Download Unit 6 – Earth

The Earth and the Universe
The exploration of space is called astronomy. Most of our
observations in space have come from using telescopes of
different kinds.
Observing the universe from the ground has problems:
The atmosphere distorts the image. Stars twinkle.
Clouds can prevent observation.
Light pollution from cities spoils the images.
The best images come from space telescopes which orbit the
Earth. The pictures are transmitted back to the ground by
microwave links, and are enhanced by computers.
E.g. images of Hubble Space Telescope
Other Kinds of Telescope
Normal telescopes rely on visible light. However astronomers
use radiation from other parts of the electromagnetic
spectrum. Radio telescopes pick up radio frequency
radiation. They have large dishes that focus radio waves onto
a receiving antenna (aerial). Other telescopes pick up:
Infra red; Ultra violet; X-rays; Gamma rays.
Obviously we can't see these radiations so astronomers rely on
computers to generate the images.
Our solar system consists of an average star we call the Sun,
the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune. It also includes (Pluto) : the satellites of
the planets and numerous comets and asteroids. The Sun is
the richest source of electromagnetic energy (mostly in the
form of heat and light) in the solar system. The Sun's nearest
other sun is a red dwarf star called Proxima Centauri, at a
distance of 4.3 light years away. The whole solar system,
together with the local stars visible on a clear night, orbits the
centre of our home galaxy, a spiral disk of 200 billion stars we
call the Milky Way.
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Day and night
We can see that the Sun seems to move across the sky
starting in the east in the morning, and then toward the west
as the day ends. We know that the Sun appears to move
across the sky because the Earth rotates on its axis.
The Earth takes 1 day , (24 hours) in order to make one
complete spin on its axis.
The Earth
The earth orbits the sun in an elliptical
orbit and the moon orbits the earth with
the same kind of orbit. Looking down
from the North pole, the earth spins in
a anti -clockwise direction once
every day . The earth’s axis is tilted
with respect to the plane of its orbit at
an angle of about 23.4 degrees. The
earth orbits the sun in a anti clockwise motion, coming to the same
position among the stars every 365.26 earth days.( 1 year)
We would also see that the moon also orbits the earth in an
anti -clockwise motion.
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Because of the tilt of the earth, not every place on earth gets
light every day. Also, some places have extremely short days.
As the earth revolves around the sun, the place where light
shines the brightest changes. This motion gives us the different
seasons. For instance, the poles receive less light than does the
equator because of the angle that the land around the poles
receive the sun’s light. When the north pole is tilted toward the
sun, the northern hemisphere is presented to the sun at a
greater angle than the southern hemisphere and the northern
hemisphere gets warmer. When this happens, the northern
hemisphere gets summer while the southern hemisphere gets
winter. When the south pole is tilted toward the sun, the two
seasons reverse hemispheres.
Because of centrifugal force involved when an object spins, the
earth is not a perfect sphere, but is somewhat flattened at the
poles and bulges out at the equator. The distance from any
point on the equator to the centre of the earth is longer than
the distance from either pole to the centre of the earth.
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The Moon
The moon is the earth’s only natural satellite. Its revolution
period around the earth is the same length and direction as its
rotation period, which results in the moon always keeping one
side turned toward the earth and the other side turned away
from the earth. The side turned away from the earth is called
the moon’s dark side, even though it is lit half of the time. The
moon’s period of revolution is about 27.32 days long.
The moon has a noticeable effect on the earth in the form of
tides, but it also affects the motion and orbit of the earth.
Phases of the the Moon
As the moon circles the Earth, the shape of the moon appears
to change; this is because different amounts of the illuminated
part of the moon are facing us. The shape varies from a full
moon (when the Earth is between the sun and the moon) to a
new moon (when the moon is between the sun and the Earth).
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How the Universe Began
Most astronomers believe that the Universe started from a
single tiny point in a titanic explosion called the Big Bang.
