowly by movements of the magma underneath. Where two plates push against each other, the crust crumples to form mountains. Where they move apart, magma can escape to form new rocks. Originally the Earth’s land was close together, but over time the crustal plates have moved the land apart.



 





 



 



 



Changing times



200 million years ago the main land masses that we know today were all grouped close together (1). Over many millions of years, crustal plates carrying the continents have moved away from one another (2). 65 million years ago the continents had moved even further apart (3). Today the continents are still moving a few centimeters each year (4). The coastlines are also being slowly eroded by the sea. How will the continents look in 100 million years time?



 



 





 



 



When crustal plates press against each other, they do not slide smoothly. Instead, they press and press until suddenly they break at a weak point. As they break, there is a sudden movement of the crust, which we feel as an earthquake.



Occasionally, some of the mantle can push through weak points in the crust, and form a volcano. The crust is usually weaker along or near the lines where plates meet.



Earthquakes can cause a great deal of destruction.



 



 



 





 



 



The Sierra Nevada Mountains, USA, were formed by two crustal plates pushing against each other.


EARTH’S OCEANS


The oceans cover about two-thirds of the Earth’s surface and have existed for more than 3,000 million years!



The top layer of the ocean has a rich plant and animal life, the ‘plankton’. Millions of these minute organisms produce food for all the other sea animals. Plankton is one of the oldest forms of life on Earth. Deeper down at about a hundred metres, it is too dark for plants to grow, so the ocean bottom is mostly bare.



Winds blowing across the ocean cause waves. When winds blow over a great distance across the sea, they build waves which may be many metres tall.



 



 





 



The tides



Tides are caused by the pull of the Moon and the Sun on the waters of the Earth. When both the Sun and the Moon are in line with the Earth, their pulls add up, and the tides are very large. These are called ‘spring tides’. When the Sun and Moon are out of line with the Earth, their pulls oppose each other, and the tides are not so large. These are called ‘neap tides’.



 



 





 



 



Cannon Beach, Oregon, USA. The action of the waves can cause erosion of the coastline.



 



 



 





 



 



Papua New Guinea islands, Pacific Ocean – the seas make up two thirds of the Earth’s surface.


SPINNING EARTH



 



 



 



The Sun appears to us as if moves around the Earth. In fact, the Earth spins around on its axis like a top, one turn every day. The place where you live, points towards the Sun in the day, and away from it at night.



The Earth spins on its axis at a great speed: the surface of the Earth moves at more than 1,500 km/h. Gravity provides the force which stops us being thrown off the Earth. Gravity extends a long way from the Earth, and pulls anything within its range towards the Earth. The Moon and artificial satellites are held in orbit round the Earth by the pull of its gravity.



 



 





 



 



The seasons



The seasons are caused by the Earth’s orbit around the Sun. The diagram shows how the Earth’s axis is slanted. During summer in the northern world, the midday Sun is over a region north of the equator, so its rays are more concentrated and feel hotter. During winter, the Sun is over a region south of the equator, and the rays reaching the north spread out over a larger area. They are less concentrated, so they feel less hot. Places south of the equator have their summer when places north of the equator have winter.



 



 



 



 



 


Continue reading "SPINNING EARTH"

CLIMATE



Different parts of the globe have very different climates. It gets hotter as you travel towards the equator because the Sun’s rays are more concentrated there. Places near the sea are usually cooler in summer and warmer in winter, than places inland. This is because the sea heats up and cools down less easily than the land, and so keeps the land near the sea warm in winter and cool in summer.



These differences in temperature cause movements of air across the Earth’s surface, called winds. Air movements, in turn, cause clouds and rain to form, as warm, moist air is cooled by rising up over hills.



 





 



 



The Sun’s rays travel about 150 million km to reach the Earth, providing our planet with light and heat. By rotating once every 24 hours, the Earth keeps the distribution of heat and light balanced with regular alternating periods of daylight and darkness.



The Sun’s rays fall at both the equator and the poles. At the equator, the Sun’s rays are spread over a smaller area than rays reaching the Earth at the poles. This means that sunlight reaching Polar Regions is less intense than sunlight reaching the equator. It also means that places near the poles are colder than places near the equator.



 



 





 



 



 



Temperatures drop considerably at the two poles, which are mainly covered in snow and ice.



 



 



 



 



 





 



 



The hottest and driest climates on Earth are found in the deserts.



 



 


THE ATMOSPHERE



 



 



 



 



The air you breathe forms a thin layer – the ‘atmosphere’ – around the Earth. This stretches up for a few hundred kilometres. As you go higher, there is less air. At the height where planes fly, there is too little air to breathe.



