The Earth, Moon & Sun (Cambridge O Level Physics)

• The Earth is a rocky planet that
  • Orbits the Sun once every 365 days (1 year)
  • Follows an approximately circular (elliptical) orbit
  • Completes one full rotation on its axis once every 24 hours (1 day)
  • Is tilted on its axis (a line through the north and south poles) at an angle of approximately 23.5°

The Earth's Axis

• The Earth's rotation on its tilted axis creates day and night
  • Day is experienced by the half of the Earth's surface that is facing the Sun
  • Night is the other half of the Earth's surface, facing away from the Sun

Day & Night on Earth

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Day and night are caused by the Earth's rotation 

The Earth's Orbit

• The Earth orbits the Sun once every year, which is approximately 365 days
• The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons

Seasons on Earth

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Seasons in the Northern Hemisphere are caused by the tilt of the Earth

• Over parts B, C and D of the orbit, the northern hemisphere is tilted towards the Sun
  
  • This means daylight hours are more than hours of darkness
  • This is spring and summer
• The southern hemisphere is tilted away from the Sun
  • This means there are shorter days than night
  • This is autumn and winter
• Over parts F, G and H of the orbit, the northern hemisphere is tilted away from the Sun
  • The situations in both the northern and southern hemispheres are reversed
  • It is autumn and winter in the northern hemisphere, but at the same time it is spring and summer in the southern hemisphere
• At C:
  
  • This is the summer solstice
  • The northern hemisphere has the longest day, whilst the southern hemisphere has its shortest day
• At G:
  • This is the winter solstice
  • The northern hemisphere has its shortest day, whilst the southern hemisphere has its longest day
• At A and D:
  • Night and day are equal in both hemispheres
  • These are the equinoxes

The Moon

• The Moon is a satellite that orbits around the Earth
• It travels around the Earth in roughly a circular orbit once a month, which takes around 28 days

• The Moon revolves around its own axis in a month so always has the same side facing the Earth
  • We never see the hemisphere that is always facing away from Earth, although astronauts have orbited the Moon and satellites have photographed it

• The Moon shines with reflected light from the Sun, it does not produce its own light

The Moon

We always see the same side of the Moon as it rotates on its axis and orbits the Earth at the same rate