The Big Bang is thought to have occurred about 18 000 million
years ago.
What is the evidence for this?
Galaxies are moving away from us as shown by red-shift;
The further they are away from us, the faster they are
going;
Cosmic background microwave radiation is described as
"the echoes of the Big Bang".
The background temperature of space is 2.73 K (= -270
o
C), a higher temperature than expected. But still pretty
cold.
Present abundances of hydrogen and helium.
Material was thrown out as the Universe expanded, and in
places came together under the influence of gravity to form
galaxies, stars, and planets. There are many thousands of
millions of stars. Latest evidence suggests that many have
planets. Who knows; there may be life.
A star is a ball of mostly hydrogen and helium gas that shines
extremely brightly. Our Sun is a star. A star is so massive that
its core is extremely dense and hot. At the high core
temperatures of a star, atoms move so fast that they make a
fusion reaction forming more massive atoms and releasing a
great amount of energy.
The definition of a planet is a celestial body that:
is in orbit around the sun;
has a nearly round shape;
has cleared the neighbourhood around its orbit.
Recently Pluto was removed from being th ninth planet but
was classified as a “dwarf planet” because it has not cleared
the neighborhood around its orbit.
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Stars and Galaxies
The reason that we are here is that we live on a planet that is
just the right distance away to give us conditions that are
suitable for life. All the other planets in the solar system are
too hot or too cold.
Astronomers believe that there are many stars that have solar
systems, if not the majority. They cannot see the planets;
they are too far away, but they can detect the wobble caused
by the gravitational pull of planets on a star.
Distances in space are big, and are measured in light years. 1
light year is about 1016 m. The nearest star, Proxima Centauri,
is 4 light years away. That means that light, travelling at 300
million meters a second, takes 4 years to get from there to
here. Flying at the speed of a supersonic plane (60 m/s) it
would take 500 000 years to get there.
Stars are in huge clusters called galaxies. Our galaxy is called
the Milky Way and is about 100 000 light years across. It
contains many millions of stars. The Sun is on one of the arms
of the galaxy.
Be careful not to confuse galaxies with constellations. These
are patterns formed by stars. These are helpful to our
observations from the Earth
Birth of a Star
Stars are formed from clouds of gas called nebulae. Particles
of dust and molecules of gas come slowly together under the
force of gravity. The process takes many millions of years to
happen, although the shock waves from the explosion of a
supernova may help the process.
If there is enough material, it will get hot enough for fusion to
occur. The temperatures involved have to be many millions of
degrees Celsius. The star lights.
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Life of a Star
The Sun has been in a stable state for about 4500 million
years. It is half way through its life now. The reaction is
nuclear. The nuclear fusion reactions in the Sun are right at the
very core. Here there are extreme temperatures and all
elements exist in a form in which all the electrons are stripped
off. This is called plasma.
Old Age of a Star
Helium is a waste product of fusion. Although some helium
nuclei are used up in fusion to larger elements, helium starts to
build up in the star. This makes the star swell into a giant star.
Red giants are so called because the surface temperature is
lower than yellow stars like the Sun. However they are still
pretty high.
Death of a Star
After a while, a red giant starts to shrink under the effect of
gravity and finally becomes a brown dwarf. It gradually cools
and ends up as a dead star. Depending on its size, it can
collapse into a very small space. It is thought that the Sun will
end up as a neutron star of extremely high density but no
bigger than the Earth. The Sun will go out with a whimper.
By contrast, stars that are bigger than the Sun can end their
lives with something of a bang. They swell to red supergiants.
Suddenly the star collapses in on itself. This collapse may take
only a few minutes. The result is an extreme explosion called a
supernova. This is the most violent event that occurs in the
Universe.
The remains of a supernova form a black hole. This is a place
where most of the mass of the star is concentrated into a
planet sized space (or smaller). The material is incredibly
dense. Gravity is so strong that even light cannot escape.
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Gravity
We feel the pull of gravity on the Earth but why gravity should
occur at all is something of a mystery.
