Sun & Earth
The Earth is a planet that
o rotates on its axis once every 24 hours
o orbits around the Sun once every 365 days
The Earth's axis is:
o a line that passes through the North and South poles
o tilted at an angle of approximately 23.5° from the vertical
The daily rotation of the Earth on its axis causes
o the periodic cycle of day and night
o 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
o the half of the Earth's surface facing the Sun experiences day
o 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
o to rise from the east
o to set in the west
o to reach its highest point above the horizon at noon (12 pm)
The length of a day is
o the number of hours a location receives sunlight, i.e. from the time the
Sun rises to the time it sets
o the same (about 12 hours) in locations near to the equator
o 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:
o day and night are approximately equal in length
o the Sun appears to rise exactly in the east and set exactly in the west
During the summer, the Sun appears:
o to rise in the northeast and set in the northwest (in the northern hemisphere)
o to rise in the southeast and set in the southwest (in the southern hemisphere)
o to move higher above the horizon
o 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:
o to rise in the southeast and set in the southwest (in the northern hemisphere)
o to rise in the northeast and set in the northwest (in the southern hemisphere)
o to move closer to the horizon
o 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
o the Earth's orbit around the Sun
o the Earth's tilted axis
The Earth's axis of rotation stays tilted at 23.5° throughout its orbit around the Sun, which
means
o one hemisphere tilts towards the Sun and receives more solar radiation
o the other hemisphere tilts away from the Sun and receives less solar radiation
o 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
o the northern hemisphere is tilted towards the Sun
o the northern hemisphere receives a greater proportion of solar radiation
o the southern hemisphere experiences winter
When it is winter in the northern hemisphere
o the northern hemisphere is tilted away from the Sun
o the northern hemisphere receives a smaller proportion of solar radiation
o 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
Northern Southern
When Daylight hours Daylight hours
Hemisphere Hemisphere
(spring) equal hours of day and (autumn) equal hours of day and
20 Mar
equinox night equinox night
days are longer than days are shorter than
Mar, Apr, nights nights
spring autumn
May hours of hours of
daylight increase daylight decrease
(summer) (winter) shortest hours of
21 Jun longest hours of daylight
solstice solstice daylight
days are longer than days are shorter than
Jun, Jul, nights nights
summer winter
Aug hours of hours of
daylight decrease daylight increase
(autumn) equal hours of day and (spring) equal hours of day and
23 Sept
equinox night equinox night
days are shorter than days are longer than
Sept, Oct, nights nights
autumn spring
Nov hours of hours of
daylight decrease daylight increase
(winter) shortest hours of (summer)
21 Dec longest hours of daylight
solstice daylight solstice
days are shorter than days are longer than
Dec, Jan, nights nights
winter summer
Feb hours of hours of
daylight increase daylight decrease
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
o orbits around the Earth in a roughly circular orbit
o takes about one month (28 days) to complete one orbit
o 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:
o the Moon is positioned between the Earth and the Sun
o the side of the Moon facing away from Earth is fully illuminated
o none of the Moon's surface is visible from Earth
On day 7, the first quarter phase is observed
o After the new moon, a thin crescent appears and becomes brighter (waxes)
o After the first quarter moon, it continues to brighten (wax) into a gibbous shape
On day 14, a full moon is observed, where:
o the Earth is positioned between the Moon and the Sun
o the side of the Moon facing towards the Earth is fully illuminated
o all of the Moon's surface is visible from Earth
On day 21, the last quarter phase is observed
o After the full moon, it becomes dimmer (wanes) back into a gibbous shape
o 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
Last updated: 1 October 2024
