My Science School

The creation, according to Archbishop James Ussher, took place at 8 PM on October 22 in 4004 BC. The Irish cleric made his calculation in the mid-17th century after study of the ages of the old Testament of patriarchs, long genealogies and other Biblical details.

His view was challenged in 1785 in the Scottish naturalist, James Harden, declared that the formation of mountains and the erosion of river beds must have taken millions, not thousands, of years.

But it was not until the discovery of radioactivity by the French physicist Antony Henri Becquerel in 1896 that an accurate idea of the Earth’s age was made possible.

Scientists now accept that the earth’s crust solidified around 4700 million years ago. This calculation has been made possible by a study of the decay of various radioactive minerals.

When rocks are formed by the cooling and solidification of volcanic lava, radioactive elements are trapped inside. These elements decay at a precise rate, defined as half-life - the time it takes for half the radioactivity to decay.

Careful study has determined the half-lives of individual elements. By measuring the amount of any radioactive element in the sample of rock, the process of decay can be used as if it were a clock which started ticking when the rock was formed.

It is not the precise quantity of the radioactive element left that matters, because that depends on how much there was originally. What is important is the ratio between the quantity of radioactive material and the substance into which it changes. The older the rock, the lower the radioactive material it will contain and the greater will be the proportion of its decay products.

In examining rock samples, several different dating systems can be used. A common one is the decay of the radioactive element potassium 14, a process with the half-life of 11,900 million years. The decay of uranium into lead (half-life 4500 million years) is also used. In the case of the earth, about half its original uranium has decayed into lead. So the age of the earth is about the half-life of uranium.

 

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Five of the planets in the solar system Mercury, Venus, Mars, Jupiter and Saturn are so bright that astronomers have known about them for thousands of years. But in the past couple of centuries, astronomers have found three more distant and fainter planets: Uranus, Neptune and Pluto. These are also indications of a 10th planet beyond Pluto. The discovery of these and new planets requires both mathematical calculations and luck.

Until 1781, no one suspected that there for planets beyond Saturn, so no one was actually looking for them. Then on March 13, the amateur astronomer William Herschel found Uranus, while looking for pairs of stars. He knew it was not a star because it had a visible disc, just like the Moon shows when it is full. As astronomers tracked its motions, they decided it had to be a planet.

After this largely accidental discovery, astronomers began to wonder if there might be another planet, even farther out. This suspicion was reinforced when they discovered that Uranus did not orbit the sun at a constant rate. It seemed that the planet was feeling the gravitational pull of a more distant and unknown planet.

Two brilliant mathematicians- John Couch Adams, in Cambridge, and Urbain Leverrier, in France calculate independently where this new planet would be. On August 31, 1846, Leverrier sent his prediction to Berlin Observatory and the astronomers there identified a star as the new planet, now called Neptune.

By the end of the last century, it was suspected that both Uranus and Neptune were being pulled by the gravity of the planet father out still. This time it was an American astronomer, Percival Lowell, who calculated where this planet X should be. In 1930, Clyde Tombaugh, working at the Observatory founded by Lowell, detected a faint speck of light that moved from night to night. It was indeed a new planet, close to Lowell’s calculated position, but much fainter than Lowell had predicted. This planet was called Pluto.

But many astronomers believe that Pluto is too small to affect the giant planets Uranus and Neptune. In 1978, astronomers and the US Naval Observatory found a moon orbiting Pluto. The motion of this moon revealed Pluto’s gravity, and it is far too weak to pull on Uranus and Neptune.

 

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Astronomers from Europe are at the moment considering building a telescope 10 times more powerful than any that exist the very large telescope. With it, they hope to look farther into the universe than ever before. Even so, no one really expects to see the edge of the universe.

Modern studies of the universe are based on Einstein’s general theory of relativity. This theory states that matter has a gravitational field which distorts space and time so that space becomes curved and time runs fast or slow. The gravity of matter also bends light.

In testing the effects of the general theory, scientists have found that it accounts for the motion of planets circling the sun and stars orbiting other stars.

Accepting that the theory can be applied to the universe as a whole, cosmologists also accept one final production of Einstein’s theory that the universe has no edge. The theory in fact says that there are two possibilities for the universe. One is that it curves round on itself, like the surface of the planet. Although it has no edge, it is finite. A space traveller setting off in one Direction and never changing course would eventually arrive at the starting point. This is a closed universe.

The other possibility is that the universe is infinite, that space goes on for ever in all directions. In this open universe, however far you travelled you would always come across new regions of space.

