The Earth, Moon & Sun (Cambridge (CIE) IGCSE Physics)
Revision Note
Written by: Lindsay Gilmour
Reviewed by: Caroline Carroll
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Sun & Earth
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) | equal hours of day and night | (autumn) | equal hours of day and night |
Mar, Apr, May | spring | days are longer than nights | autumn | days are shorter than nights |
21 Jun | (summer) | longest hours of daylight | (winter) | shortest hours of daylight |
Jun, Jul, Aug | summer | days are longer than nights | winter | days are shorter than nights |
23 Sept | (autumn) | equal hours of day and night | (spring) | equal hours of day and night |
Sept, Oct, Nov | autumn | days are shorter than nights | spring | days are longer than nights |
21 Dec | (winter) | shortest hours of daylight | (summer) | longest hours of daylight |
Dec, Jan, Feb | winter | days are shorter than nights | summer | days are longer than nights |
Examiner Tips and Tricks
It is a common misconception that summer is warm because the Sun is closer to Earth and that winter is cold because the Sun is further away - this is not correct! The Earth does have a slightly elliptical orbit around the Sun, but this does not cause a significant temperature variation.
Remember that seasons are caused by the Earth's tilted axis of rotation and its yearly revolution around the Sun.
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Moon & Earth
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
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