GCSE Astronomy Revision Notes

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  1. What three features distinguish Earth from other planets in our solar system?
  • Liquid water, atmosphere, life
  1. Explain how Raleigh scattering makes the sky appear blue.
  • When blue light hits oxygen and nitrogen in the atmosphere, it is absorbed and reemitted in any direction. This is due to the shorter wavelength of blue light compared to other colours, meaning that it is scattered more often than other colours, which pass through more easily. This is why the sun appears more orange at sunset – the light passes through more atmosphere, so more light is scattered, so only the most red light passes through, as all of the blue light is scattered.
  1. Why is Earth’s atmosphere beneficial to humans?
  • The atmosphere keeps the Earth at a relatively warm temperature suitable for life. It also protects Earth’s surface from ultraviolet radiation. This is due to the ozone gas filtering out these rays. It contains oxygen and carbon dioxide, which are needed for life.
  1. Name three causes of light pollution.
  • Shopping centres, floodlights, street lights, etc
  1. Why is light pollution undesirable for astronomers?
  • It makes it difficult to see fainter stars and galaxies.
  1. What shape is the Earth?
  • Oblate spheroid (squashed sphere). The diameter is greater at the equator than it is at the poles.
  1. What is the diameter of the Earth?
  • 13,000 kilometres
  1. Describe how lunar eclipses show that the Earth is not flat.
  • During a lunar eclipse, the shadow of the Earth on the moon is curved.
  1. Describe how sailing ships show that the Earth is not flat.
  • They appear to disappear bottom-first over the horizon as they move away.
  1. Describe how shadows show that the Earth is not flat.
  • Using two places north and south of each other a few hundred kilometres apart, you can measure the angle of shadows cast by a vertical metre stick at each location. You can use the shadows to calculate the angle between the sticks. When you take the distance between the sticks into account, you can calculate the curvature of the Earth, because they are not parallel. Eratosthenes of Cyrene used this experiment to calculate the circumference of the Earth.
  1. How long is the rotation period of the Earth?
  • 23 hours and 56 minutes
  1. How long does it take the Earth to rotate by one degree?
  • 4 minutes
  1. What is one rotation of the Earth in relation to the stars called?
  • A sidereal day
  1. Define:
  • Equator – The line equidistant from Earth’s poles, parallel to Earth’s axis of rotation. The sun is directly above the equator at noon on the Equinoxes.
  • Tropics of Cancer and Capricorn – The Tropic of Cancer is 23°27′ North of the equator, and the Tropic of Capricorn is 23°27′ South of the equator. They represent the furthest north and south that the sun appears in the Zenith at noon on the solstices. In the summer solstice, it appears directly above the Tropic of Cancer.
  • Latitude – The angle between the equator, the centre of the Earth and the observer
  • Longitude – The angle of the observer east or west of the Greenwich Meridian.
  • Pole – The points on the Earth’s surface where the rotational axis passes through
  • Horizon – The circular boundary of part of Earth’s surface visible from a particular point. It is the line where the Earth’s surface appears to meet the sky.
  • Meridian – The imaginary line connecting the poles through the observer. The Prime Meridian passes through Greenwich and is used to determine longitude.
  • Zenith – The imaginary line passing through the centre of the Earth and the observer. The point in the sky directly above the observer.
  • Equinox – When the sun appears directly overhead at the equator at noon. This happens twice a year – on the autumn and spring equinoxes.
  • Summer solstice – When the sun appears directly overhead at the Tropic of Cancer at noon. This happens in the Northern Hemisphere summer.
  • Winter solstice – when the sun appears directly overhead at the Tropic of Capricorn at noon. This happens in the Northern Hemisphere winter.
  1. Why does the atmosphere cause problems for astronomers?
  • The atmosphere does not let all wavelengths of the electromagnetic spectrum to pass through, so we cannot observe all wavelengths. We can only observe visible light, microwaves and radio waves.
  1. Why do we put telescopes on mountains?
  • There is less atmosphere between the telescope and space, so less radiation is absorbed. There is also less light pollution as generally there are fewer people living in mountainous areas.
  1. Why do we send telescopes to space?
  • There is no atmosphere, so we can observe all wavelengths. However, this is very expensive and difficult to fix when things go wrong.
  1. What are the two types of telescope?
  • Reflecting and refracting
  1. How does a reflecting telescope work?
  • Light is reflected in a concave mirror at the back of the telescope, which focuses light onto the secondary mirror, which reflects it into the eyepiece.
  1. Who invented the reflecting telescope?
  • Isaac Newton
  1. How does a refracting telescope work?
  • Two convex lenses are placed so that their foci are in the same place. The larger lens is the Objective lens and the smaller one is the eyepiece. This focuses light to increase its intensity.
  1. What type of telescope are large telescopes and why?
  • Reflecting telescopes. They are easier and cheaper to make for the same magnifying power. They do not have the problems of chromatic aberration. They produce better quality images due to the large apertures.
  1. Give an advantage of larger telescopes.
  • They are not affected by Chromatic aberration.
  1. Which types of waves pass through the atmosphere?
  • Visual light, microwaves and some radio waves.
  1. What happens to infrared, ultraviolet and X-rays?
  • They are absorbed by the atmosphere.
  1. Where are observatories measuring these wavelengths located? Why?
  • In space, because there is no atmosphere to absorb these wavelengths.
  1. What are the Van Allen belts?
  • They are bands of radiation 1000 miles above the Earth’s surface. They can harm astronauts passing through them. Astronauts passing through the Van Allen belts see flashes with their eyes shut when passing through them.
  1. How were the Van Allen belts discovered?
  • They were confirmed when the Geiger counter on-board Explorer 1 was overwhelmed by strong radiation from a belt of charged particles trapped by Earth’s magnetic field.


