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6.
THE SUN, EARTH AND MOON
The Moon is a satellite that was possibly formed when a giant
object planet, named Theia, hit the Earth 4.5 billion (4500
million) years ago. A piece of the Earth broke off and remained
in orbit around the Earth, held by the force of gravity.
The Moon has an average radius of 1738 km. The average
distance between the Moon and the Earth is 384400 km,
although every year it moves away from our planet by almost
4 cm.
Like other celestial bodies, the Moon moves in two ways:
revolution and rotation. Its revolution around the Earth takes
27.32 Earth days. This is the same time it takes the Moon to rotate once on its axis.
So, a lunar day is the same length as a lunar year. For this reason, the side of the
Moon we see from Earth is always the same.
6.1.
Phases of the Moon
The Moon does not produce its own light, but it reflects the light from the Sun.
Depending on the position of the Moon in its orbit around the Earth, we see a
different part of the side that is illuminated.
The
phases of the moon
are the different images of the Moon we can see
as it moves around the Earth. These phases are: new moon, first quarter, full
moon and last quarter.
Analyse
29.
If you could look at the
Earth from the Moon,
would you see ‘phases
of the Earth’? Discuss
and explain your answer.
7
The Moon’s revolution
Phases of the moon
Remember
30.
Look at the diagram. Listen and name the phases of the Moon.
First quarter
: the Moon is
D-shaped. It can be seen from
midday to midnight.
Newmoon
: the side of the
Moon that is not illuminated.
It can’t be seen because it
receives no light. It is at its
highest point at midday.
Last quarter
: the Moon is
C- shaped. It can be seen
frommidnight to midday.
Full moon
: the visible side
of the Moon is completely
illuminated. It can be seen
all night long.
The repetition of the phases of the Moon is called a
lunar month
.
One lunar month lasts about 29 days. So there are 12 lunar months.
To adjust the days to the calendar, as we do with leap years, every two or three
years there is a lunar year with 13 full moons.
