The Sun's Energy: An Essential Part of the Earth System
Energy from the Sun is studied as part of heliophysics, which relates to the Sun’s physics and the Sun’s connection with the solar system.
How Does Energy from the Sun Reach Earth?
It takes solar energy an average of 8 ⅓ minutes to reach Earth from the Sun. This energy travels about 149 million km (93 million miles) through space to reach the top of Earth’s atmosphere. Waves of solar energy radiate, or spread out, from the Sun and travel at the speed of light through the vacuum of space as electromagnetic radiation. The majority of the Sun’s radiation reaching Earth is in the form of visible light we can see and invisible infrared energy that we can’t see. A smaller portion of sunlight is made up of ultraviolet radiation, which is also invisible to our eyes.
Other forms of radiation from the Sun can reach Earth as part of the solar wind, but in smaller quantities and with longer travel times. The solar wind contains plasma and particles and can also include gamma rays and x-rays resulting from solar storms or other bursts of energy from the Sun’s surface.
The Energy We Receive Depends on Distance From the Sun
The Sun’s energy we receive is electromagnetic radiation that travels through space or a medium in the form of waves or particles. The visible light we see from the Sun is similar to the visible light from a light bulb, but the Sun provides much more of it. The energy released from the Sun is mind-bogglingly large, equal to about 36 octillion (3.6 x 1028) or 36 thousand trillion trillion lumens, where a lumen is a measure of the total amount of visible light. This brightness (or intensity) falls off with distance from the source by a factor of 1/d2, where d2 is the square of the distance. From the surface of Mercury, depending on the planet's position in its orbit, the Sun would appear from seven to eleven times brighter as it does when viewed from Earth. By the time the Sun's energy reaches Earth's surface, it has an average brightness of about 127,000 lumens per square meter, or about 127 light bulbs if we consider that a typical light bulb produces about 650-1600 lumens of visible light.
The Sun’s Energy is Important to Life on Earth
Energy from the Sun makes it possible for life to exist on Earth. It is responsible for photosynthesis in plants, vision in animals, and many other natural processes, such as the movements of air and water that create weather. Most plants need at least some sunlight to grow, so without light, there would be no plants, and without plants, there would not be oxygen for us to breathe. Infrared radiation from the Sun is responsible for heating the Earth’s atmosphere and surface. Without energy from the Sun, Earth would freeze. There would be no winds, ocean currents, or clouds to transport water.
How do Humans Benefit from the Sun's Energy?
Throughout history, humans have used technology to harness the Sun’s energy as a source of light and heat and for growing crops. As early as 30 CE, people were constructing greenhouses to grow plants out of season. Did you know that one of the earliest greenhouses was built for the Roman emperor Tiberius using translucent sheets of mica, all so that he could eat cucumbers in any season?
Early civilizations knew that setting up shelters to capture or block sunlight could help with heating and cooling. In cold climates, they would position buildings to face southward, allowing interior spaces to gather heat and light. In warm climates, shelters might be constructed under cliffs or natural overhangs to protect the residents from the Sun’s radiation in the hottest part of the day. Using energy from the Sun effectively also helped with growing crops.
Today, we can intentionally position windows and skylights to help heat or cool our homes through passive solar design. Solar panels can also capture energy from the Sun by gathering sunlight and converting it to electricity. As of 2023, solar power is the third largest source of renewable energy worldwide, behind hydropower and wind.
How is Energy from the Sun Harmful?
Some of the Sun’s energy reaches Earth in the form ultraviolet (or UV) radiation. Fortunately, the ozone layer high in Earth’s atmosphere absorbs a lot of this UV radiation and blocks it from reaching Earth’s surface. But some UV still makes it through. UV radiation from the Sun causes sunburn and skin damage but can be blocked with clothes and sunscreen. Wearing hats and sunglasses in bright sunlight and trying to stay in the shade when possible can also help protect you from harmful UV rays. Note that it’s not realistic or advisable to avoid all exposure to sunlight, as sunlight also helps our bodies produce vitamin D, which we need to be healthy.
UV radiation can damage plants and limit photosynthesis. Scientists have found that overexposure to UV radiation reduces size, productivity, and quality in several crop species, including many varieties of rice, soybeans, winter wheat, cotton, and corn. Sometimes, UV can cause bleaching of plants, visible as damage or discoloration. Planting tender young plants in the shadow of larger plants can help protect them from UV radiation.