"The Hitchhiker's Guide to the Galaxy," Arthur Dent has
trouble getting his mind around the Vogon Constructor Fleet's
destruction of the Earth. He can't process it -- it's just too
big. Arthur tries to narrow it down, but thinking of England,
New York, Bogart movies and the dollar produces no reaction.
Only when he considers the extinction of McDonald's hamburgers
does it finally sink in.
Image courtesy NASA
The Earth, as seen from the
deciding to write about how the Earth works, we felt a little
like Arthur Dent. Even though it's tiny compared to the rest
of the universe, the Earth is enormous, and it's extremely
But instead of collectively going out for a burger, we
decided to take another approach. Rather than examining each
of the Earth's parts, we'll look at what ties it all together.
Power and Light
Compared to the rest of the
universe, the Earth is very small. Our planet and eight (or
maybe nine) others orbit the sun, which
is only one of about 200 billion stars in our galaxy. Our
galaxy, the Milky Way, is part of the universe, which includes
millions of other galaxies and their stars and planets. By
comparison, the Earth is microscopic.
Compared to a person, on the other hand, the Earth is
enormous. It has a diameter of 7,926 miles (12,756 kilometers)
at the equator, and it has a mass of about 6 x 1024 kilograms. The Earth orbits the sun at
a speed of about 66,638 miles per hour (29.79 kilometers per
second). Don't dwell on those numbers too long, though; to a
lot of people, the Earth is inconceivably, mind-bogglingly
big. And it's just a fraction of the size of the sun.
Image courtesy NASA
The Earth and moon are tiny compared to the
sun, but the moon's shadow can completely cover the sun
From our perspective on Earth, the sun looks very small.
This is because it's about 93 million miles away from us. The
sun's diameter at its equator is about 100 times bigger than
Earth's, and about a million Earths could fit inside the sun.
The sun is inconceivably, mind-bogglingly bigger.
But without the sun, the Earth could not exist. In a sense,
the Earth is a giant machine, full of moving parts and complex
systems. All those systems need power, and that power comes
from the sun.
The sun is an enormous nuclear power source -- through
complex reactions, it transforms hydrogen into helium,
and heat. Because of these reactions, every square meter of
our planet's surface gets about 342 Watts of energy from the
sun every year. This is about 1.7 x 1017 Watts total, or as much as 1.7 billion
large power plants could generate [ref].
The only way the Earth could generate more power than the sun
would be if every three people had a power plant of their own
(and the planet were big enough to hold them all). You can
learn about how the sun creates energy in How the Sun
When this energy reaches the Earth, it provides power for a
variety of reactions, cycles and systems. It drives the
circulation of the atmosphere and the oceans. It makes food
for plants, which many people and animals eat. Life on Earth
could not exist without the sun, and the planet itself would
not have developed without it.
A World of
think of the Earth as a "blue marble" or a sphere,
although it's really shaped more like a pumpkin. But
scientists classify the Earth as several spheres:
- Atmosphere: the air we breathe
- Biosphere or ecosphere: the life on earth
- Geosphere: the layers of the planet itself
- Hydrosphere: all of the water, including oceans,
rivers, and lakes
- Cryosphere: the ice at the poles
- Anthrosphere: the people who live on the Earth
To a casual observer, the sun's most visible contributions
to life are light, heat and weather. Now we'll look at how the
sun powers each of those.
Night and Day
biggest impacts on our planet are also its most obvious. As
the Earth spins on its axis, parts of the planet are in the
sun while others are in the shade. In other words, the sun
appears to rise and set. The parts of the world that are in
daylight get warmer while the parts that are dark gradually
lose the heat they absorbed during the day.
You can get a sense of how much the sun affects the Earth's
temperature by standing outside on a partly cloudy day. When
the sun is behind a cloud, you feel noticeably cooler than
when it isn't. The surface of our planet absorbs this heat
from the sun and emits it the same way that pavement continues
to give off heat in the summer after the sun goes down. Our
atmosphere does the same thing -- it absorbs the heat that the
ground emits and sends some of it back to the Earth.
