P1,P2,P3

The Earth is one of the eight planets orbiting the Sun, and there are many other members of the Solar System including asteroids, moons and planets. Data provides the answers to many questions on this subject, but some questions remain unanswered.

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Earth

The Earth is one of the eight planets orbiting the Sun, and there are many other members of the Solar System including asteroids, moons and planets. Data provides the answers to many questions on this subject, but some questions remain unanswered.

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Earth

The Universe is considered to be everything there is, though most of it is thought to be empty.

Much is now known about the Earth and the place of the Earth in the Universe, for example:

the diameter of the Earth is 12,800km (7,953 miles)

the diameter of the Sun is 109 times that of the Earth’s
the Earth is 150 million km (93 million miles) from the Sun
the distance to the nearest star is four light years.The Universe is considered to be everything there is, though most of it is thought to be empty.

  • Much is now known about the Earth and the place of the Earth in the Universe, for example:

- the diameter of the Earth is 12,800km (7,953 miles)
- the diameter of the Sun is 109 times that of the Earth’s
- the Earth is 150 million km (93 million miles) from the Sun
- the distance to the nearest star is four light years.

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Solar System

The Earth is just one of the eight planets orbiting the Sun, which is a star. The orbits all lie in the same plane, and the planets all go round in the same direction.

There are many other members of our Solar System:

  • asteroids are much smaller than planets, and orbit the Sun. Most of the asteroids are between the planets Mars and Jupiter, but some come close to the Earth
  • moons orbit planets. Most are tiny. Only a few are as large as our Moon, which is nearly a sixth of the diameter of the Earth
  • comets have different orbits to those of planets, spending much of their orbital time far from the Sun. Comets are similar in size to asteroids, but are made of dust and ice. The ice melts when the comet approaches the Sun, and forms the comet’s tail.
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The Sun

Nearly all of the mass in our Solar System is in the Sun. The Sun is very large. Its diameter is 109 times the Earth's. The Sun is the source of nearly all the energy we receive. For many years, it was a mystery as to where this came from and this baffled the leading scientists. It is now understood that the nuclear fusion is the energy source. In nuclear fusion, smaller nuclei come together and form larger nuclei. For example hydrogen nuclei are joined together to make helium nuclei. This releases enormous amounts of energy.

hydrogen nucleus + hydrogen nucleus   →   helium nuclei

In stars larger than our Sun helium nuclei can be fused together to create larger atomic nuclei. As the Earth contains many of these larger atoms, like carbon, oxygen, iron, etc, scientists believe that our Solar System was made from the remains of an earlier star.

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How stars and planets are formed

As the gas falls together, it gets hot. A star forms when it is hot enough for anuclear fusion reaction to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. This happened about 5 billion years ago. This is quite recent in the history of the Universe, which is currently believed to be 14 billion years old. Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the star.

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The Sky

The radiation that distant stars and galaxies produce gives us information about the distances to stars, and about how they are changing. In the future, this may allow us to find out if life exists on planets around some of these stars.

Everything we know about stars and galaxies has come from the light, and other radiations, that they give out. This has become more difficult to see from the Earth’s surface, as light pollution from towns and cities interferes with observations of the night sky.

Looking at the sky with the naked eye shows the Sun, Moon, stars, planets and a few cloudy patches called nebulae. When telescopes were invented and developed, astronomers could see that some of the nebulae were in fact groups of millions of stars. These are galaxies.

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Parallax

Powerful telescopes allowed astronomers to answer a question that had baffled scientists since the astronomer Copernicus (1473-1543) first suggested that the Earth moved around the Sun. If the Earth moves, you would expect to see a different view of the stars at different times of the year, in the same way as the room you are in looks slightly different if you move your head to one side. That is to say everything seems to move in the opposite direction to your head, but the objects close to you seem to move more. This effect is called parallax. So if the Earth was moving, why did the stars always look the same?

The answer to the question was revealed by more powerful telescopes. These showed that nearby stars do seem to move from side to side and back every year when compared with very distant stars, but that the amount of movement is tiny.

The second nearest star to us is Proxima Centauri. The Sun is the nearest.

It seems to move through an angle of 1.5 seconds between January and June. As one second = 1/60 of a minute, and one minute = 1/60 of a degree, this tiny movement, which is less than a thousandth of the diameter of the Moon, needed powerful telescopes and accurate measurement to observe.

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Light pollution

In the last 200 years, it has become very difficult to make astronomical observations in industrialised countries such as the UK. This is not just because of cloudy weather or air pollution. It is due to the bright lights found in cities and towns, and on roads. This light pollution means that it is hard for many people to see more than a few of the very brightest stars at night.

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Telescopes

Telescopes are now placed in the few remote, dark places left on our planet, or out in orbit around the Earth.

