Sunlight

Sunlight is the energy that comes from the Sun. It is a kind of light and warmth that we feel when the Sun is shining. This light is part of something called electromagnetic radiation, which includes things we can see, like the bright daytime sky, and things we cannot see, like warmth and certain types of energy that can affect our skin.

When sunlight reaches Earth, it changes a little as it moves through the air. On clear days, we feel strong, direct sunlight, which makes us warm. On cloudy days, the sunlight is spread out more, making things feel softer and less bright.

Sunlight is very important for life on Earth. It helps plants make their food through a process called photosynthesis. It also helps our bodies make vitamin D, which is good for our bones and health. Even though too much sunlight can sometimes harm our skin, it plays a big role in keeping everything alive and healthy.

Measurement

Researchers can measure the strength of sunlight using special tools like a sunshine recorder, pyranometer, or pyrheliometer. They need to consider how far Earth is from the Sun and how Earth’s air affects the sunlight. Scientists have formulas to calculate how much sunlight reaches the ground.

Sunlight at the top of Earth’s air is very strong, but when it passes through the air, some of its strength is lost. The total energy from the Sun that reaches Earth’s surface changes slightly during the year. Most of this energy comes as visible light, with smaller amounts of warmth-giving infrared light and ultraviolet light, which can sometimes cause sunburn. Sunlight is a very efficient way to light up spaces, producing less heat than many types of artificial lights.

Composition and power

Sunlight is the energy the Sun sends out, and it includes many types of light. Most of it is visible light — the kind we see — but it also has infrared, which makes us feel warm, and ultraviolet, which can cause sunburns.

The Sun sends out energy in many wavelengths. Only a small part of the ultraviolet light reaches the ground, but it can still affect our skin. The biggest part of sunlight is infrared and visible light. Together, they make up most of the energy we get from the Sun.

Intensity in the Solar System

Different objects in the Solar System get light from the Sun, but the strength of this light changes depending on how far away they are. The farther an object is from the Sun, the weaker the light becomes.

For example, Venus's thick atmosphere reflects most of the sunlight it gets, so its surface isn't very bright. On Mars, the sunlight looks a bit like a cloudy day on Earth. Even faraway places like Pluto still get enough light to feel almost like being inside a room with normal lighting.

Variations in solar irradiance

Further information: Insolation and Sunshine duration

Sunlight changes depending on where you are on Earth and the time of year. In summer, areas far from the equator get more sunlight, while in winter, some places near the poles can have very little or no sunlight at all. When the sun shines without clouds blocking it, we feel warmth because the ground absorbs the sun's energy.

The amount of sunlight Earth gets also changes slightly over many years. Scientists have measured this using satellites since 1978. They’ve also studied older patterns by looking at spots on the sun and other natural clues, finding that sunlight levels go through cycles — some lasting about 11 years, and others much longer. These changes affect Earth’s climate over time.

Solar irradiance

Main article: Solar irradiance

Main article: Solar constant

The solar constant is the amount of energy from the Sun that reaches a certain area in space. It includes all kinds of sunlight, not just what we can see. Scientists have measured this amount and found it to be about 1,361 watts for every square meter.

Since 1978, special tools on satellites have measured how much sunlight reaches the top of Earth's atmosphere. These measurements help us understand how the Sun's energy affects our planet's climate. The tools also watch how the Sun's energy changes over time, which is important for learning about climate change.

Surface illumination and spectrum

See also: Diffuse sky radiation

The color of sunlight changes depending on where the Sun is in the sky. During sunrise and sunset, the sunlight appears red because it travels through more of Earth’s atmosphere. Blue light is seen more clearly when the Sun is high in the sky. These changes happen because of how Earth’s air scatters and absorbs light.

Sunlight helps us see colors because it contains many different colors of light. When sunlight reaches Earth directly, it shows a balanced mix of colors. But when sunlight passes through more air, like during sunrise or sunset, blue light gets scattered away, leaving mostly red and yellow colors. Water vapor, dust, and other particles in the air also affect which colors we see.

Main article: Natural light photography

Life on Earth

The Sun’s light is essential for almost all life on Earth. Plants, called autotrophs, use sunlight along with carbon dioxide and water to make sugars through a process called photosynthesis. These sugars help plants grow and provide food for other organisms.

Animals, known as heterotrophs, get their energy by eating plants or other animals. When they break down these foods, they release the stored energy from sunlight, which helps them survive. Long ago, humans learned to use plant and animal materials in new ways, such as using animal skins for clothing or wooden tools for hunting. This helped our ancestors make better use of the Sun’s energy. Later, during the Neolithic Revolution, people began to farm plants and raise animals, which gave them even more access to the energy stored in sunlight. Even today’s fossil fuels come from ancient plants and animals that once used the Sun’s energy.

Cultural aspects

Sunlight has inspired many artists, like Édouard Manet and Claude Monet, who painted beautiful outdoor scenes and landscapes.

Many people find direct sunlight too bright and can even hurt their eyes if they look straight at the Sun. To stay comfortable, people wear sunglasses, and cars, helmets, and caps often have visors to block the Sun’s glare. Buildings use walls, window blinds, awnings, shutters, curtains, or shade trees to keep sunshine out. However, a little sunlight is good for our bodies because it helps us make Vitamin D, which keeps our bones and muscles strong.

In many religions, like Hinduism and in Ancient Egypt, the Sun is valued as a source of life and energy.

Sunbathing

Main article: Sun tanning

Sunbathing is a fun activity where people sit or lie in the sunshine. Popular places for sunbathing include beaches, swimming pools, parks, gardens, and sidewalk cafes. Some people sunbathe without much clothing to get an even tan, while others use special indoor machines called sunbeds that create similar light. Sunbathing can help treat certain skin conditions and gives the skin a darker color, which some people like. This happens because of special cells in our skin that respond to sunlight.

Effects on human health

Main article: Health effects of sunlight exposure

Sunlight, especially its ultraviolet radiation, has both good and bad effects on our health. It helps our bodies make vitamin D₃, which is important for strong bones and may help prevent some diseases. However, too much sunlight can harm us. Long-term exposure can lead to skin aging, skin cancer, and eye problems. Short-term overexposure can cause sunburn and temporary eye issues.

Experts say it's important to find a balance. While sunlight can be harmful if we get too much, avoiding it completely might also be bad for our overall health. It's best to enjoy sunlight in moderation and protect our skin when needed.

Effect on plant genomes

Strong sunlight, especially a type called UV-B, can change the DNA inside plants like Arabidopsis thaliana and tobacco (Nicotiana tabacum). This change happens more often when there is more UV-B light. Plants respond by making more of a special enzyme that helps fix their DNA, which keeps their genes stable. This shows how important sunlight is for plant health.

Main article: DNA