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igcse physics notesprinted 122 · PDF 122
124 pagesrevision 46936
printed 122 · PDF 122
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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 re ected 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
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 sunligh
Orbital Speed EXTENDED When planets move around the Sun, or a moon moves
around a planet, they orbit incircular motion This means that in one orbit, a planet
travels a distance equal to the circumference of a circle (the shape of the orbit) This is
equal to 2 π r where r is the radius a circle The relationship between speed, distance
and time is: Speed = distance time the average orbital speed of an object can be dened
by the equation: v= Where: v= orbital speed in metres per second (m/s) r = average