It
It
It
It
occurs between any objects that have mass;
is a very tiny force;
has a very long range (infinite in fact);
is always attractive.
Gravity always pulls objects together. It NEVER pushes
them apart, so we say that it is an attractive force. Between
objects of "normal" size the force is so small that you cannot
feel it.
Gravity is only felt where the objects are very large. Planets
are big objects. The Earth has a mass of 6 × 1024 kg. Being
such a big object, the Earth has a gravity of 10 N/kg. When we
look at things like atoms, gravity is far too small.
Gravity and the Universe
Many of the processes that occur in the universe happen
because of the effect of gravity. Although gravity is so tiny, its
range is infinite. This means that it is possible for tiny dust
particles millions of kilometers apart to come together. The
forces involved may be tiny, but space is a friction-free
environment. This means that dust particles will accelerate
towards each other. The process will be slow, taking millions of
years, but they will get there in the end. Gravity decreases
with distance. If you double the distance, the force goes down
by 4 times.
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Satellites in Orbit
Orbits occur because of gravity, not because there is no
gravity. Where satellites orbit the Earth, the acceleration due
to gravity is between about 2 to 9 m/s2. So there is still an
appreciable pull.
An orbiting satellite is constantly falling to the Earth. However
it travels at such a speed that it always misses the Earth.
Since space is a vacuum, there is no friction, so the satellite
does not slow down.
Cosmonauts in satellites feel weightless. This is because they
are falling at the same rate as the satellite, not because there
is no gravity.
Types of Orbit
The picture shows two common kinds of orbit.
Polar orbit
Radio signals to ground
station.
Geostationary
Orbit
Geostationary satellites have an orbit time equal to the
rotational period of the Earth. This means that relative to a
point on the ground they appear stationary. The orbital speed
is zero. It is about 7 km/s.
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A satellite in geosynchronous equatorial orbit (GEO) is located
directly above the equator, exactly 22,300 miles out in space.
At that distance, it takes the satellite a full 24 hours to circle
the planet. Since it takes Earth 24 hours to spin on in its axis,
the satellite and Earth move together.
So, a satellite in GEO always stays directly over the same spot
on Earth and so we always know where GEO satellites are. If
our antenna points in the right direction, we'll always have
direct contact with the satellite. Many communications
satellites travel in geosynchronous orbits, including those that
relay TV signals into our homes.
Because they're so far away, GEO satellites have a very broad
view of Earth. Geostationary satellites are used for
communication.
Satellites in polar orbit are used for monitoring. They
orbit the Earth every 90 minutes and as they make each orbit,
a new section of the Earth passes underneath. This allows all
the ground to be "swept".
Polar orbits are used by satellites that carry out monitoring,
for example for weather and spying.
A Polar orbit is a particular type of Low Earth Orbit. The only
difference is that a satellite in polar orbit travels a north-south
direction, rather than the more common east-west direction.
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Social and economic benefits of space explorations.
1. Space exploration will eventually allow us to establish a
human civilization on another world (e.g., Mars) as a hedge
against the type of catastrophe that wiped out the dinosaurs.
2. We explore space and create important new technologies to
advance our economy. Space exploration can also serve as a
stimulus for children to enter the fields of science and
engineering.
3. Space exploration in an international context offers a
peaceful cooperative venue that is a valuable alternative to
nation state hostilities. International cooperation is also a way
to reduce costs.
4. Exploration of space will provide humanity with an answer to
the most fundamental questions: Are we alone? Are there other
forms of life beside those on Earth?
It is these last two arguments that are the most compelling .It
is challenging to make the case that humans are necessary to
the type of scientific exploration that may bring evidence of life
on another world. There are strong arguments on both sides.
Personally, I think humans will be better at unstructured
environment exploration than any existing robot for a very long
time.
Humans will be exploring space. The challenge is to be sure
that they accomplish meaningful exploration.
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