How the earth works
article from “How Stuff Works”
Earth's Moon: Image Gallery
The Earth, as seen from the moon.
In "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.
After 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 complex.
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. Just about
everything on Earth happens because of the presence of the sun. You'll get a basic idea of how
vital the sun is to life on Earth and the wide variety of roles it plays in the next section.
The Earth and moon are tiny compared to the sun, but the moon's shadow can completely
cover the sun during an eclipse.
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, mindbogglingly big. And it's just a fraction of the size of the sun.
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, releasing light 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 [source: NASA].
You can learn about how the sun creates energy in How the Sun Works.
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.
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.
A World of Spheres
People generally 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:
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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
The Earth's tilt creates its seasons. The captions in this image relate to the northern
hemisphere.
Night and Day
Some of the sun's 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.
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:
1. 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.
2. 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.
3. 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.
4. 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 lowpressure 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 enormous scale.
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 suppose.)
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. There
also might not be breathable air at all. We'll look at the reasons for this next.
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
Sugar and Carbon
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.
But 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
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. We'll look at this cycle on the next
page.
The Carbon Cycle
Carbon 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 carbon cycle.
The basics of the water cycle
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.
Here's what happens:
1. 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.
2. 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.
3. 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 under them.
4. 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 fall.)
5. 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.
Without 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 frozen.
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.
The Earth's Atmosphere
Some people 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.
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.
The 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.
Oxygen molecule + light = two atoms of oxygen. Oxygen atom + oxygen molecule = ozone
molecule.
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 field.
The Earth's layers include the inner core, outer core, mantle and crust.
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
volcanoes.
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 giant
electromagnet. When the solar wind reaches the Earth, it collides with the magnetic field, or
magnetosphere, rather than with the atmosphere.
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.
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:
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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].
Solar Nebula
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.
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 billion years.
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 a sun.
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.
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.
Through earthquakes, volcanic 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.
Aurora
The Earth's magnetic field is the source of the aurora borealis, 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.