Day and Night


Why are times different in other countries?



Clocks in other countries need to be adjusted so that it gets light and dark at approximately the same time everywhere. Without this adjustment, people might find that dawn was at 10 o’ clock in the evening, for example.



In 1884 time zones were set up around the world, measured from Greenwich in London, England. Every time zone east or west of Greenwich has noon at a different time. Each zone is either one hour ahead or behind of its neighbouring zone – it is one hour earlier to the west of each zone, and one hour later to the east.



Picture credit: google



 





 



What are the northern lights?



           The northern lights are streaks and sheets of pale, flickering, coloured lights that are sometimes seen in the night sky in far northern regions. Their proper name is the aurora borealis. The same effect also occurs in the far south, where it is known as the aurora Australis.



            Auroras take place between 97 and 1,000 km above the Earth’s surface. They are caused by electrically charged particles from the Sun. These particles strike the Earth’s atmosphere and release energy in the form of light. Auroras are most common when there are many sunspots, which increase the amount of energy released by the Sun.



Picture credit: google


Day and Night


 



Why do we have coloured sunsets?



Coloured sunsets happen when light is scattered by dust and water particles in the air, as the Sun sets. At sunset, the light passes through a much greater thickness of air, because it strikes the atmosphere at a shallow angle. The farther the light has to pass through the air, the more likely it is to be scattered by the dust particles, causing the red colouration.



 



 



 





 



 



 



Why does the Sun rise in the east?



The direction of the Earth’s rotation means that the Sun appears to rise in the east and set in the west. If you imagine that you are standing at the North Pole and looking down on the Earth, it would rotate anti clockwise. In other words, the Earth rotates towards the east, so the Sun first becomes visible from that direction.



 



 



 





 



 



 



 



What is the Earth’s axis?



The Earth’s axis is an imaginary line through the geographic centre of the Earth about which it spins. You can think of the axis as being like a stick pushed through the middle of an orange. The axis is tilted at an angle of 23.5 degree from the vertical, in relation to the Sun. We still do not know the reason for this tilt.



 


Day and Night



 



Why do we have day and night?



As the Earth spins on its axis, the Sun always shines on one side giving us daylight. It is night on the shaded side. As the Earth continues to turn, the shaded side moves into the Sun’s light and the sunlit side turns away from the light. It takes 24 hours for the Earth to make one complete turn on its axis, and our clocks are based on this principle.



In the 1940s, people discovered that the Earth speeds up and slows down a little as it spins, although the reason for these changes is not fully understood. We have now developed atomic clocks that can measure time exactly.



 



 



 





 



Why do we have seasons?



We have seasons because the Earth is tilted on its axis. As the Earth moves around the Sun, the hemisphere tilted towards the Sun receives more sunlight and this is summer time. The days are longer and the weather is warm because of the extra sunlight. The hemisphere tilted away from the Sun receives less sunshine, has shorter days and is cooler – it is winter time. The area near the Equator is always exposed to the Sun’s rays, so it is warm all the year round. This means that there is little difference between the seasons.


Our Planet Earth



 



What are metals?



Metals are one of the major groups of elements. Most of them are shiny and hard. Mercury is the only metal that is a liquid at normal temperatures. Most metals can be bent and stretched, and they can be mixed to make alloys.



Metals are usually found with other elements in the form of compounds. Bauxite is an ore, or mixture of substances, that contains aluminium, the commonest metal. It makes up about eight percent of the Earth’s crust. Osmium is the heaviest metal, and is twice as heavy as lead. Lithium is the lightest metal. It is half the weight of the sane volume of water, so it floats in water.



 



 



 





 



 



How much of the Earth is covered by water?



More than two-thirds of the Earth’s surface is covered by seas and oceans. About 72 percent of the Earth’s surface is water. This water is either in the oceans, locked away as ice at the poles, or held as water vapour in the atmosphere. All of the Earth’s water is known as the hydrosphere. 



 



 



 



 



 





 



 



 



Why is the Earth round?



Gravity pulled the Earth and the other planets into a sphere when they were being formed. The Earth is not truly round, but is slightly flattened making it bulge out at the Equator. This effect is due to the speed with which the Earth spins, causing the Equator to try to fly out from the axis of the spinning Earth. It is rather like a heavy object whirling round on the end of a piece of string. The shape of some of the other planets is distorted in the same way.


