The Earth, Moon & Sun (CIE IGCSE Physics)

• The Earth is a planet that
  • rotates on its axis once every 24 hours
  • orbits around the Sun once every 365 days

• The Earth's axis is:
  • a line that passes through the North and South poles
  • tilted at an angle of approximately 23.5° from the vertical

• The daily rotation of the Earth on its axis causes
  • the periodic cycle of day and night
  • the apparent daily rising and setting of the Sun

Day and night

• Day and night are caused by the Earth's rotation on its axis
• One full rotation takes approximately 24 hours, which means
  • the half of the Earth's surface facing the Sun experiences day
  • the other half of the Earth's surface, facing away from the Sun, experiences night

Day and night on Earth

Day and night are caused by the rotation of the Earth on its axis once every 24 hours

The rising and setting of the Sun

• The apparent daily motion of the Sun is also caused by the Earth's rotation on its axis
• Each day, the Sun appears 
  • to rise from the east
  • to set in the west 
  • to reach its highest point above the horizon at noon (12 pm)

• The length of a day is
  • the number of hours a location receives sunlight, i.e. from the time the Sun rises to the time it sets
  • the same (about 12 hours) in locations near to the equator
  • variable in locations north and south of the equator

Apparent motion of the Sun

The Sun rises in the east and sets in the west. Its apparent motion across the sky changes throughout the year

• During equinoxes in both hemispheres:
  • day and night are approximately equal in length
  • the Sun appears to rise exactly in the east and set exactly in the west

• During the summer, the Sun appears:
  • to rise in the northeast and set in the northwest (in the northern hemisphere)
  • to rise in the southeast and set in the southwest (in the southern hemisphere)
  • to move higher above the horizon
  • to reach its greatest height above the horizon on the summer solstice, the day when daylight hours are the longest

• During the winter, the Sun appears:
  
  • to rise in the southeast and set in the southwest (in the northern hemisphere)
  • to rise in the northeast and set in the northwest (in the southern hemisphere)
  • to move closer to the horizon
  • to reach its lowest height above the horizon on the winter solstice, the day when daylight hours are the shortest

The seasons

• Throughout the year, most locations on Earth experience four seasons; summer, autumn, winter and spring
• These seasons are caused by
  • the Earth's orbit around the Sun
  • the Earth's tilted axis

• The Earth's axis of rotation stays tilted at 23.5° throughout its orbit around the Sun, which means
  • one hemisphere tilts towards the Sun and receives more solar radiation
  • the other hemisphere tilts away from the Sun and receives less solar radiation
  • six months later, the hemispheres tilt in the opposite direction

Seasons on Earth

Seasons are caused by the tilt of the Earth and the orbital motion around the Sun. When it is summer in the northern hemisphere (NH), it is winter in the southern hemisphere (SH)

• When it is summer in the northern hemisphere
  • the northern hemisphere is tilted towards the Sun
  • the northern hemisphere receives a greater proportion of solar radiation
  • the southern hemisphere experiences winter

• When it is winter in the northern hemisphere
  • the northern hemisphere is tilted away from the Sun
  • the northern hemisphere receives a smaller proportion of solar radiation
  • the southern hemisphere experiences summer

• When it is spring or autumn, both hemispheres receive about the same amount of solar radiation

The effect of the Earth's tilt on solar radiation

The amount of solar radiation received by the northern hemisphere in winter is less than the amount of solar radiation received by the southern hemisphere in summer

• The variation in daylight hours throughout the year in the northern and southern hemispheres is shown below: 

Seasons, equinoxes and solstices

When	Northern Hemisphere	Daylight hours	Southern Hemisphere	Daylight hours
20 Mar	(spring) equinox	equal hours of day and night	(autumn) equinox	equal hours of day and night
Mar, Apr, May	spring	days are longer than nights hours of daylight increase	autumn	days are shorter than nights hours of daylight decrease
21 Jun	(summer) solstice	longest hours of daylight	(winter) solstice	shortest hours of daylight
Jun, Jul, Aug	summer	days are longer than nights hours of daylight decrease	winter	days are shorter than nights hours of daylight increase
23 Sept	(autumn) equinox	equal hours of day and night	(spring) equinox	equal hours of day and night
Sept, Oct, Nov	autumn	days are shorter than nights hours of daylight decrease	spring	days are longer than nights hours of daylight increase
21 Dec	(winter) solstice	shortest hours of daylight	(summer) solstice	longest hours of daylight
Dec, Jan, Feb	winter	days are shorter than nights hours of daylight increase	summer	days are longer than nights hours of daylight decrease

• The Moon is a natural satellite that
  • orbits around the Earth in a roughly circular orbit
  • takes about one month (28 days) to complete one orbit
  • rotates on its axis once every 28 days so the same side always faces the Earth

Phases of the Moon

• The Moon does not produce its own light
• It is visible in the night sky because it reflects the light from the Sun
• As it orbits around the Earth, it can be seen to undergo different phases

Motion of the Moon

Exactly half of the Moon is always illuminated by the Sun, but its appearance varies when viewed from Earth as it completes its monthly orbit

• On day 0, a new moon is observed, where:
  • the Moon is positioned between the Earth and the Sun 
  • the side of the Moon facing away from Earth is fully illuminated
  • none of the Moon's surface is visible from Earth

• On day 7, the first quarter phase is observed
  • After the new moon, a thin crescent appears and becomes brighter (waxes)
  • After the first quarter moon, it continues to brighten (wax) into a gibbous shape

• On day 14, a full moon is observed, where:
  • the Earth is positioned between the Moon and the Sun 
  • the side of the Moon facing towards the Earth is fully illuminated
  • all of the Moon's surface is visible from Earth

• On day 21, the last quarter phase is observed 
  • After the full moon, it becomes dimmer (wanes) back into a gibbous shape
  • After the last quarter moon, it continues to dim (wane) into a crescent

• On day 29, a new moon is observed and the cycle starts again

Phases of the Moon as seen from Earth

The Moon undergoes eight phases as it orbits the Earth. The time between new moons is about 29 days