Obe Danumata (ඔබේ දැනුමට)

Clocks and watches are used to measure time. Clocks can stand on their own or are hung on the wall, but watches are strapped on to the wrist. There have been many ways of measuring time for many thousands of years. Time was probably first measured by man several thousand years ago by observing the position of the Sun in the sky . Water clocks and sand glasses or hourglasses used changing levels of water or sand in a container to measure time. Candle clocks used to measure time with the burning of candles. Sundials used the movement of the shadow cast by the Sun. The first mechanical time piece was made in China in the 8th century. The clocks previously used springs or weights, but the modern ones use batteries for power. The digital watches show time by numbers on a small screen . 

Galileo Galilei (1564-1642) lived in Italy and is remembered as the first person to turn a telescope to the sky and make important astronomical observations. He built the first telescope in 1609 and then went on to make many. He observed that Moon was not smooth and perfect as it looks, but it was covered with mountains and craters. He also discovered that a pendulum could be used to measure time. He found this by watching a hanging lamp swinging in Pisa Cathedral. He realized that it took exactly the same time for each swing , whether the swings were large or small . He also observed bright satellites of Jupiter , sunspots and other things. The four largest moons of Jupiter are now known as the Galilean satellites. He made himself unpopular because he lived in a time when people believed that the Earth did not move and the Sun and Moon revolved around it. People believed what was written in the Bible. He was imprisoned and ended his life in a house arrest.

Sextant is an instrument that navigators use to find their position. It was invented by John Hadley of Britain and Thomas Godfrey of the United States in 1731. It made navigation easier, and sea travel much safer. The sextant measures how high the Sun or star , such as the Pole Star, is above the horizon. The navigator looks through lenses and mirrors fixed to the sextant and views the Sun or Star and the horizon. A scale on sextant gives the height of the Sun or star. From this and the time of observation , the navigator can work out the position.

Radar was invented during 1930s. Many countries worked on radar at this time, but the first successful radar was built by Britain in 1935. Radar works by sending out radio signals that bounce off distant objects. The radar set detects the signals that return and produces a picture indicating the position of the objects . British and American scientists worked together to invent methods of using short waves called microwaves.

People in the olden days used to spin thread by hand . They pulled fibres out of cotton and wool and twisted them together into thread. Spinning machines were invented in Britain in the 1700s. These machines had rollers turning at different speeds to twist the fibres into yarn. James Hargreaves in 1764 produced the first successful spinning machine. It was the hand-powered jenny which produced thin yarn. Richard Arkwright's water frame of 1769, powered by water wheels, gave a sturdy thread. Samuel Crompton combined these two spinning machines in 1779 , to make the spinning mule, which could spin many kinds of yarn. These machines were the precursors of the Industrial Revolution . 

The first mechanical clocks are believed to have been made in China in 1088 AD. Later in 1200s, they were first made in Europe. Such clocks made in Europe were driven by a weight on the end of a cord wound round a drum. As the drum rotated, it turned the hands of the clock. The earliest known clock was made in Spain in 1276 AD and the oldest mechanical clock that still works is in Salisbury cathedral in Britain. It was made in 1368. These clocks had devices called escapements to keep the hands turning at the correct rate, so that the clocks depicted the correct time.

A transistor is an electronic device used as a switch or to amplify an electric current or voltage. It was discovered by three American scientists in 1948, namely, William Shockley, John Bardeen and Walter Brattain. The transistor replaced the valve in electronic machines. Being much smaller and more reliable, this made portable radio and television sets possible. Transistors are made of pieces of silicon and can be connected together to produce small, but complex circuits that can handle signals, such as those used in radio and television. In 1970s, single transistors began to be replaced by integrated circuits containing a large number of electronic components.

The distance between the Earth and the moon can be measured accurately by timing how long it takes a laser beam to reach the moon and be reflected back again. A large mirror has been placed on the moon. The speed of light is known, time is measured by this method and distance is calculated by the formula : Speed = Distance/time

The world's 20 highest mountains are all in the Himalaya-Karakoram range in Asia. All these mountains are over 8,000 metres high, the highest of them being Mount Everest at 8,863 metres. The highest peak of each continent are Mt. Elbrus in Europe at 5,633 m, Mt. Kosciusko in Australia at 2,228 m, Mt. Vinson Massif , Antarctica at 5,140 m , Mt. Kilimanjaro in Africa at 5,895 m, Mt. Aconcagua in South America at 6, Kinley in North America at 6, 194 m.