Our Planet Earth


 



 



What is the Earth made of?



The Earth is made up of elements. These are the simplest possible substances, which are composed of one kind of atom. Elements cannot normally be broken down into other substances, except under the special conditions that you might find inside the hot core of a star. A total of 92 different elements are found in nature. Other elements can be made in laboratories, but they have only a very short life.



Minerals, which make up the solid surface of the Earth, consist mostly of combinations of elements. The minerals gold, diamond and graphite are examples of single elements, while most others contain several elements. 



 



 



 



 





 



What are crystals?



Crystals are solid substances that are naturally formed into regular geometric, angular shapes. All crystals can be classified into seven systems. Sometimes a crystal is formed when water evaporates, and substances dissolved in the water gradually grow into a crystal. Other crystals are formed under great pressure. Diamond crystals are made when carbon dissolves in molten rock deep down below the Earth’s surface. The diamond crystallizes out and is later uncovered on the surface either by volcanic action or by erosion of the Earth’s crust. Other valuable gems are formed in a similar way, and most of them contain compounds of aluminium.



 



 



 





 



 



What is the Equator?



The Equator is an imaginary line drawn around the outside of the Earth. It lies midway between the North Pole and the South Pole, at the Earth’s broadest point.



The Equator was invented by map-makers because it makes a convenient point from which to measure distances, together with the geographic North Pole and South Pole. On a map, the Equator is positioned at 0 degrees of latitude. It divides the Earth into two halves, which we call the north and south hemispheres. 


Our Planet Earth



What is inside the Earth?



The Earth is not solid rock all the way through. It has an inner core of solid rock, which is mostly iron. The temperature here is probably around 4,500°C. Beyond the inner core is a layer of liquid rock, called the outer core, which extends about halfway to the surface. Beyond this is a thick layer of rock called the mantle, which is partly molten and runny. The thinnest layer, or crust, is on the Earth’s surface, floating on the red-hot liquid mantle. The crust is about 6 km thick under the oceans, but 30 to 40 km thick beneath the land. 



 



 





 



 



Why is the Earth like a magnet?



The Earth acts as if it is a huge magnet. When the solid rocky core moves inside the liquid rock above it, it creates a magnetic field with a north pole and a south pole. This field surrounds the Earth and extends right out into space. The Earth’s magnetic field is changing constantly. This means that the magnetic north pole is not always the same as the geographic, or true, North Pole that you find on a map, although it wanders about in the same region. Millions of years ago, the north magnetic pole lay in what is now the Sahara desert. 



 



 



 





 



How old is our Earth?



The Earth is thought to be about 4,600 million years old. The oldest rocks so far discovered are up to 3,800 million years old. The Universe is much older, and probably began about 15,000 million years ago. We can calculate the ages of the Earth and the Universe by examining meteorites, and also by looking at changes in the atomic structure of some of the elements found on the Earth. Radioactive elements decay at a steady rate, and these changes can indicate the age of the Earth and other planets, as well as the age of the stars. 


Space Travel



 



 



 



Has any spacecraft ever left the Solar System?



Pioneer 10 is the first man-made object to leave the Solar System. This probe was launched towards Jupiter in 1972 and by 1983 it had reached deep space. Pioneer 10 carried messages about life on Earth to be read by any extraterrestrial travellers who might meet the probe. 



 



 





 



 



What is a space station?



Space stations allow the crew to work in space for long periods in conditions of zero gravity. While conditions in space capsules and the space shuttle are cramped, space stations are more suitable for longer stays in space. Rockets or the space shuttle bring supplies of air and food to the space station, and often a new crew. Space stations are usually made up of several modules that are sent into orbit, one at a time, and assembled in space. Some space stations, such as the Russian Mir, stayed up for many years, and their crews remained in space for months at a time. 



 


Continue reading "Space Travel "

Space Travel


 



 



Rocker power!



All space flight depends on the use of rockets that burn huge amounts of fuel. They burn high-energy fuel in a chamber which directs the burning gases through a nozzle. As the gases stream out through the nozzle they provide the thrust to push the rocket up into space.