Whichever possibilities correct depends on the amount of matter in the universe. If there is enough matter, its gravity will bend space around so that the universe is closed. In this case, the gravity is strong enough eventually to halt the expansion of the universe, and draw galaxies together into a big crunch.

The most recent estimates of the amount of matter indicate that there is not enough matter to close the universe. The universe is there is likely to be infinite in size, with no end. This also means that the universe will keep on expanding for ever.

 

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In the 1920s the American astronomer Edwin Hubble made and astounding discovery about galaxies: they all moving away from the earth at speeds that depend on their distance - the father away they are, the faster they are receding. This is known as Hubble law and it occurs because the universe itself is expanding, which means that all the galaxies are rushing apart from one another.

It is quite easy to measure a galaxy’s speed, by looking at its light and seeing how the wavelength changes according to the Doppler effect. Hubble’s law tells astronomers how to calculate the distance from the speed. So if a distant galaxy is found to be receding at, say, 3,000,000 km/h, by multiplying Hubble’s constant (about 13, when calculating in kilometer per hour and light years) by the galaxy’s speed in kilometer per hour, they can work out that it is 40,000,000 light years away. In this way, astronomers have measured distances to galaxies that lie a staggering 12,000 million light-years away.

 

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A particularly bright kind of star acts as a beacon for measuring the distance to the near galaxies. These stars are called ‘Cepheid variables’, which change in brightness in the regular way.

Astronomers cannot measure the time it takes a Cepheid to flash from maximum brightness, down to minimum, and back to maximum. This time is called its period. Brightness Cepheid flash more slowly than fainter ones, so once Cepheid period has been established, its brightness can be deduced. If a Cepheid is found with a period of two weeks, for example astronomers can say that it is 4000 times brighter than the sun. By investigating the apparent dimness of Cepheid in distant galaxies they can tell how far away the galaxies life.

 

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To establish the distance of stars that are even farther away, astronomers use the stars temperature and brightness. They can measure a star’s temperature surprisingly easily: a bluish star is hot, around 20,000°C; or white or yellow star has a medium temperature and orange or red stars are cool about 3000°C.

The hotter a star, the brighter it is. A star with a temperature of 10,000°C, for example, is 40 times brighter than the sun (which has a temperature of 5500°C). So if a star of 10,000°C is found which appears very dim, then it must be a long way off in space, its brightness diminished by its great distance. Before astronomers can use this relatively simple method they need to know the relationship between brightness and temperature, and the distance from earth. This is why the first use methods such as Parallax, on nearby stars. After measuring the brightness of those stars they can then use what they know as a guide to ascertain the relative brightness of more distant stars.

The measurement of star brightness allows astronomers to measure distances to any star in the Milky Way, some lying as such as 100,000 light years away.

 

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For stars more distant than 300 light years, astronomers need a different technique. One method involves finding out the direction a Star is moving in, and its actual speed.

To establish the direction, it is much easier to work with the cluster of stars than with a single star. Many stars belong to clusters that consist of hundreds, or even thousands, of stars moving through space. Perspective makes the star in each cluster appear to converge. The angle of their converging lines reveals the direction the cluster is heading in space - towards earth, away from Earth at a 45° angle, and so on.

A Star’s speed can be deduced from its light. The stars motion towards or away from Earth changes the wavelength of the light that it emits - so that it becomes blue if it is coming towards and red if it is moving away (a phenomenon called the Doppler effect).

By combining the rate of change in the star’s spectrum with the direction of movement of the cluster, astronomers can work out its real speed through space, and hence calculate the distance to the cluster.

 

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The stars lie millions of times farther away than the sun, so astronomers have to use different techniques to establish their distance. The most important is the method of Parallax, which involves measuring the angle of the movement of a star between two points and relating it to the Earth’s orbit.

A simple experiment illustrates the method. Hold a finger in front of your face; in relation to the background. Now close that eye and open the other. The finger seems to have moved. The nearer the finger is to your face, the greater distance it seems to move.

In astronomy, the finger is the nearby star whose distance is being measured. Astronomers observe its position relative to very distant stars, looking at it from two different positions in the Earth’s orbit. By measuring the angle of the stars apparent movement between these two positions, also known as the Parallax angle, and knowing the diameter of the Earth’s orbit, astronomers can calculate the distance.