  1. What do the highlands of the Moon look like?
  • The light areas of the Moon’s surface
  1. What do the lunar seas of the Moon look like?
  • The dark areas of the Moon’s surface
  1. Label this picture of the Moon with:
  1. Sea of Tranquility
  2. Ocean of Storms
  3. Sea of Crises
  4. Tycho
  5. Copernicus
  6. Kepler
  7. Apennine mountains

  1. What is the Moon’s diameter?
  • 3500 kilometres
  1. How far is the Moon from Earth?
  • 380000 kilometres
  1. What is the orbital period of the Moon?
  • 27.3 days
  1. What is the rotational period of the Moon?
  • 27.3 days
  1. What do the rotational and orbital periods mean for those observing the Moon from Earth?
  • We only ever see the near side of the Moon.
  1. Why is the far side of the Moon not visible from Earth?
  • The same side of the Moon is always facing the Earth.
  1. How do we know what the far side of the Moon looks like?
  • Space probes have captured images of it.
  1. How does the appearance of the far side of the Moon differ? What causes this?
  • There are fewer maria and more craters. This is due to thicker crust on the far side of the moon being thicker, so molten rock is less likely to flow to the surface. There are more craters because the far side of the Moon is not shielded from debris.
  1. What is the Latin name for seas on the Moon?
  • Maria
  1. What is Latin name for the highlands of the Moon?
  • Terrae
  1. How do lunar seas form?
  • Molten rock rises to the surface of the Moon.
  1. How do craters form?
  • Impact from debris
  1. How can we tell that the lunar seas were formed after the highlands?
  • There are fewer craters in the Maria.
  1. Describe rilles.
  • Rilles are trenches on the surface of the moon caused by the collapse of lava tunnels.
  1. Describe wrinkle ridges.
  • They are ridges up to 200m high which formed as the crust of the Moon cooled.
  1. Why does the Moon not have an atmosphere?
  • Its gravity is not strong enough to hold one.
  1. Describe the Apollo space programme.
  • The Apollo space programme used ALSEPS to learn about the geology of the Moon. This stands for Apollo Lunar Surface Experiments Packages, which consist of a data transmitter, a radioactive decay powered power supply and experiment modules. These measure seismic activity, charged particles in solar wind, the lunar atmosphere, the rate at which heat flows through the lunar rock and the strength of the Moon’s gravity. There is also a device to detect a laser from Earth to measure the distance between Earth and the Moon.
  1. What was the purpose of the Apollo space programme?
  • To learn about the geology and conditions on the Moon.
  1. How do astronomers think the Moon was formed?
  • 4.3 billion years ago, an object the size of Mars collided with the Earth, ejecting debris into a disc around Earth. The rocks in this disc were attracted to one another due to gravity, forming the Moon.
  1. What evidence is there to support the Moon formation theory?
  • Rocks found on the Moon by Apollo missions contained the same amount of an isotope of oxygen as those found on Earth suggesting a common origin. The surface of the Moon used to be molten – a giant impact could have provided the energy to melt it. Belts of warm dust similar to the one that the Moon could have formed from are found around other stars.


  1. Describe the use of a Sunspotter.
  • Point the gnomon in the direction of the sun until its shadow disappears. Adjust the sunspotter to ensure the dot of light is on the aiming dot. Trace the sunspots and the sun’s disc onto a piece of paper.
  1. Describe the use of a hydrogen alpha filter.
  • Attach the filter to the objective end of the telescope and aim the telescope at the sun. You can attach a camera to the telescope to browse the sun using a computer. This allows us to see a lot of detail.
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  1. What is the diameter of the Sun?
  • 1.4 million kilometres – about 109 times the diameter of the Earth
  1. How far is the Sun from Earth?
  • 150 million kilometres – 1 Astronomical Unit
  1. What is the temperature of the photosphere?
  • 5800 Kelvin
  1. Which two parts of the Sun make up its atmosphere? Describe them.
  • The Chromosphere and the Corona. The Chromosphere is a dense layer of hydrogen and helium gas, around 5000 kilometres thick and with a red/pink tint. The Corona is millions of kilometres of thin, faint gas. When bubbles of billions of tonnes of ...

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