Some rockets are powered by burning kerosene or liquid hydrogen with liquid oxygen, but this requires very careful handling. Other rockets burn solid fuel in a controlled explosion.



 



 





 



One giant leap!



The Apollo project was the United States’ plan to get humans on the Moon during the 1960s. It used the world’s most powerful rocket, Saturn V, to launch the three-man Apollo spacecraft. While orbiting the Moon, the Apollo craft would separate into two parts. The landing module carried two astronauts down onto the Moon’s surface, while the main part remained in orbit with the third astronaut. 



 



 



 



 





 



 



How do satellites stay up?



The speed with which a satellite is launched helps to keep it in orbit. To stay up above the Earth, a satellite must be launched at a speed of about 8 km per second. If the orbit is less than 200 km above the Earth, faint traces of air will gradually slow the satellite so it loses height and eventually crashes to the ground. Satellites that are much higher can stay up indefinitely. Some satellites are placed in a geostationary orbit. This means that their speed exactly matches that of the Earth’s rotation, and they seem to stay above a fixed point on the ground. 


Space Travel


 



 



Who was the first man into space?



Yuri Gagarin was the first man to orbit the Earth. On April 12, 1961 the Soviet Union launched him into space, carried by a converted intercontinental ballistic missile. He only made a single orbit of the Earth and landed safely by parachute. 



 



 





 



 



 



What is the space shuttle?



The space shuttle is the first re-usable spacecraft. It was developed to provide a re-usable, and therefore cheaper, vehicle for launching satellites and for other work in space. The shuttle is a bulky delta-winged aircraft with powerful rocket motors. At launch, two solid-fuel booster rockets are strapped to its sides, and a giant fuel tank is fixed to its belly. The rockets and fuel tank fall away after launch, and the rockets are recovered and re-used. In orbit, the shuttle’s cargo bay opens to release satellites or allow the crew to work in space. The shuttle lands on a runway like a conventional aircraft. 



 



 



 





 



What do space satellites do?



Space satellites have revolutionized communications, making possible everyday developments such as mobile phones and television. Communications satellites receive signals beamed at them from the Earth, and send them on to other places. They transmit television and telephone signals around the world, even to remote areas. They are also used for defence communications, including checking on the movement of military forces. Satellites can survey the Earth’s surface, predict weather changes and track hurricanes. They can also help to examine resources such as crops, forests and even minerals. Navigation satellites enable people on land or at sea to work out their exact map position, to within a few metres. 


Our Moon


What does the surface of the Moon look like?



The Moon’s surface is covered with a thick layer of gravel-like rock and dust. This material was created by the impact of millions of meteorites. The Moon also has mountain ranges, many of which are the remains of impact craters and volcanoes that were active when the Moon was still hot. There are some huge valleys called rilles, which can be hundreds of kilometres in length and look like river-beds. However, scientists do not yet fully understand their origins. 



 



 





 



 



What’s on the far side of the Moon?



The far side of the Moon was a mystery until 1959, when a Soviet space probe took the first photographs of it.



The actual appearance of the far side was something of a disappointment, because it had far fewer craters and other physical features than the familiar side that always faces the Earth. 


Our Moon



 



 



 



 



 



What is the Moon made of?



The Moon is a rocky satellite, and is made of similar material to the Earth. It has an outer layer, or mantle, of rock, and a core that is probably made up mostly of iron. Unlike the Earth’s liquid mantle, the interior of the Moon is cool and solid. There is little or no volcanic activity on the Moon. However, while it was cooling early in its life, floods or streams of lava ran out across the Moon’s surface. 



 



 



 





 



Dead planet!



Living things need air and water to stay alive, and neither of these is available on the Moon. The atmosphere of the Moon is a higher vacuum than can be created in most scientific laboratories. The Moon also suffers extreme temperature swings between the lunar day and night. Lunar conditions have been re-created in laboratories on the Earth, but no form of terrestrial life has been able to survive them. There is some water on the Moon’s surface, in the form of frost that can be found in shadowed craters near the Moon’s poles. 



 



 



 


Our Moon


 



 



 



What is a lunar month?



