The Earth, Moon & Sun (CIE IGCSE Physics)
Revision Note
Author
Lindsay GilmourExpertise
Physics
The Earth's Axis
- 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
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
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's Orbit
- 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
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
Moon & Earth
- 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
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
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