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Discovery, Symbol, Properties,
Uses, & Facts


The Power of the Sun | National Geographic Society

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 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.

Solar Radiation Basics | Department of Energy

Solar radiation, often called the solar resource or just sunlight, is a general term for the electromagnetic radiation emitted by the sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource.


Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one spot on the Earth’s surface varies according to:

• Geographic location

• Time of day

• Season

• Local landscape

• Local weather.

Because the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the sun’s rays are vertical, the Earth’s surface gets all the energy possible. The more slanted the sun’s rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year.

The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy. The Earth is nearer the sun when it is summer in the southern hemisphere and winter in the northern hemisphere. However, the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.

The 23.5° tilt in the Earth’s axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other 6 months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.

Countries such as the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun’s rays are far more slanted during the shorter days of the winter months. Cities such as Denver, Colorado, (near 40° latitude) receive nearly three times more solar energy in June than they do in December.

The rotation of the Earth is also responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon, when the sun is at its highest point. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon.


As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by:

• Air molecules

• Water vapor

• Clouds

• Dust

• Pollutants

• Forest fires

• Volcanoes.

This is called diffuse solar radiation. The solar radiation that reaches the Earth’s surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.


Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface,or as total radiation on a surface tracking the sun.

Radiation data for solar electric (photovoltaic) systems are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).

Radiation data for solar water heating and space heating systems are usually represented in British thermal units per square foot (Btu/ft2).


The solar resource across the United States is ample for photovoltaic (PV) systems because they use both direct and scattered sunlight. Other technologies may be more limited. However, the amount of power generated by any solar technology at a particular site depends on how much of the sun’s energy reaches it. Thus, solar technologies function most efficiently in the southwestern United States, which receives the greatest amount of solar energy.

What is the NSRDB

Solar system designers, planners, and engineers use NSRDB data to predict the potential solar energy available in a location based on what has been available in the past. This helps to optimize the production for solar installations and reduce the risk for investors. Photo by Dennis Schroeder, NREL 31437

The NSRDB is a serially complete collection of hourly and half-hourly values of the three most common measurements of solar radiation—global horizontal, direct normal, and diffuse horizontal irradiance—and meteorological data. The current NSRDB is modeled using multi-channel measurements from geostationary satellites. The older versions of the NSRDB were modeled using cloud and weather information primarily collected at airports. Sufficient number of locations and temporal and spatial scales were used to represent regional solar radiation climates accurately.

Data flow from satellite to solar radiation measurement

Using the NSRDB data, it is possible to estimate the amount of solar energy that has been historically available at a given time and location anywhere in the United States; the NSRDB is also expanding to encompass a growing list of international locations. Using the long-term NSRDB data in various models, it is possible to predict the potential future availability of solar energy in a location based on past conditions.

Typical Meteorological Year (TMY) data can be derived from the NSRDB time-series datasets. Visit the TMY page for detailed information about this data type and its uses.

The latest addition to the NSRDB is spectral datasets. Spectral datasets are calculated on demand based on user specifications of tilt and orientation. Please visit the Spectral Datasets page for more information.

All NSRDB data was processed using Peregrine system which as of 2019 substituted by Eagle is the newest high-performance computing (HPC) system at NREL.Photo by Dennis Schroeder, NREL 31717


The NSRDB and TMY data are compatible with many system performance and economic models, including the following products created by NREL:

• System Advisor Model (SAM)

• PVWatts

• Renewable Energy Potential (reV) model

Contributing Partners

Data collection for the NSRDB is a collaborative effort. Contributors include:

• National Renewable Energy Laboratory

• U.S. Department of Energy

• National Oceanic and Atmospheric Administration

• National Aeronautics and Space Administration

• University of Wisconsin

• Solar Consulting Services, Colebrook, New Hampshire

• Atmospheric Sciences Research Center, State University of New York at Albany

• National Climatic Data Center, U.S. Department of Commerce

• Solar Radiation Monitoring Laboratory, University of Oregon.

Radiometry for light Measurement | ILT (


Radiometry light meters

Radiometry is the detection and measurement of light waves in the optical portion of the electromagnetic spectrum which is further divided into ultraviolet, visible, and infrared light. All light measurement is considered radiometry with photometry being a special subset of radiometry weighted for a typical human eye response.

Manipulating light for more accurate measurement

Light waves behave like all electromagnetic waves in that they interfere with each other, become directionally polarized, or bend when passing an edge. Because of these properties, light waves can be predictably manipulated for increased or decreased amplitude or even filtered by wavelength or a band of wavelengths by means of various filters, mirrors, lenses, prisms, gratings, and other input optics.

These principals make it feasible to tailor a light detector to match a particular application.