Mars is the planet nearest to the Earth. It is half the size of the Earth, but has a mass of only a tenth of Earth's. It is colder and its atmosphere is very thin. It has very high mountains and deep valleys. It's highest peak Olympus Mons is 23 km. high and its largest valley is 4000 km long. Mars has a stony and dusty landscape and dust is blown up and carried for thousands of kilometres by storms . Ice deposits and melts at the pole with changing seasons. The atmosphere is largely carbon dioxide and the surface of the planet is covered with craters , volcanoes and gorges. Mars has two small satellites.

We get the light from the Sun. The sunlight looks white, but is actually a mixture of seven colours namely violet, indigo, blue, green, yellow , orange and red. When the light from the Sun pass through the atmosphere, some rays are scattered by the dust particles suspended in the air. Blue rays are scattered most and reach our eyes from all angles. So we see more blue than any other colour and this makes the sky look blue .

A binary star or a double star is a pair of stars that revolve around each other and are held together by gravity . The closer a pair of stars are, the faster they revolve. Some binary stars are almost touching and go around each other in a few hours, while others, which are separated by hundreds of times the diameter of the solar system, may take a million years to orbit each other. About a quarter of all known stars belong to binary systems . Most of these, particularly the close pairs, must have been born together. In most cases, one star is large , cool and reddish while the other star is small, white and hot . 

Artificial satellites sent from the Earth orbit around the earth. The ones near the Earth orbit faster than the ones far away. A satellite 35,900 Km above the Earth's surface, takes 24 hours to orbit once. The satellite is always above the same point on the Earth's surface as the planet rotates. Geostationary orbit is also called as geosynchronous ' orbit. Such satellites are used to receive telephone and television signals and retransmit them over long distances.

The Big Bang theory explains the origin of the Universe . Scientists believe that the Universe was created about 15,000 million years ago in an explosion called the Big Bang . Matter was formed in the tremendous heat that followed and galaxies were created. It is believed that the Universe may expand for ever, or it may eventually stop expanding and then collapse inwards. It is predicted that before this, in about 5000 million years, the Sun will expand and burn the Earth.

A supernova is the result of the violent death of a massive star, larger than the Sun . A star shines as a round ball because of the radiation coming from its centre. When the centre cools down, it collapses producing as much energy in the explosion as a galaxy containing thousands of millions of stars, for a short time. In the first ten seconds of explosion, the star produces 100 times more energy than the Sun has during the whole of its 4.6 billion years lifetime. All that remains of the explosion is an expanding cloud of gas and a neutron star , a few hundred kilometres across.

Sunspots are huge cool patches on the Sun's surface . Beneath the Sun's brilliant surface, the churning gases can produce strong magnetic fields. This magnetism siphons away heat, forming cooler and darker sunspots. Sunspots range in size from a few thousand kilometers across to tens of thousands (much much larger than the earth) kms . Most of them last for less than a month. They are more common about every 1 1 years, a period known as sunspot cycle. During it's peak a dozen sunspots may be visible. Vast explosions known as flares sometimes occur near or inside sunspots. These send out extra bursts of particles reaching beyond the Sun's corona.

If the planets did not revolve, the Sun's gravitational pull would drag them inwards . If they moved very quickly, they would fly off into the space. The closer a planet is, the faster it must move . All the planets move in the same direction, because probably they were formed from the same spinning cloud of material that also made the Sun. All the planets except Venus and Uranus spin in anti- clockwise direction.

When the moon passes completely in front of the Sun during an eclipse, a glowing halo shines out. This is known as Corona or Sun's atmosphere. But it, in no way, resembles our atmosphere. It has a temperature of a million degrees centigrade and is a million times thinner than air. Normally , it hides from the view due to the Sun's brilliance except during an eclipse The Corona contains hydrogen, nitrogen oxygen and other common elements. They are very hot and scattered far apart. Unlike our atmosphere, which is held to the Earth by gravity, corona is pouring off into space at a speed of about 400Km/s. These clouds of atoms pass the planets and the Sun towards the stars. This movement of atomic particles is called solar wind.

