The Earth, Moon & Sun (CIE IGCSE Physics)

• The Earth is a rocky planet that rotates in a near circular orbit around the Sun
• It rotates on its axis, which is a line through the north and south poles
  • The axis is tilted at an angle of approximately 23.5° from the vertical
• The Earth completes one full rotation (revolution) in approximately 24 hours (1 day)

   

• This rotation creates the apparent daily motion of the Sun rising and setting
  • Rotation of the Earth on its axis is therefore responsible for the periodic cycle of day and night

Day and Night

• The Earth's rotation around its 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

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Day and night are caused by the Earth's rotation

 

Rising and Setting of the Sun

• The Earth's rotation on its axis makes the Sun looks like it moves from east to west
  • At the equinoxes the Sun rises exactly in the east and sets exactly in the west  
  • Equinox (meaning 'equal night') is when day and night are approximately of equal length
• However, the exact locations of where the Sun rises and sets changes throughout the seasons

   

• In the northern hemisphere (above the equator): 
  • In summer, the sun rises north of east and sets north of west
  • In winter, the sun rises south of east and sets south of west

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The Sun rises in the east and sets in the west. Its approximate area changes throughout the year

• The Sun is highest above the horizon at noon (12 pm)
• In the northern hemisphere, the daylight hours are longest up until roughly the 21st June
  • This day is known as the Summer Solstice and is where the Sun is at its highest point in the sky all year
• The daylight hours then decrease to their lowest around 21st December
  • This is known the Winter Solstice and is where the Sun is at its lowest point in the sky all year

• The Earth orbits the Sun once in approximately 365 days
  • This is 1 year
• The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons

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Seasons in the Northern hemisphere 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 hemisphere 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 is a satellite around the Earth
• It travels around the Earth in roughly a circular orbit once a month
  • This takes 27-28 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 satellite have photographed it
• The Moon shines with reflected light from the Sun, it does not produce its own light

Phases of the Moon

• The way the Moon's appearance changes across a month, as seen from Earth, is called its periodic cycle of phases

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Phases of the Moon as it orbits around Earth

• In the image above, the inner circle shows that exactly half of the Moon is illuminated by the Sun at all times
• The outer circle shows how the Moon looks like from the Earth at its various positions
• In the New Moon phase:
  • The Moon is between the Earth and the Sun 
  • Therefore, the sunlight is only on the opposite face of the Moon to the Earth
  • This means the Moon is unlit as seen from Earth, so it is not visible
• At the Full Moon phase:
  • The Earth is between the Moon and the Sun 
  • The side of the Moon that is facing the Earth is completely lit by the sunlight
  • This means the Moon is fully lit as seen from Earth
• In between, a crescent can be seen where the Moon is partially illuminated from sunlight