The sun is the closest star to Earth. Even at a distance of 150 million kilometers (93 million miles), its gravitational pull holds the planet in orbit. It radiates light and heat, or solar energy, which makes it possible for life to exist on Earth. 

Plants need sunlight to grow. Animals, including humans, need plants for food and the oxygen they produce. Without heat from the sun, Earth would freeze. There would be no winds, ocean currents, or clouds to transport water.

Solar energy has existed as long as the sun—about 5 billion years. While people have not been around that long, they have been using solar energy in a variety of ways for thousands of years.

Solar energy is essential to agriculture—cultivating land, producing crops, and raising livestock. Developed about 10,000 years ago, agriculture had a key role in the rise of civilization. Solar techniques, such as crop rotation, increased harvests. Drying food using sun and wind prevented crops from spoiling. This surplus of food allowed for denser populations and structured societies. 

Early civilizations around the world positioned buildings to face south to gather heat and light. They used windows and skylights for the same reason, as well as to allow for air circulation. These are elements of solar architecture. Other aspects include using selective shading and choosing building materials with thermal mass, meaning they store heat, such as stone and concrete. Today, computer programs make applications easier and more precise.

The greenhouse is another early solar development. By converting sunlight to heat, greenhouses make it possible to grow plants out of season and in climates that may not be suited for them. One of the earliest greenhouses dates to 30 CE, before glass was even invented. Constructed from translucent sheets of mica, a thin mineral, it was built for the Roman emperor Tiberius, who wanted to be able to eat cucumbers all year. The general technique is the same today, although there have been many improvements to increase the variety and amount of crops grown.

Once food is harvested, solar energy can be used to cook it. The first solar box cooker was built in 1767 by Horace de Saussure, a Swiss physicist. It reached temperatures of 87.8 degrees Celsius (190 degrees Fahrenheit) and was used to cook fruit. Today, there are many different types of solar cookers being used for cooking, drying and pasteurization, which slows the growth of microbes in food. Because they do not use fossil fuels, they are safe, do not produce pollution or cause deforestation.

Solar cookers are used in many parts of the world in growing numbers. It is estimated that there are half a million installed in India alone. India has the world’s two largest solar cooking systems, which can prepare food for 25,000 people daily. According to Indian Prime Minister Manmohan Singh, “Since exhaustible energy sources in the country are limited, there is an urgent need to focus attention on development of renewable energy sources and use of energy efficient technologies.”

In Nicaragua, a modified solar cooker is being used to sterilize medical equipment at clinics.

Solar thermal energy can be used to heat water. First introduced in the late 1800s, the solar water heater was a big improvement over stoves that burned wood or coal because it was cleaner and cost less to operate. They were very popular for American homes in sunny places, including Arizona, Florida, and California. However, in the early 1900s, low-cost oil and natural gas became available and solar water systems began to be replaced. Today, they are not only popular again; they are becoming the norm in some countries, including China, Greece, and Japan. They are even required to be used in any new construction in Australia, Israel, and Spain.

Besides heating water, solar energy can be used to make it potable, or suitable for drinking. One method is solar disinfection (SODIS). Developed in the 1980s, SODIS involves filling plastic soda bottles with water then exposing them to sunlight for several hours. This process reduces the viruses, bacteria and protozoa in water. More than 2 million people in 28 developing nations use this method daily for their drinking water.
Solar power—the conversion of sunlight into electricity—is yet another application of solar technology. This can be done in a number of ways. The two most common are photovoltaic (solar cells) and concentrating solar power.

Solar cells convert sunlight directly into electricity. The amount of power generated by each cell is very low. Therefore, large numbers of cells must be grouped together, like the panels mounted on the roof of a house, to generate enough power. 

The first solar cell was constructed in the 1880s. The earliest major application was on the American satellite Vanguard I, launched in 1958. A radio transmitter powered by solar cells operated for about seven years; one using conventional batteries lasted only 20 days. Since then, solar cells have become the established power source for satellites, including those used in the telecommunications industry.

On Earth, solar cells are used for everything from calculators and watches to homes, commercial buildings, and even stadiums. Kaohsiung World Stadium in Taiwan, completed in 2009 to host the World Games, has more than 8,800 solar panels on its roof. Charles Lin, director of Taiwan’s Bureau of Public Works, said, “The stadium's solar energy panels will make the venue self-sufficient in electricity needs.” When the stadium is not in use, it can power 80 percent of the surrounding neighborhood.

Unlike solar cells, which use sunlight to generate electricity, concentrating solar power technology uses the sun’s heat. Lenses or mirrors focus sunlight into a small beam that can be used to operate a boiler. That produces steam to run turbines to generate electricity. This method will be used at the Solana Generating Station, which is being built by the APS utility company outside of Phoenix, Arizona, in the United States. When completed in 2012, Solana will be one of the largest solar power stations in the world. Once operating at full capacity, it will serve 70,000 homes.

“This is a major milestone for Arizona in our efforts to increase the amount of renewable energy available in the United States,” said former Arizona Gov. Janet Napolitano.

