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1. EARTH, STARS, GALAXIES AND SPACE

   • 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
     • 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.
   • How stars and planets are formed: As the gas falls together, it gets hot. A star forms when it is hot enough for a nuclear 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. Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the star.
   • If the Earth moves, you would expect to see a different view of the stars at different times of the year, but the objects close to you seem to move more. This effect is called parallax.
     • A light-year is the distance light travels in a year. Light travels very fast, 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. We say that the distance to the next nearest star is four light years. However, measuring distances to other stars, and to very distant galaxies, is not easy, so the data is uncertain.
   • 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.

2. HOW THE EARTH IS CHANGING

   • 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. James Hutton presented his idea of a rock cycle, he detailed ideas of erosion and sedimentation taking place over long periods of time, making massive changes to the Earth’s surface. Geologists use other evidence from the rocks such as: looking at cross-cutting features, using fossils (species existed/ became extinct during certain time periods), and the deepness of the rock.
   • Alfred Wegener proposed the theory of continental drift. 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.
     • 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.
       • 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.
     • 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. Wegener suggested that mountains were formed when the edge of a drifting continent collided with another, causing it to crumple and fold.
       • The Earth is almost a sphere. These are its main layers, starting with the outermost: The crust, which is relatively thin and rocky. The mantle, which has the properties of a solid, but can flow very slowly. The outer core, which is made from liquid nickel and iron. The inner core, which is made from solid nickel and iron
   • In the centres of oceans, there are ridges. At these places, the tectonic plates are moving apart. 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.

3. SEISMIC WAVES

   • 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
       • 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.
     • 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.
   • The amplitude is not the distance between the top and bottom of a wave. It is the distance from the middle to the top.
     • The wavelength of a wave is the distance between a point on one wave and the same point on the next wave.
       • 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.
       • Distance = speed x time
         • wave speed = frequency x wavelength