The Very Large Telescope is part of the Paranal Observatory that is built on top of the Cerro Paranalmountain, which is 2,635 m high, in the Atacama Desert in Chile.

Telescopes in space, such as the Hubble Space Telescope, can observe the whole sky. They are above light pollution and above dust and clouds in the atmosphere. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.

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Beyond the solar system

The Sun is 150 million km(93 million miles) from the Earth, but that’s a tiny distance compared with the distance to other stars, or other galaxies. Larger units of distance are used for these measurements. One popular measurement is the light-year.

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Light years

A light-year is the distance light travels in a year. Light travels very fast (300,000 km/186,282 miles per second), and takes only about eight minutes to reach us from the Sun. It takes over four years to reach us from the next nearest star (Proxima Centauri), and 100,000 years to cross the Milky Way galaxy. We say that the distance to the next nearest star is four lightyears, and the diameter of the Milky Way is 100,000 light years.

The most distant galaxies observed are about 13,000 million light-years away. However, measuring distances to other stars, and to very distant galaxies, is not easy, so the data is uncertain.

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Measurement uncertaincies

When initial distances to stars were being established more than one method was employed. After establishing distances of nearby stars using the parallax method, the 'brightness method' was used to approximate distances to further stars. Other methods were also used. 

Each method had its own assumptions. For example, with the parallax method anassumption made is that during the total time in which the measurement is taking place, distance remains constant between the two stars.

As methods were reliant on each other, a certain level of uncertainty is found in the results.

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Galaxies

Galaxies contain thousands of millions of stars. For many years, it was thought that our galaxy, which is the Milky Way, was the only one that existed, and that the blurry nebulae that could be seen were clouds of dust and gas in the Milky Way.

Observations of many of these nebulae by astronomers such as Edwin Hubble showed they were in fact galaxies outside the Milky Way, and that distant galaxies are all moving away from us.

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Beginning and end of the universe

Hubble’s observations led to the ‘Big Bang’ explanation of the beginning of theUniverse, and set a date for this at 14,000 million years ago.

There are many questions left unanswered about the beginning and end of the Universe. Observations suggest it contains a lot of ‘dark matter’ that cannot be seen, and this is not yet clearly understood.

Perhaps the Universe will continue to expand in the way it is at the moment. Perhaps gravity will eventually win and pull all the fleeing galaxies back together again. Better observations of very distant galaxies and a better understanding of the mysterious ‘dark matter’ are needed before these will be understood.

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Age of the universe

The development of powerful telescopes allowed astronomers to see distant galaxies. The light observed was shifted towards the red end of the spectrum. This phenomenon is known as red-shift. The degree to which light has been shifted indicates how fast the galaxies are moving away.

In general, the further away the galaxy is, the faster it is moving away from the Earth. The motions of the galaxies themselves suggest that space itself is expanding.

It is estimated that the Universe is approximately 13.7 billion years old. Evidence suggests that our Solar System formed around 4.5 billion years ago, so it is around one-third the age of the Universe.

The eventual fate of the Universe is hard to predict due to the uncertainty in measuring such large distances and studying motion of distant objects. A better idea of the mass of the Universe would lead to better predictions.

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How the earth is changing

The theory of plate tectonics is now well established. Continental drift is happening as tectonic plates move, with earthquakes and volcanoes often occurring around their edges.

Rocks provide evidence for changes in the Earth. In 1785 James Hutton presented his idea of a rock cycle to the Royal Society. He detailed ideas oferosion and sedimentation taking place over long periods of time, making massive changes to the Earth’s surface.

Geologists can use other evidence from the rocks themselves such as:

  • looking at cross-cutting features (rock that cuts across another is younger)
  • using fossils (species existed/ became extinct during certain time periods)
  • deepness of the rock (younger rocks are usually on top of older ones).

This kind of evidence only shows that some rocks are older than others. To get a more accurate idea of the age of rocks radioactive dating is used.

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Wegener's theory

Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century. His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists.

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Before wegener

Before Wegener developed his theory, it was thought that mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth's crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case. The heating effect of radioactive materials inside the Earth prevents it from cooling.

Wegener suggested that mountains were formed when the edge of a drifting continent collided with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia.

Wegener’s evidence for continental drift was that:

  • the same types of fossilised animals and plants are found in South America and Africa
  • the shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle
  • matching rock formations and mountain chains are found in South America and Africa.
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Seafloor spreading

In the centres of many oceans, there are mid-ocean ridges. At these places, the tectonic plates are moving apart. Molten material, known as magma from inside the Earth oozes out and solidifies. This movement of the mantle is referred to as convection due to heating by the core of the Earth. This process is called seafloor spreading. It results in seafloors spreading by a few centimetres each year.

