# How the Earth Works

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.

Contents

## 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.

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.

## 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 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 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.

## 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.

## 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.

## 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.

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 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.

## 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 -- 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.

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.

### Related HowStuffWorks Articles

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