The Parallax angle is measured in arc seconds. One arc second is 1/3600 of a degree in the sky, or roughly 1/2000 the apparent size of the moon. The distance to a star in light-years is 3.26 divided by the Parallax angle. The result is given in parsecs, which is the unit of distance that corresponds to a parallax of one second of arc, or 3.26 light years. Using this method, astronomers have found the distance to hundreds of the nearest stars. For example, the nearest star to the sun is a faint one called Proxima Centuari, which lies 4.22 light years away or 1.2 parsecs. The brightest star in the sky, Sirius, is 8.6 light years away, or 2.64 parsecs.

 

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Radar cannot be used to work out the distance of the sun because it is not solid. Instead, astronomers base their calculations on the law of planetary motion. This is the third law discovered by the astronomical Johannes Kepler (1571-1630) in 1618. It states that the square of the time it takes for a planet to complete a journey around the sun (the orbital period) is equal to the cube of the planets mean distance from the sun.

Using Kepler’s law, astronomers could calculate the average distance of the earth from the sun. This value is now known to be 92,955,630 miles (149,597,870km). The Earth-Sun distance is defined as one unit, called an astronomical unit, or AU. Astronomers use this unit to calculate how far the other planets are from the Sun. First they must find the distance from the Earth to the planet. To do so they can use either Parallax or radar.

Using radar, it is possible to tell, for example, that the distance of Venus from Earth, when the two are at their closest, is 26,000,000 miles (42,000,000 km). But astronomers also know that it takes Venus 224.7 days (or .615 of the year) to orbit the sun. According to Kepler‘s law, then, Venus’s distance from the sun is .72AU (since .615 of a year’s equals .72AU cubed).

 

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When it comes to the planets in the solar system, astronomers don’t have reflectors to return pulses of light. Instead, they use radar. Before radar was available they used the speed of light and the Parallax method to calculate the distance of planets. Today, however, they send a pulse of radio waves towards a planet, and wait for the faint echo to return after the waves have bounced of the planets rocky surface. Radio waves travel at the speed of light, so the calculation is the same as for measuring the distance to the Moon.

Radio astronomers have picked up radar reflections from all the planets with rocky surfaces - Mercury , Venus and Mars - and even from the rings of Saturn, which are made of billions of tiny lumps of ice. They cannot detect a radar echo from Saturn itself, or Jupiter, because both consist of gases and do not reflect radar.

 

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The Moon is the nearest object to Earth in the Universe – an average of 238,900 miles (384,400km) away. Its distance varies slightly because it follows an egg-shaped (elliptical) orbit around the Earth.

When the Apollo astronauts visited the Moon, from 1969 to 1972, they left behind small ‘retro’reflectors’, rather like the reflectors on the back of a car. Astronomers on Earth shoot a powerful pulse of laser light at these retro-reflectors, and about two and a half seconds later their telescopes pick up a faint flash as the pulse of light returns to Earth. They then multiply the time it takes for the pulse to leave Earth and return, by the speed of the light, and divide the result by two to arrive at the Moon’s distance from Earth.

The Moon and the Earth are drawing apart because the friction between the Earth’s ocean floor and the water heaped up in the tides is gradually slowing its rotation. This is making it lose energy. In return, the bulges of the Earth’s ocean tides pull the Moon forward in its orbit, making it gain energy. The Moon is therefore gradually moving away from the Earth as it is pulled into a larger orbit.

 

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Circling around the Earth far out in space are giant cameras which can see details on the ground only 12in (300mm) across. The cameras are fixed to satellites as big as a single-Decker bus 50ft (15m) long, and take up half the area of the satellites as ‘spies in the sky’ to check the extent of other countries’ arsenals.

But satellite photography has other purposes. Every day, TV weather forecasts show pictures of the Earth photographed by cameras on board satellites. Geologists and economists study photographs taken from space that reveal rocks and crops on Earth. And astronomers look at distant stars and galaxies, unhindered by the Earth’s atmosphere. but how do these images reach Earth?

The most common way to send photographs from space is to use radio waves and beam the pictures down in the same way that TV pictures are sent. The amount of detail that you can see depends on the spacing between the lines that make up the picture: the more lines, the more detail you can make out.

The world’s most advanced commercial satellite for surveying the ground, the French SPOT satellite, transmits 6000 lines per picture – nearly ten times as many as the 625 lines used on most of the world’s TV sets. This means that in a picture which covers an area of 40sq miles (100 sq km) and taken from a height of 570 miles (920 km

), details as small as 30ft (10m) across are visible. In a photograph of the whole of Paris, for example, you could pick out the Arc de Triomphe.

Military intelligence experts generally want to be able to make out even finer detail. When monitoring a war, they need detailed photographs that will enable them to count the number of troops on a battlefield, or reveal different types of aircraft or ship.