Photo courtesy NOAA
The Earth's tilt creates its seasons. The
captions in this image relate to the northern
The Earth's relationship with the sun also creates seasons.
The Earth's axis tips a little -- about 23.5 degrees. One
hemisphere points toward the sun as the other points away. The
hemisphere that points toward the sun is warmer and gets more
light -- it's summer there, and in the other hemisphere it's
winter. This effect is less dramatic near the equator than at
the poles, since the equator receives about the same amount of
sunlight all year. The poles, on the other hand, receive no
sunlight at all during their winter months, which is part of
the reason why they're frozen.
Most people are so used to the differences between night
and day (or summer and winter) that they take them for
granted. But these changes in light and temperature have an
enormous impact on other systems on our planet. One is the
circulation of air through our atmosphere. For example:
- The sun shines brightly over the equator. The air gets
very warm because the equator faces the sun directly and
because the ozone layer is thinner there.
- As the air warms, it begins to rise, creating a low
pressure system. The higher it rises, the more the air
cools. Water condenses as the air cools, creating clouds and
rainfall. The air dries out as the rain falls. The result is
warm, dry air, relatively high in our atmosphere.
- Because of the lower air pressure, air rushes toward the
equator from the north and south. As it warms, it rises,
pushing the dry air away to the north and the south.
- The dry air sinks as it cools, creating high-pressure
areas and deserts to the north and south of the equator.
This is just one piece of how
the sun circulates air around the world -- ocean currents,
weather patterns and other factors also play a part. But in
general air moves from high-pressure to low-pressure areas,
much the way that high-pressure air rushes from the mouth of
an inflated balloon when you let go. Heat also generally moves
from the warmer equator to the cooler poles. Imagine a warm
drink sitting on your desk -- the air around the drink gets
warmer as the drink gets colder. This happens on Earth on an
Sun and Moon
The sun has a relationship
with the moon, too. The light we see when the moon
shines at night is really reflected light from the sun.
The relative positions of the sun and moon also create
solar and lunar eclipses. This might make it seem like
the moon is nothing without the sun, but it does some
important jobs for the Earth. The moon regulates the
Earth's orbit, and it causes the ocean tides.
The Coriolis Effect, a product of the Earth's
rotation, affects this system as well. It causes large weather
systems, like hurricanes, to rotate. It helps create
westward-running trade winds near the equator and
eastward-running jet streams in the northern and
southern hemispheres. These wind patterns move moisture and
air from one place to another, creating weather patterns. (The
Coriolis Effect works on a large scale -- it doesn't really
affect the water draining from the sink like some people
The sun gets much of the credit for creating both wind and
rain. When the sun warms air in a specific location, that air
rises, creating an area of low pressure. More air rushes in
from surrounding areas to fill the void, creating wind.
Without the sun, there wouldn't be wind -- and there might not
be breathable air at all. We'll look at the reasons for this
Atmosphere and the Water Cycle
The Earth's atmosphere is mostly composed of
nitrogen. Oxygen makes up just 21 percent of the air we
breathe. Carbon dioxide, argon, ozone, water vapor and other
gasses make up a tiny portion of it, as little as 1 percent.
These gasses probably came from several processes as the Earth
evolved and grew as a planet.
some scientists believe that the Earth's atmosphere would
never have contained the oxygen we need without plants. Plants
(and some bacteria) release oxygen during
photosynthesis, the process they use to change water
and carbon dioxide into sugar they can use for food.