For thousands of years people have used the Moon to measure the passing of time and the seasons. The Moon revolves around its own axis every 29  days, which is also the time it takes to complete one orbit around the Earth. This period of time is known as a lunar month. Our calendar has been devised to follow this astronomical pattern fairly closely. It has to be adjusted slightly to round up the number of days in each month to a more convenient figure. The sidereal month, of 27.32 days, measures the time it takes for the stars to return to an identical position in the sky.



 



 





 



 



 



Wobbling Moon



We always see the same side of the Moon as it rotates around the Earth. However, it wobbles slightly as it orbits the Earth, giving us a glimpse of some of the features around the edges of its hidden side. Because of this wobble, we can actually see about 59 percent of the Moon’s surface from the Earth. 



 



 



 



 



 



 


Continue reading "Our Moon"

Our Moon



 



 



 



 



Why does the Moon shine?



The Moon is by far the brightest object in the night sky, but it has no light of its own. Moonlight is simply the reflected light of the Sun. Parts of the Moon that are not in sunlight are invisible against the deep blackness of space. Although the Moon appears bright and silvery, only a small proportion of the light that falls on it from the Sun is reflected back towards us. This is because the Moon’s surface is grey and rocky, and does not reflect light well. 



 



 



 



 





 



 



Why does the Moon seem to change shape each month?



The appearance of the Moon alters, depending on the position of the Sun in relation to the Moon. The Sun lights up only one side of the Moon. As the Moon orbits the Earth, we see this lit-up area from different angles. When the Sun is almost behind the Earth, it lights up the whole of the Moon; this is called a Full Moon. When the Sun is off to one side, part of the Moon is in deep shadow and all we can see is a thin slice, or crescent, of the Moon’s lit surface. 


Our Moon



 



What is the Moon?



The Moon is the Earth’s only satellite, and it has been orbiting our planet for at least 4,000 million years. It is a rocky sphere about 3,476 km in diameter, which is about one-quarter the size of the Earth.



Scientists believe that the Moon formed when another planet about the size of Mars collided with the Earth. The collision splashed a huge mass of molten (liquid) rock into space. This molten rock quickly formed into a sphere, and the Moon rapidly cooled into its solid form. The Moon’s surface is heavily pitted by collisions with debris such as asteroids. 



 



 





 



What is a lunar eclipse?



The Earth casts a long shadow into space, and when this shadow passes over the Moon it causes an eclipse.



A lunar eclipse can only happen during a full Moon, when the Sun is directly behind the Earth. It happens only occasionally, because the orbit of the Moon is slightly tilted and so the shadow usually misses it. Lunar eclipses can last for over one hour, but they do not completely black out the shadowed part of the Moon. Some sunlight always filters through, making the shadow look a reddish-brown colour. 


Small Worlds



What happens when a meteorite hits the ground?



When a large meteorite hits the ground it can produce a crater. Meteorite craters are rare on the Earth, because the atmosphere slows the meteorite and also usually burns it up. Many ancient meteorite craters have been worn away by water and by weather over thousands of years.



On planets and moons with no atmosphere, huge numbers of meteorites strike with enormous power. Our own Moon is estimated to have 3,000,000 million meteorite craters measuring 1 m or more in diameter. Some of the large geographical features on the Moon and other planets and moons throughout the Solar System are probably the result of strikes by large meteors hitting their surfaces. 



 



 





What are meteorites made of?



Meteorites are made of rock or metal. They enter the Earth’s atmosphere at speeds of at least 11 km per second, which makes them glow brightly. Several thousand meteorites enter the Earth’s atmosphere every year, but very few of them reach the ground. The largest known meteorite is made of iron and weighs 66 tonnes. It probably fell to the Earth in prehistoric times in what is now Namibia, southwest Africa.



In general it is hard to find meteorites. Recently, researchers have been locating them on the ice sheets in the Arctic and Antarctic, where they are easier to find. 



 



 





 



 



Why is Europa especially interesting?



Europa is one of Jupiter’s moons. In 1979 the Voyager spacecraft passed by Europa and found that it had a very smooth surface covered with ice. It has very few craters, which has led astronomers to suspect that there may be liquid water beneath the surface. In theory, there might even be a form of primitive life hidden beneath the ice, where water remains as a liquid.