高一英语人教版必修三Unit 4 课文内容上课讲义
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高一英语人教版必修
三Unit 4
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Unit 4
Astronomy: the science of the stars
HOW LIFE BEGAN ON THE EARTH
No one knows
exactly how the earth began, as it happened so
long ago. However,
according to a widely
accepted theory, the universe began with a “Big
Bang” that
threw matter in all directions.
After that, atoms began to form and combine to
create
stars and other bodies.
For
several billion years after the “Big Bang”, the
earth was still just a cloud of dust.
What it
was to become was uncertain until between 4.5 and
3.8 billion years ago
when the dust settled
into a solid globe. The earth became so violent
that it was not
clear whether the shape would
last or not. It exploded loudly with fire and
rock. They
were in time to produce carbon
dioxide, nitrogen, water vapour and other gases,
which were to make the earth’s atmosphere.
What is even more important is that as
the
earth cooled down, water began to appear on its
surface.
Water had also appeared on other
planets like Mars but, unlike the earth, it had
disappeared later. It was not immediately
obvious that water was to be fundamental to
the development of life. What many scientists
believe is that the continued presence
of
water allowed the earth to dissolve harmful gases
and acids into the oceans and
seas. This
produced a chain reaction, which made it possible
for life to develop.
Many millions of years
later, the first extremely small plants began to
appear on the
surface of the water. They
multiplied and filled the oceans and seas with
oxygen,
which encouraged the later development
of early shellfish and all sorts of fish. Next,
green plants began to grow on land. They were
followed in time by land animals.
Some were
insects. Others, called amphibians, were able to
live on land as well as in
the water. Later
when the plants grew into forests, reptiles
appeared for the first time.
They produced
young generally by laying eggs. After that, some
huge animals, called
dinosaurs, developed.
They laid eggs too and existed on the earth for
more than 140
million years. However, 65
million years ago the age of the dinosaurs ended.
Why
they suddenly disappeared still remains a
mystery. This disappearance made possible
the
rise of mammals on the earth. These animals were
different from all life forms in
the past,
because they gave birth to young baby animals and
produced milk to feed
them.
Finally
about 2.6 million years ago some small clever
animals, now with hands and
feet, appeared and
spread all over the earth. Thus they have, in
their turn, become the
most important animals
on the planet. But they are not looking after the
earth very
well. They are putting too much
carbon dioxide into the atmosphere, which prevents
heat from escaping from the earth into space.
As a result of this, many scientists
believe
the earth may become too hot to live on. So
whether life will continue on the
earth for
millions of years to come will depend on whether
this problem can be solved.
A VISIT TO THE MOON
Last month I was lucky
enough to have a chance to make a trip into space
with my
friend Li Yanping, an astronomer. We
visited the moon in our spaceship!
Before we
left, Li Yanping explained to me that the force of
gravity would change
three times on our
journey and that the first change would be the
most powerful. Then
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we were off. As the rocket
rose into the air, we were pushed back into our
seats
because we were trying to escape the
pull of the earth’s gravity. It was so hard that
we
could not say anything to each other.
Gradually the weight lessened and I was able to
talk to him. “Why is the spaceship not falling
back to the earth? On the earth if I fall
from
a tree I will fall to the ground.” I asked. “We
are too far from the earth now to
feel its
pull,” he explained, “so we feel as if no gravity
at all. When we get closer to
the moon, we
shall feel its gravity pulling us, but it will not
be as strong a pull as the
earth’s.” I cheered
up immediately and floated weightlessly around in
our spaceship
cabin watching the earth become
smaller and the moon larger.
When we got
there, I wanted to explore immediately. “Come on,”
I said. “If you are
right, my weight will be
less than on the earth because the moon is smaller
and I will
be able to move more freely. I
might even grow taller if I stay here long enough.
I
shall certainly weigh less!” I laughed and
climbed down the steps from the spaceship.
But
when I tried to step forward. I found I was
carried twice as far as on the earth and
fell
over. “Oh dear,” I cried, “walking does need a bit
of practice now that gravity has
changed.”
After a while I got the hang of it and we began to
enjoy ourselves.
Leaving the moon’s gravity
was not as painful as leaving the earth’s. But
returning
to the earth was very frightening.
We watched, amazed as fire broke out on the
outside of the spaceship as the earth’s
gravity increased. Again we were pushed hard
into our seats as we came back to land. “That
was very exhausting but very exciting
too,” I
said. “Now I know much more about gravity! Do you
think we could visit
some stars next time?”
“Of course,” he smiled, “which star would you like
to go to?”
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