Photosynthesis is a complex reaction. In a lot of ways,
it's similar to the way your body breaks down food into fuel
that it can store. Essentially, using energy from the sun, a
plant can transform carbon dioxide and water into glucose and
oxygen. In chemical terms:
6CO2 + 12H2O + Light –> C6H12O6 + 6O2+ 6H2O
CycleCarbon is fundamental
to life -- all organic forms of life contain it. On
Earth, carbon cycles through the atmosphere and the
planet itself. This cycle has two components. The
geological component involves carbon-containing
compounds eroding from the land, washing into the sea,
entering the Earth's mantle layer and being expelled
through volcanoes. The biological component
involves plants' and animals' inspiration and
expiration. Since carbon is a greenhouse gas, its
presence affects how warm or cool the planet is. The NASA
Earth Observatory has a thorough explanation of the
In other words, while we inhale oxygen and exhale carbon
dioxide, plants inhale carbon dioxide and exhale oxygen. Some
scientists believe that our atmosphere had little to no oxygen
before plants evolved and started releasing it.
Without the sun to feed plants (and the plants to release
oxygen), we might not have breathable air. Without plants to
feed us and the animals most people use for food, we'd also
have nothing to eat.
Obviously, plants are important, but not just because they
give us food to eat and oxygen to breathe. Plants help control
the amount of carbon dioxide, a greenhouse gas, in the
atmosphere. They protect the soil from wind and from water
runoff, helping to control erosion. In addition, they release
water into the air during photosynthesis. This water, along
with the rest of the water on the planet, takes part in a huge
cycle that the sun controls.
Water and Fire
The sun has a huge effect on our
water. It warms the oceans around the tropics, and its absence
cools the water around the poles. Because of this, ocean
currents move large amounts of warm and cold water,
drastically affecting the weather and climate around the
world. The sun also drives the water cycle, which moves
about 18,757 cubic miles (495,000 cubic kilometers) of water
vapor through the atmosphere every year [ref].
If you've ever gotten out of a swimming pool on a hot day
and realized a few minutes later that you were dry again, you
have firsthand experience with evaporation. If you've
seen water form on the side of a cold drink, you've seen
condensation in action. These are primary components of
the water cycle, also called the hydrologic cycle,
which exchanges moisture between bodies of water and land
masses. The water cycle is responsible for clouds and rain as
well as our supply of drinking water.
The basics of the water
Here's what happens:
the sun to start the process of evaporation, the water cycle
wouldn't exist. We wouldn't have clouds, rain, or weather. The
water on the planet would be stagnant. It would also be solid,
since without the sun to warm it, the Earth would be entirely
- The sun shines on the surface of oceans and lakes,
exciting molecules of water. The more the sun excites the
molecules, the faster they move, or evaporate.
- The molecules rise through the atmosphere as water
vapor. Plants add to this water vapor through
transpiration, a byproduct of photosynthesis, which
also depends on the sun. In some locations, water
sublimates, or changes directly from ice to vapor.
- All of this water vapor rises into the atmosphere. The
higher it rises, the cooler it gets. The molecules of water
slow down and stick together, or condense, as they cool.
This forms clouds. Depending on how high and thick they are,
clouds can either warm or cool the surface of the planet
- Droplets continue to combine inside the clouds. When
they get big and heavy enough, they fall as precipitation.
(Pollution in clouds can decrease the amount of rainfall by
requiring droplets to be bigger and heavier before they can
- Precipitation falls as rain, snow, sleet or hail,
depending on the temperature and other conditions. Over
land, it falls onto the ground and into rivers and lakes.
Some of the water seeps into the soil, nourishing plants and
joining the groundwater. Much of it flows into rivers and
lakes, which eventually run into the ocean.