Most plants and animals need atmospheric gases in order to live. Animals rely on ‘oxygen’ to breathe and plants rely on ‘carbon dioxide’ to make their food. The air also contains nitrogen, small amounts of other gases, and some water vapour and dust. The ‘stratosphere’ and ‘ionosphere’ contain small amounts of ‘ozone’, which is a form of oxygen. A layer of ozone prevents harmful ultraviolet rays from the Sun reaching the Earth’s surface.



 



 



 



 





 



 



 



The aurora is caused by charged particles from the Sun hitting the atmosphere.



 



 



 



 



 



 





 



Exosphere



The highest, outer region of the Earth’s atmosphere is called the ‘exosphere’. Here, conditions are not very different from outer space as there is only very little air indeed.



Ionosphere



The ‘ionosphere’ is made up of electrically charged particles produced when radiation from the Sun hits the upper atmosphere. Near the poles, this causes a brilliant display of lights, the ‘aurora’.



Man-made pollution is now causing a hole to appear in the ozone layer.



Stratosphere



The ‘stratosphere’ extends up to about 80 km above Earth. As you travel upwards in this layer, the temperature rises slightly; nevertheless temperatures are below freezing point.



Troposphere



The ‘troposphere’ contains the air we breathe. Clouds rain and snow all form in this layer. As you travel up through the troposphere, it becomes colder.


INTRODUCTION – THE SOLAR SYSTEM


 



 



Look at the sky on a clear night and you can see thousands of stars. The universe contains countless millions more stars which are too far away to see. Our own part of this vast universe is called the Solar System. If we could travel from one side of the Solar System to the other it would take many years.



Telescopes enable us to see faraway objects such as galaxies.


WHAT IS THE SOLAR SYSTEM?


The Solar System is made up of the Sun, nine planets, several moons (the number changes as more are discovered) and a band of rocks called the Asteroid Belt. The planets and the Asteroid Belt all travel round the Sun. we say they ‘orbit’ the Sun. They each take a certain time to round the Sun. The time is the planet’s ‘year’. As the planets orbit, they themselves spin. They all spin at different speeds. The time they take to spin round once is called a ‘day’. Many of the planets have moons. While the planets orbit the Sun, the moons orbit the planets.



All the planets are different sizes. The Sun is so big compared to the planets that only a tiny part of it fits on the page!



The planets vary greatly in size and all are a very long way from the Sun. The planets closest to the Sun have the shortest years as they do not have as far to travel. The planets with the shortest days are the ones that spin round fastest.


ORIGINS OF THE SOLAR SYSTEM


Scientists think that the Sun and planets grew out of a cloud of dust and gas about 4,600 million years ago. Part of this cloud collapsed and shrank and got very hot. This was the beginning of the Sun. The planets formed from the left-over gas and dust that circled the Sun.



Our Sun is a star. Stars form in enormous groups called ‘galaxies’. Our Sun is part of the Milky Way galaxy. Stars are so far apart that we use special units called ‘light years’ to measure distances between them. Light travels faster than anything else in the universe. But light takes about 80,000 years to cross from one side of the Milky Way to the other! We say the Milky Way measures 80,000 light years across.



 



 



 





 



 



Birth of the Solar System



These diagrams show how the Solar System probably began. The Sun formed first at the centre of the cloud. Specks of material bumped into each other and gradually built up into lumps. These grew to form the planets.



Close to the Sun, where it was hottest, rocky planets grew. They had iron at their centres. These are the ‘inner planets’. Further from the Sun, where it was cooler, giant gas planets grew. All the planets moved around the Sun.



 



 



 





 



 



 



This spiral galaxy at the top of the photo is a similar shape to our own galaxy.


THE SUN



 



The Sun keeps the planets in their orbits. Like other stars, the Sun emits radiation in the form of heat, part of which is visible as light. Energy from the Sun is essential for life on Earth.



The Sun is huge. A hollow ball the size of the Sun could hold about a million Earths! It is also very hot. The ‘surface’ of the Sun reaches about 6,000 degree Celsius. The centre of the Sun is about 13 million degrees centigrade!



The Sun is made of the gases hydrogen and helium. At its centre, hydrogen is constantly being turned into helium. This is a nuclear reaction which releases huge amounts of energy. This energy travels to the surface of the Sun and then into space as radiation.



 



 



 



 





 



 



 



 



The surface of the Sun is called the ‘photosphere’. Here there are often dark patches called ‘sunspots’. These are areas of gas that are cooler than the rest of the surface. Although we call them spots, they are many times larger than the Earth. Giant jets of gas shoot out from the Sun. They are called ‘flares’. Sometimes arches of gas loop across the surface. These are known as ‘prominences’.