There are some challenges with solar power. First, it is intermittent, or not continuous. When there is no sun—at night, for example—power cannot be generated. In order to provide continuous power, either storage or other energy sources, such as wind power, must be used. Second, while both photovoltaic and concentrating solar power can be used virtually anywhere, the equipment they require takes up a lot of space. Installation, except for on existing structures, can have a negative impact on the ecosystem by displacing plants and wildlife. Lastly, the cost to collect, convert and store solar power is very high. However, as technological advancements are made and demand rises, the costs are dropping.

Fossil fuels, such as coal, oil and natural gas, currently produce most of our electric and engine power. They also produce almost all of our pollution. Plus, they are non-renewable, meaning there is a limited supply.

The sun, on the other hand, offers free and clean energy in abundance. In fact, it gives much more energy than we can ever possibly use. The only questions are how and when we will take full advantage of it.

The Power of the Sun
The solar panel to the right probably can't help provide electricity to the shack on the left, but it can help lower the energy costs for the neighborhood.

African Energy
16,000 square kilometers (9,942 square miles) of solar power plants in North Africa could generate enough electricity for all of Europe.

Electrified Nation
Ranked third in the world in population, the United States uses more electricity than any other country, even the entire European Union of 27 nations.

Solar Power
In 15 minutes, the sun radiates as much energy as people use in all forms in an entire year.

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

autoclave
Noun

an airtight, steam-heated device used to sterilize medical instruments.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

CE
Adjective

Common Era. CE designates the years following 1 BCE, including the current year.

Noun

complex way of life that developed as humans began to develop urban settlements.

Noun

all weather conditions for a given location over a period of time.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

coal
Noun

dark, solid fossil fuel mined from the earth.

convert
Verb

to change from one thing to another.

Noun

agricultural produce.

crop rotation
Noun

the system of changing the type of crop in a field over time, mainly to preserve the productivity of the soil.

cultivate
Verb

to prepare and nurture the land for crops.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

deforestation
Noun

destruction or removal of forests and their undergrowth.

dense
Adjective

having parts or molecules that are packed closely together.

developing world
Noun

nations with low per-capita income, little infrastructure, and a small middle class.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Noun

community and interactions of living and nonliving things in an area.

electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.

essential
Adjective

needed.

fossil fuel
Noun

coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

gravitational pull
Noun

physical attraction between two massive objects.

greenhouse
Noun

building, often made of glass or other clear material, used to help plants grow.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

Horace de Saussure
Noun

(1740-1799) Swiss scientist and mountain climber.

installation
Noun

placing or setting up for use.

intermittent
Adjective

starting and stopping, not consistent.

livestock
noun, plural noun

animals raised for sale and profit.

Manmohan Singh
Noun

prime minister of India since 2004.

mica
Noun

type of mineral that can be split into thin, see-through sheets.

microbe
Noun

tiny organism, usually a bacterium.

mount
Verb

to climb up or get on top of.

Noun

type of fossil fuel made up mostly of the gas methane.

oil
Noun

fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

oxygenate
Verb

to fill with oxygen.

pasteurize
Verb

to heat a liquid to a high temperature to destroy harmful microorganisms.

photovoltaic
Adjective

able to convert solar radiation to electrical energy.

Noun

introduction of harmful materials into the environment.

population
Noun

total number of people or organisms in a particular area.

potable
Adjective

suitable for drinking.

precise
Adjective

exact.

protozoa
Noun

one-celled organisms in the kingdom protista, such as amoebas. (singular: protozoan)

radiate
Verb

to move outward from a central spot.

radio transmitter
Noun

device that sends out sound signals.

renewable energy
Noun

energy obtained from sources that are virtually inexhaustible and replenish naturally over small time scales relative to the human life span.

Roman emperor
Noun

supreme ruler of ancient Rome in its imperial period (the Roman Empire) from 27 BCE to the fall of the empire in 476 CE.

society
Noun

large community, linked through similarities or relationships.

solar architecture
Noun

the planning and design of buildings to make the most use of the sun's heat and light.

solar cooker
Noun

oven that uses sunlight to heat food.

Noun

radiation from the sun.

solar power
Noun

rate of producing, transferring, or using solar energy.

south
Noun

direction to the right of a person facing the rising sun.

spoil
Verb

to rot or ruin.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

sun
Noun

star at the center of our solar system.

surplus
Noun

more than what is needed or wanted.

telecommunications
Noun

the science and technology of sending and receiving information over long distances using electric, radio, or light signals.

thermal mass
Noun

the ability of a substance or structure to store heat.

Tiberius
Noun

(42 BCE-37 CE) second emperor of the Roman Empire.

translucent
Adjective

almost clear.

turbine
Noun

machine that captures the energy of a moving fluid, such as air or water.

Vanguard I
Noun

(1958) oldest satellite still in orbit, and the first that was solar-powered.

virus
Noun

pathogenic agent that lives and multiplies in a living cell.

wind
Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.