The sun powers the processes that control our climate and
the content of our atmosphere. Without it, we wouldn't have
oxygen or liquid water on our planet. We wouldn't have weather
or seasons. But the sun's immense source of power also has
some drawbacks. Next, we'll look at some of phenomena that
protect Earth from the power of the sun.
imagine the Earth's atmosphere as a blanket of air that
gets thinner and colder the higher you go. While the
atmosphere does tend to get thinner, it's made of
distinct regions, and some outer layers are much hotter
than our planet's surface. NASA
has a great description of the layers of the atmosphere
with links to images of each.
the Big Bang
Heat and Wind
The sun's massive power source has
two main disadvantages -- ultraviolet light and the
solar wind. Ultraviolet light can cause cancer,
cataracts and other health problems. The solar wind, a stream
of charged, or ionized, particles that stream off of
the sun, could strip away our atmosphere. Fortunately, the
Earth has some natural defenses against both. Our ozone
layer protects us from ultraviolet (UV) light, and our
magnetic field protects us from the solar wind.
stratosphere, the layer of atmosphere just above the
one in which we live, contains a thin layer of ozone
(O3). This layer wouldn't
exist without the sun. Ozone is made of three atoms of oxygen.
It's not a very stable molecule, but it takes a lot of power
to create it. When UV light hits a molecule of oxygen
(O2) of, it splits it into two
atoms of oxygen (O). When one of these atoms comes into
contact with a molecule of oxygen, they combine to make ozone.
The process also works in reverse -- when UV light hits ozone,
it splits it into a molecule of oxygen and an atom of oxygen.
Images courtesy NASA
Oxygen molecule + light = two atoms of
Oxygen atom + oxygen molecule = ozone
This process is called the ozone-oxygen cycle, and
it converts UV light into heat, preventing it from reaching
the surface of the Earth. Without the sun, the Earth wouldn't
have an ozone layer -- but without the sun, the Earth also
wouldn't need it.
But while the sun creates the ozone layer, the Earth itself
creates its defense against the solar wind. Without the
Earth's magnetic field, ionized particles from the solar wind
could strip the planet's atmosphere away. This magnetic field
comes from deep inside the Earth's core. Interactions between
the inner and outer core create the magnetic
Image courtesy USGS
The Earth's layers include the inner core,
outer core, mantle and
The planet's inner core is made of solid iron. Surrounding
the inner core is a molten outer core. These two layers are
very deep within the Earth, separated from its crust by
the thick mantle. The mantle is solid but malleable,
like plastic, and it's the source of the magma that comes from
The Earth's inner core spins, much like the Earth spins on
its axis. The outer core spins as well, and it spins at a
different rate than the inner core. This creates a dynamo
effect, or convections and currents within the core. This
is what creates the Earth's magnetic field -- it's like a
When the solar wind reaches the Earth, it collides with the
magnetic field, or magnetosphere, rather than with the
Image courtesy SOHO
SOHO is a project of international
cooperation between ESA and NASA.
The poles actually change places periodically -- about 400
times in the last 330 million years. The field weakens while
the shift takes place. But computer simulations predict that
the sun might come to the rescue, interacting with the
atmosphere to supplement the magnetic field, while the shift
is in process.
The Earth's physical composition generates its magnetic
field. That composition is a product of the Earth's creation,
which would not have been possible without the sun.
Planets and Stars
The most prominent scientific theory about
the origin of the Earth involves a spinning cloud of dust
called a solar nebula. This nebula is a product of the
Big Bang. Philosophers, religious scholars and
scientists have lots of ideas about where the universe came
from, but the most widely-held scientific theory is the Big
Bang Theory. According to this theory, the universe originated
in an enormous explosion.
magnetic field is the source of the aurora borealis
(question471.htm), the dramatic lights that appear when
solar radiation bounces off the Earth's magnetic field.
This happens at the South Pole as well. In the southern
hemisphere, the lights are called the aurora australas.
Before the Big Bang, all of the matter and energy now in
the universe was contained in a singularity. A
singularity is a point with an extremely high temperature and
infinite density. It's also what's found at the center of a black
hole. This singularity floated in a complete vacuum until
it exploded, flinging gas and energy in all directions.
Imagine a bomb going off inside an egg -- matter moved in all
directions at high speeds.
