P7.1 Naked-eye astronomy

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What is the solar system?

  • Solar system - Collection of planets, comets and all other objects that orbit the Sun.
  • The Earth takes 365 1/4 days to complete one orbit (a Year).
  • It also rotates around an imaginery line called its axis.
  • The moon orbits the earth and takes 28 days for one orbit.
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The movement of the Sun

  • The sun appears to move from East to West as the Earth is spinning on its axis.
  • Solar day - The Sun reappears in the same place every 24 hours.
  • Sidreal day - Time it takes for the Earth to spin around once on its axis (23 hours and 56 mins).
  • Earth also moves further along its orbit of the Sun  so for the same part of the Earth to be facing the Sun, it needs to turn for an extra 4 mins.
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The movement of the stars

  • As the Earth rotates on its axis, the stars appear to be moving in circles around the Pole star.
  • They appear to be back in the same place after 23 hours and 56 mins ( Sidereal day).
  • Constellation - A group of stars that form a pattern.
  • We see different constellations in summer and winter as the Earth moves around the Sun.
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The movement of the Moon

  • Like the Sun, the Moon appears to move from East to West.
  • The moon takes longer, it reappears in the same postition every 24 hours and 49 mins.
  • As well as the Earth's rotation allowing a different view of the moon, the Moon is also orbiting the Earth (every 28 days).
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Moving planets

  • Mercury, Venus, Mars, Jupiter and Saturn can be seen with the naked eye.
  • As the planets orbit the Sun, their positions appear to change each night against the background of fixed stars.
  • Most of the time, planets seem to move from East to West like the sun.
  • Sometimes the planets seem to go backwards - Retrograde motion.
  • Retrograde motion happens as different planets take different amounts of time to orbit the Sun.
  • Their postion depends on where the planet and the Earth are in their orbits.
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The phases of the Moon

  • We can only see the part of the Moon that is lit up by the sun.
  • As the Moon orbits the Earth, we can see different parts of it lit up by the Sun.
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What causes an eclipse

  • Solar eclipse - Moon blocks the Sun's light.
  • Lunar eclipse - Moon moves into the Earth's shadow.
  • Moon and Earth both have shadows with areas of total darkness (Umbra) and areas of partial darkness (penumbra).
  • When the Moon's Umbra touches the surface of the Earth, there is a solar ecipse formed inside the Umbra.
  • When the moon passes into the Earth's Umbra, there is a lunar eclipse which are more common as the Earth's shadow is bigger than the Moon's.
  • Eclipses are rare as the Moon doesn't often line up exactly with the Sun. Its orbit is tilted by 5' relative to the Earth's.
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Locating objects in the sky

  • Stars can be described like lights on the inside of a spinning sphere (the celestial sphere).
  • It has an axis from the Pole star through the axis of the Earth.
  • Also, a celestial equator which is an extension of the Earth's equator.
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