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Solar radiation is the spectrum of light emitted by the sun due to nuclear fusion in its core. It is studied for its effects on Earth’s weather and astronomy. Only a small fraction of solar radiation reaches Earth, but it is still much greater than energy consumed on Earth. Solar radiation is measured in watts per square meter using instruments such as pyranometers and pyrheliometers. Variations in solar radiation are caused by Earth’s orbit and periodic fluctuations, but they are usually small.
Solar radiation is the entire spectrum of light emitted by the sun. Includes visible light and all other frequencies of radiation in the electromagnetic spectrum. Compared to the familiar energy sources on Earth, the sun emits a huge amount of energy into space. The type of radiation emitted by the sun is a product of its high temperature, which is caused by nuclear fusion within the sun’s core. Solar radiation is studied by scientists for its effects on phenomena on Earth, such as the weather, and for the science of astronomy.
The sun was formed about 4.5 billion years ago when a cloud of gas collapsed on itself. Gravity caused the large amount of matter, which was mostly hydrogen, to coalesce tightly; very high pressure resulted in the core of the sun. This pressure became so great that the hydrogen atoms began to fuse together, a process that releases a lot of heat energy. The temperature of any collection of matter, including the sun, is what gives rise to the radiation it emits.
Only a small fraction of solar radiation reaches the Earth; the majority are radiated into empty space. Even the fraction that reaches Earth, however, is much greater than the amount of energy consumed on Earth from sources such as fossil fuels. The average power consumed by humans during 2008 was approximately 1.5 x 1013 watts. In comparison, the average power reaching the Earth through sunlight is more than 10,000 times greater. The enormous amount of energy radiated by the sun can be attributed to its large mass and high temperature.
Sunlight is measured in different ways. An instrument that measures sunlight from a full 180 degree field of view is called a pyranometer. A pyrheliometer is a device aimed directly at the sun for measuring radiation. Solar radiation is measured in a quantity called irradiance, which has standard units of watts per square meter. Average solar irradiance at Earth’s distance from the sun is about 1.366 watts per square meter.
Measurements of solar radiation tend to vary slightly over time. Part of the reason for this fluctuation is that Earth’s orbit is not perfectly circular and ground-based observers are constantly changing their distance from the sun. Actual variations in solar radiation also cause fluctuations in measurements, but these quantities are usually small. Some of these variations have been observed to follow an 11-year cycle. Periodic fluctuations like these have been measured to affect solar irradiance levels by 0.1%
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