As the gas from the explosion cooled, various physical
forces caused particles to stick together. As they continued
to cool, they slowed down and became more organized,
eventually growing into stars. This process took about a
About five billion years ago, some of this gas and matter
became our sun. At first, it was a hot, spinning cloud of gas
that also included heavier elements. As the cloud spun, it
collected into a disc called a solar nebula. Our planet
and others probably formed inside this disc. The center of the
cloud continued to condense, eventually igniting and becoming
Image courtesy NASA
There's no concrete evidence for exactly how the Earth
formed within this nebula. Scientists have two main theories.
Both involve accretion, or the sticking together of molecules
and particles. They have the same basic idea - about 4.6
billion years ago, the Earth formed as particles collected
within a giant disc of gas orbiting what would become our sun.
Once the sun ignited, it blew all of the extra particles away,
leaving the solar system as we know it. Our moon formed in the
solar nebula as well -- read "Where
Did the Moon Come From?" to learn more.
At first, the Earth was very hot and volcanic. A solid
crust formed as the planet cooled, and impacts from asteroids
and other debris caused lots of craters. As the planet
continued to cool, water filled the basins that had formed in
the surface, creating oceans.
eruptions and other factors, the Earth's surface
eventually reached the shape that we know today. Its mass
provides the gravity that holds everything together and its
surface provides a place for us to live. But the whole process
would not have started without the sun.
Check out the links on the next page to learn more about
the Earth, the sun and related topics.
How Do We
Know?As with evolution,
the Big Bang Theory has caused some controversy. Here
are a few of the reasons scientists think it's accurate:
To learn more about the Big Bang, check out this
information from NASA, the University
of Michigan and the University
of California at Berkeley.
- All of the matter in the universe is moving away
from all the other matter at a very fast rate.
Scientists have proven this by measuring stars'
Hubble red shift, or how light waves get
stretched out as they rush away from us.
- Scientists can detect and measure low-level
radiation called cosmic microwave background
(CMB) or primordial background radiation.
This seems to be an aftereffect of the Big Bang. New
analysis of the CMB suggests that the universe changed
from a microscopic point to an enormous system in a
fraction of a second [ref].
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Our World Go 'Round." Space.com, March 18, 2003.
- Britt, Robert Roy. "When North Becomes South: New Clues
to Earth's Magnetic Flip-Flops." Space.com, April 7, 2004.
- "The Carbon Cycle." NASA Earth Observatory.
- Chown, Marcus. "Solar Wind to Shield Earth During Flip."
New Scientist, May 15, 2004.
- "Cracking the Ice Age." NOVA Online.
- "Earth's Atmosphere." NASA.
- "The Earth's Layers." North Dakota University System.
- Glasner, Joanna. "Earth Hurtles Toward 6.5 Billion."
Wired, February 21, 2006.
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EPA, April 2000.
- "Global Weather Patterns." BBC.
- Graham, Steve, et al. "The Water Cycle." NASA Earth
- "The Great Ice Age." U.S. Geological Survey.
- Hawkes, Nigel. "Waterworld: How will Life on Earth Look
in 1000 Years." Times Online, February 17, 2006.
- Herring, David. "Ocean and Climate." NASA Earth
- Illinois State Museum. "Why Are there Ice Ages?"
- Larson, Josh. "Understanding Global Climate Patterns."
USA Today, August 6, 2004.
- NASA Earth Observatory.
- Seeds, Michael A. "Foundations of Astronomy." Wadsworth
Publishing Company. 1994.
- Simkin, Tom, et al. "This Dynamic Planet." USGS. 1994.
- Smith, Lewis and Ben Hoyle. "World Has Only 20 Years to
Stop Climate Disaster." Times Online, January 31, 2006.
- "Steam Explosions, Earthquakes, and Volcanic Eruptions -
What's in Yellowstone's Future?" Fact Sheet 2005-3024, U.S.
- Vastag, Brian. "North Magnetic Pole is Shifting Rapidly
Toward Russia." National Geographic. December 15, 2005.
- "What Is Photosynthesis?" Arizona State University,
February 7, 2006.
- "Why Isn't the Earth Hot as an Oven?" NASA Earth