A neutron star is the smallest and densest kind of star . It is about 20 km in diameter and have more than 1.4 times the mass of our Sun. This means that a teaspoonful would weigh a billion tons! As long as a star shines, the power of its radiation holds all the parts of atoms apart. But when the star begins to die, the gravity in the star makes the outer layer collapse inwards with such force that the centre is crushed to solid neutrons because the electrons and protons are forced together to make neutrons. A neutron star is one of the strangest objects in the universe. It becomes a million times heavier than lead. A pinhead size piece of a neutron star would weigh as much as a huge multi-storeyed building .

The surface of the Sun is about four times hot as a furnace. The lenses or corner of our eyes act like a burning glass when we look straight at the Sun. The lens could be destroyed for life making us blind. Smoked glasses have been believed to be safe to look at the Sun. But even they are not safe. The Sun may look dimmer through it, but it cannot stop the harmful heat rays passing through it. Watching a solar eclipse with naked eye is very dangerous. To observe the Sun, we can project its image through binoculars or a telescope on to a sheet of white card. This can be viewed together by many people .

The moon spins on its axis and also orbits the Earth. Hence, the moon sometimes passes between the sun and the Earth and sometimes the Earth gets in between the Sun and the Moon. All the while, one side of the Moon is always away from the Earth. The Moon has no light of its own, but it shines because it reflects the light of the Sun. Different amounts of sunlit on the face of the moon are visible to us as it travels around the Sun, along with the Earth. It ranges from a thin crescent to a full moon . At new moon, a slim crescent area of moon on which light falls can be seen . As it moves around the Earth, this crescent slowly increases or waxes. Then the Moon passes through first quarter and the gibbous phases to the fully illuminated one. Then the Moon begins to wane to become waning crescent and then disappears . It happens so because the side of the Moon facing us has no sunlight falling on it. The Moon then appears as a New Moon. This cycle of Moon is known as lunar month, which...

The Sun is a huge source of energy. This energy travels into space in the form of waves of light and heat. The hydrogen is the main fuel that burns in the Sun. The temperature at the centre may be 15 million o C. The surface of the Sun is known as the photosphere. This is the coolest part of the Sun with a temperature of about 60000C. It takes about 8.3 minutes for the light to travel from the Sun to the Earth. The Sun is a star which is about 4.6 billion years old. It spins around its axis once every 27.4 days. It has a diameter of about 1,380,000 million km which is about 109 times the diameter of the earth. The Sun can easily house about million earths in itself.

Sirius is the brightest star in the sky. It is at a distance of 9.5 light years from the Earth. It is also known as the Dog Star because it lies in the constellation of Canis Major , the Greater Dog. Sirius is actually a binary star and it is made up of two stars that orbit around each other. Only the brighter of the two can be seen with the naked eye. Sirius was important in ancient Egypt before the calenders were developed . The appearance of Sirius coincided with the annual flooding of river Nile.

Proxima Centauri is the closest known star in the Solar System and is about 10,000 times dimmer than the Sun. It is about 4.3 light years away and is visible with binoculars. It is the faintest member of a triple star system called Alpha Centauri present in the constellation of Centaurus. The other two stars form a brilliant double star when viewed through a small astronomical telescope . They are each about as bright as the Sun and take 80 years to orbit each other. The Proxima Centauri flares up every now and then and appears brighter than normal for just a few minutes . It takes about a million years to orbit the bright stars.

Neptune is the eighth planet of the our Solar System, Pluto being the outermost one. However, once every 248 years, for 20 years, Pluto's orbit swings it closer than Neptune to the Sun. Neptune was the outermost planet till 1999. Neptune was discovered in 1846 and is four times the diameter of the Earth, but because it is very far, it appears as very small even through powerful telescopes. It is mainly made up of liquid and frozen gases. Voyager, a space probe passed Neptune in August 1989 and has provided much information about it. Neptune has a Great Dark Spot which is a long lived oval cloud, a smaller cloud and rapidly changing feathery white methane clouds. Winds blow at a speed of 2000Km/h. Voyager showed that Neptune has two narrow rings and other fainter ones and eight satellites.

Meteorites are bodies (a lump of stone or metal) from space that are large enough to reach the ground without burning up in the atmosphere. The largest known meteorite weighed about 60 tones and landed in Namibia , Southern Africa. Meteorites were probably formed at the same time as the Solar System. A falling meteorite leaves a brilliant trail in the sky known as a fireball. Some meteorites are mostly metal (iron and nickel ) while others are made of stone.