The most modern American ‘spy satellites’. The KH-11 series, relay their pictures by television techniques. But, in general television images cannot show as much detail as a fine-grained 16mm or 35mm film. When film is used, it has to be returned to Earth physically. If the photographs are being taken in manned spacecrafts, the cosmonauts can bring the film back with them, but this is obviously impossible with unmanned spacecraft. So the Americans and the Russians – and more recently the Chinese – have developed satellites that return a film to Earth automatically.

The American ‘Big Bird’ satellites have perfected this technique. The exposed film is out into one of six re entry capsules, which is then jettisoned and drops back into the Earth’s atmosphere. as it parachutes down, the capsule is captured, or lassoed, in a wire loop which trails behind a C-130 Gercules transport plane.

 

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So it was only pure luck that Lovells calculation on the motions of Uranus and Neptune had led to the discovery of Pluto! Astronomers are now asking what else could be pulling on Uranus and Neptune. The answer seems to be a massive planet lying much farther out in the solar system.

Bob Harrington, of the US Naval Observatory, has calculated that this planet is currently in the southern part of the sky. Every few weeks, telescope in New Zealand, at the Black Birch Astrometric Observatory near Blenheim, takes photographs of Harrington’s suspect part of the sky.

Harrington has allies in his research that no previous planet hunter could call on space probes. If planet 10 is pulling on Uranus and Neptune, it should also disturb the parts of the three spacecraft - Pioneer 10, Pioneer 11 and Voyager 1 - that are currently leaving the solar system. Scientists are measuring theirs motions carefully, to see if planet 10 is pulling them off-course. So far, the results are negative.

Other astronomers are not convinced by the calculation made so far. They believe that planet 10 could be anywhere in the sky, and so they are taking a different approach. Planets produce copious amount of infrared radiation. In 1983, the infrared astronomical satellite scanned the whole sky, looking for objects in the universe that produce infrared radiation. If planet 10 exists, then the satellite will probably have picked it up. The results from this survey were recorded on 60 miles (100 km) of computer tape. Astronomers, using this vast amount of data, have located many interesting objects comets, asteroids and newborn stars but planet 10 has still to come to light.

 

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On April 12, 1981, the 1st to space shuttle, Columbia, lifted off from Cape Canaveral on its maiden flight into space. Columbia was powered by three liquid fuelled engines and a pair of giant strap on, solid fuel boosters, and was controlled by five sophisticated, interlinked computers. But despite the space shuttles apparent complexity, the basic principle that makes it work is exactly the same as that behind a simple firework rocket or a balloon that zooms across the room when you let go of its neck. It is a principle of action and reaction.

In the 17th century, the English physics to Sir Isaac newton summed up one of the basic rules of the universe in the statement: ‘action and reaction are equal and opposite’. For example, when the neck of an inflated balloon is released, and air rushes out through the aperture, the equal and opposite reaction to the escaping rush of air pushes the balloon forward.

Unlike a balloon, a rocket does not contain compressed gas. Instead, it manufactures gas by burning solid or liquid fuels. But once the gas has been produced, the principle is the same. As the hot exhaust gases escape from its rear, the rocket is pushed forward in an equal and opposite reaction to the rush of escaping gases. But, unlike a balloon, which darts in all directions, the rocket is designed to keep a stable course.

Colombia’s three liquefied fuelled engines, which together burn 100 tons of fuel a minute, produce a downward stream of gases that cause an opposite, upward force or reaction 640 tons. The gases from two solid fuel boosters produce a reaction of 2400 tons. The total upward reaction on the shuttle is therefore more than 3000 tons. But the few fully fuelled shuttle weighs only 2000 tons, so the reaction is sufficient to lift it off the ground and the accelerate it towards space. Once in space, the shuttle goes into its regulated orbit around the earth.

 

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Each of the two Voyager craft gets its electrical power from a miniature nuclear generator, which consists of three metal cylinders, each 17in (430mm) long and 13in(330mm) wide, connected end to end. The cylinders are packed with plutonium dioxide, a radioactive substance.

Surrounding the core of each cylinders are hundreds of thermocouples. There are miniature electric circuits, consisting of a piece of silicon and a piece of germanium. One end of each thermocouple is heated by the decaying plutonium; the other faces outwards into the cold of space.

The difference in temperature between the two ends of the thermocouple causes an electrical current to flow through it. When electron in the silicon are distributed by heating, free electrons will tend to move to the germanium, creating a current.

When each Voyager was launched, in 1977, the three cylinders together produced 475 watts of electric power. But the power decreases by 7 watts per year, as the plutonium decays.

 

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