In most cases, when making radiometric measurements with silicon photodiodes, a flat spectral response filter is used to even out the detector’s innate sensitivity to the red portion of the spectrum versus the blue. This flat response allows the detector to accurately measure light at one wavelength versus another. This is particularly useful if the spectral output of the light source is unknown or likely to vary with operating conditions or application requirements.

In the event that a particular section of the output from a light source needs to be isolated, a detector with a narrower response can be used, or a band pass filter can be placed on a broad response detector to tailor it’s response to measure just the required portion of light. Narrow band filters can also be used to isolate a very narrow portion of light such as that of a specific mercury emission line.

Radiometry instrumentation for nearly any application

International Light Technologies (ILT) offers a broad array of radiometry detectors to allow configuration of a radiometer system to measure nearly any application from the UV (160 nm) to far-Infrared (40 um). We also have high-gain radiometry detectors for very low level light detection and our powerful ILT950 Wideband Spectroradiometer or economical ILT550 Spectroradiometer for complete, turnkey spectral measurements.

Use the table below to identify the system (meter + detector) that meets your specific application. Use the table to find the spectral range you wish to measure. The table can be filtered to show our meters by type, (e.g., hand-held), as well as searching on the minimum and maximum spectral range you wish to measure. The tables can also be sorted to group systems by meter type, spectral range, measurement range, and units. Click the product link of the system to view it’s details.

* All Radiometers/Photometers/Spectroradiometers are NIST Traceable.

* If units of measure are not shown please contact us (empirical units also available i.e. fc, fL, nits, lm/ft²).

Ultraviolet Radiation: How it affectslife on earth (

The sun radiates energy in a wide range of wavelengths, most of which are invisible to human eyes. The shorter the wavelength, the more energetic the radiation, and the greater the potential for harm. Ultraviolet (UV) radiation that reaches the Earth’s surface is in wavelengths between 290 and 400 nm (nanometers, or billionths of a meter). This is shorter than wavelengths of visible light, which are 400 to 700 nm.

People and plants live with both helpful and harmful effects of ultraviolet (UV) radiation from the sun. (Photograph courtesy Jeannie Allen)

UV radiation from the sun has always played important roles in our environment, and affects nearly all living organisms. Biological actions of many kinds have evolved to deal with it. Yet UV radiation at different wavelengths differs in its effects, and we have to live with the harmful effects as well as the helpful ones. Radiation at the longer UV wavelengths of 320-400 nm, called UV-A, plays a helpful and essential role in formation of Vitamin D by the skin, and plays a harmful role in that it causes sunburn on human skin and cataracts in our eyes. The incoming radiation at shorter wavelengths, 290-320 nm, falls within the UV-B part of the electromagnetic spectrum. (UV-B includes light with wavelengths down to 280 nm, but little to no radiation below 290 nm reaches the Earth’s surface). UV-B causes damage at the molecular level to the fundamental building block of life— deoxyribonucleic acid (DNA).

Electromagnetic radiation exists in a range of wavelengths, which are delineated into major divisions for our convenience. Ultraviolet B radiation, harmful to living organisms, represents a small portion of the spectrum, from 290 to 320 nanometer wavelengths. (Illustration by Robert Simmon)

DNA readily absorbs UV-B radiation, which commonly changes the shape of the molecule in one of several ways. The illustration below illustrates one such change in shape due to exposure to UV-B radiation. Changes in the DNA molecule often mean that protein-building enzymes cannot “read” the DNA code at that point on the molecule. As a result, distorted proteins can be made, or cells can die.

Ultraviolet (UV) photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent bases bond with each other, instead of across the “ladder.” This makes a bulge, and the distorted DNA molecule does not function properly. (Illustration by David Herring)

But living cells are “smart.” Over millions of years of evolving in the presence of UV-B radiation, cells have developed the ability to repair DNA. A special enzyme arrives at the damage site, removes the damaged section of DNA, and replaces it with the proper components (based on information elsewhere on the DNA molecule). This makes DNA somewhat resilient to damage by UV-B.

In addition to their own resiliency, living things and the cells they are made of are protected from excessive amounts of UV radiation by a chemical called ozone. A layer of ozone in the upper atmosphere absorbs UV radiation and prevents most of it from reaching the Earth. Yet since the mid-1970s, human activities have been changing the chemistry of the atmosphere in a way that reduces the amount of ozone in the stratosphere (the layer of atmosphere ranging from about 11 to 50 km in altitude). This means that more ultraviolet radiation can pass through the atmosphere to the Earth’s surface, particularly at the poles and nearby regions during certain times of the year.

Without the layer of ozone in the stratosphere to protect us from excessive amounts of UV-B radiation, life as we know it would not exist. Scientific concern over ozone depletion in the upper atmosphere has prompted extensive efforts to assess the potential damage to life on Earth due to increased levels of UV-B radiation. Some effects have been studied, but much remains to be learned.