 



 



 



 



 





 



 



 



 



This picture of the Sun shows a giant loop prominence.


THE INNER PLANETS


The inner planets – Mercury, Venus, Earth and Mars – are all made of rock. They are quite small compared with some of the other planets. Mercury is closest to the Sun. It is extremely hot during the day but the temperature falls to -175 degree C at night. Mercury is only a little bigger than our Moon.



Although Venus is further from the Sun than Mercury, it is the hottest planet in the Solar System. Venus spins in the opposite direction to all the other planets. It also spins so slowly that its ‘day’ is longer than its ‘year’!



Earth has water, oxygen and moderate temperatures. Because of this it can support life. Mars is about half the diameter of Earth. Temperatures here are never above zero degree C and the only water is ice at the frozen poles.



 



 





 



Venus



On the surface of Venus, below, there are deep cracks and dead volcanoes. The atmosphere is thick carbon dioxide gas and the planet is surrounded by clouds of sulphuric acid.



 



 



 



 





 



 



Mercury



Like the Moon, the surface of Mercury is pitted with crates and covered in dust and stones. There is no air or water. It is bathed in dangerous radiation from the Sun.



 



 



 



 



 



 





 



 



Earth



About three-quarters of the Earth’s surface is covered by water. The atmosphere contains the gases oxygen, nitrogen and a small amount of carbon dioxide.



 



 



 



 



 





 



 



 



Mars



Mars is made of red rocks. It has craters and dead volcanoes on its surface. The atmosphere is dusty and made of carbon dioxide. There are strong winds which blow up dust storms.


THE GIANT PLANETS


 



 



Jupiter and Saturn are the two largest and fastest spinning planets in the Solar System. They are both made mostly of the gases hydrogen and helium. Jupiter is the largest. It weighs two and a half times as much as all the other planets put together. Its outer layer of gas clouds is about 1,000 km thick. Fierce winds blow these clouds and huge streaks of lightning flash between them. Below the cloud layer the gases get denser and denser until they become liquid.



Saturn is made of less dense gases. It could even float on water! The planet looks beautiful as it is surrounded by rings. The rings aren’t solid. They are probably made of pieces of rock and ice which orbit the planet.



 



 



 





 



 



 



Here we see Jupiter with two of its moons. It has at least 39 moons in total.



 



 



 



 





 



 



 



This picture is made up of photographs of Saturn and several of its moons.


THE OUTER PLANETS


Uranus, Neptune and Pluto are the planets furthest from the Sun. they get very little of the Sun’s radiation so they are all dark and cold places. Uranus and Neptune are large planets made of gas. They look greeny-blue because they contain a gas called methane. Uranus is circled by nine rings, which are smaller than the rings around Saturn. The rings seem to be made of rocks and ice.



Pluto is the greatest mystery of all. It was only discovered in 1930 and is even smaller than our Moon. It is probably made of rock and covered in ice. Pluto is usually the outermost planet. But sometimes its orbit crosses Neptune’s for a period of 20 years, and then Neptune is the outermost planet. This last happened in 1979.



 



 





 



 



 



 



 



This artwork shows the Voyager 2 space probe as it passes Neptune’s North Pole.



 



 



 



 



 


Continue reading "THE OUTER PLANETS"

THE MOONS



A moon is a ball of rock that orbits a planet. Mercury and Venus are the only planets that do not have moons. Jupiter and Saturn each have at least 16 moons. Our Moon is our closest neighbour in space. Moons may be lumps of material that were left over when the planets formed. Like planets, moons can only be seen when they are lit up by the Sun.



We know more about our own Moon than any other member of the Solar System, apart from the Earth. Astronauts have even landed on the Moon. The Moon is a bare, dead place where nothing lives or grows. Its surface has hills and mountains as well as flat plains and deep craters. There is no water or air on the Moon and it is covered with a layer of fine dust. From the Moon, the sky always looks black and the Earth seems to change shape and go through phases.



 



 





 



 



Phases of the Moon



Our Moon takes  days to orbit the Earth. During this time it seems to change shape. This is because only the side of the Moon that faces the Sun is lit up. And as the Moon orbits the Earth we see different amounts of this lit-up side.



When the Moon is between the Sun and the Earth we cannot see it. This is called a ‘New Moon’. A ‘Full moon’ is when the Earth is between the Sun and the Moon. The diagram shows phases of the Moon seen in the Northern Hemisphere.



 



 



 





 



 



 



Seen from the Moon, (foreground), the Earth appears to change shape.