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Inside the Earth

All our evidence for changes in the Earth comes from looking at rocks. Folds and fossils in sedimentary rocks, radioactive dating and the weathering of ancient craters show that the oldest rocks are about 4000 million years old. That means the Earth must be at least as old as this.

The only thing that we have been able to observe directly is the Earth’s crust, which is the very thin outer rocky layer.

Evidence from earthquakes shows that the Earth has a very dense core surrounded by a solid mantle.

The Earth is almost a sphere. These are its main layers, starting with the outermost:

  1. The crust, which is relatively thin and rocky
  2. The mantle, shown here as dark red, which has the properties of a solid, but can flow very slowly
  3. The outer core, shown as orange, which is made from liquid nickel and iron
  4. The inner core, shown as yellow, which is made from solid nickel and iron
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The Earth's magnetic field

The typical speed of seafloor spreading is slow: about 10 cm per year. When the magma oozing out of mid-ocean ridges solidifies into rock, the rock records the direction of the Earth’s magnetic field. The Earth’s magnetic field changes with time, and sometimes even reverses its direction. These changes are recorded in the rocks. The same magnetic patterns are seen on both sides of the mid-ocean ridges.

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Plate tectonics

The Earth’s crust, with the upper region of the mantle, consists of huge slabs of rock (tectonic plates). These fit together. Although the mantle below the tectonic plates is solid, it does move. This movement is a few centimetres every year. The continents have changed their positions over millions of years. Volcanoes, mountains and earthquakes occur at the edges of tectonic plates - their creation depends on the direction the plates are moving. If the plates are moving apart, as at mid-ocean ridges, volcanoes are produced as molten magma is allowed to escape. This happens in Iceland. If the plates are moving towards each other, the edges of the plates crumple, and one plate ‘dives’ under the other (subduction). It produces mountains, like the Himalayas. The friction of the movement can also melt rocks and produce volcanoes. This is also part of the rock cycle, because the plate that dives under the other one becomes part of the mantle and emerges much later from volcanoes and in seafloor spreading. There are 2 other ways in which mountains can be formed. At destructive margins mountain chains can be formed as plates push against each other. If an ocean closes completely then continents can collide. This occurs slowly but the collision would still result in the formation of a mountain chain.

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Earthquakes and detecting wave motions

In California on the western coast of the USA, the San Andreas fault at the edge of the North American tectonic plate marks the point at which two plates are moving sideways.

Earthquakes are common in this region. They include the Great San Francisco Earthquake of 1906. The vibrations of an earthquake are detected using a seismometer that records the results in the form of a seismogram.

The vibrations that are detected from the site of an earthquake are known as seismic waves.

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Seismic waves

Vibrations from an earthquake are categorised as P or S waves. They travel through the Earth in different ways and at different speeds. They can be detected and analysed. A wave is a vibration that transfers energy from one place to another without transferring matter (solid, liquid or gas). Light and sound both travel in this way. Energy released during an earthquake travels in the form of waves around the Earth. Two types of seismic wave exist, P- and S-waves. They are different in the way that they travel through the Earth.

P-waves (P stands for primary) arrive at the detector first. They are longitudinal waves which mean the vibrations are along the same direction as the direction of travel. Other examples of longitudinal waves include sound waves and waves in a stretched spring. 

S-waves (S stands for secondary) arrive at the detector of a seismometer second. They are transverse waves which mean the vibrations are at right angles to the direction of travel. Other examples of transverse waves include light waves and water waves. Both types of seismic wave can be detected near the earthquake centre but only P-waves can be detected on the other side of the Earth. This is because P-waves can travel through solids and liquids whereas S-waves can only travel through solids. This means the liquid part of the core blocks the passage of S-waves.

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Amplitude, wavelength and frequency

Amplitude - As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care: the amplitude is not the distance between the top and bottom of a wave. It is the distance from the middle to the top.

Wavelength -The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.

Frequency -The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies. For example, most people cannot hear a high-pitched sound above 20kHz, radio stations broadcast radio waves with frequencies of about 100MHz, while most wireless computer networks operate at 2.4GHz.

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Wave speed

Wave speed is the velocity at which each wave crest moves and is measured in metres per second (m/s). The wave speed only depends on the material the wave is travelling through. The distance travelled by a wave is calculated using this equation:

Distance = speed x time

The speed of a wave - its wave speed (metres per second, m/s)- is related to its frequency (hertz, Hz) and wavelength (metre, m), according to this equation:

wave speed = frequency x wavelength

For example, a wave with a frequency of 100Hz and a wavelength of 2m travels at 100 x 2 = 200m/s.

The speed of a wave does not usually depend on its frequency or its amplitude.

Check your understanding of the equation by having a go at this activity.

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