Daylight

Daylight is light from the sun.

It happens when the sun is up. The sun's light hits the earth. This light helps us see. It can be very bright. The sun at noon is very bright. It can even hurt your eyes. Sometimes the light is dim. Thick storm clouds make it dark. Smoke or dust can dim it too. Moonlight is not daylight. Daylight is a wonderful thing to see.

Daylight is all the light we get from the sun.

It includes light that hits us directly. It also includes light that bounces off the sky. This light can hit the land or buildings too.

Even if the sun is below the horizon, there is still some daylight. This is because the air bends the light. We call this refraction. This means more than half of Earth has daylight at any time.

Daylight can be very bright or very dim. At noon, direct sunlight is very bright. It can reach 120,000 lux. This much light might cause eye pain. On a cloudy day, it is much dimmer. Thick storm clouds can make it less than 5 lux. Smoke, dust, or ash can also make things dark. A solar eclipse can make it dark too. Moonlight is not daylight. This is because moonlight is light that reflects off the moon. Daylight must come from the sun itself.

Daylight is the light we see during the day.

It is a mix of different kinds of light. Some light comes straight from the sun. This is called direct sunlight. Other light comes from the sky. This is called diffuse sky radiation. Light also bounces off the land and buildings. We do not call moonlight daylight. Moonlight is light that reflects off the moon. Daylight must come from the sun itself.

Daylight works in a very interesting way. It is present whenever the sun is above the horizon. The sun is the line where the sky meets the land. Even when the sun is below that line, we see light. This happens because the atmosphere refracts the light. Refraction means the air bends the light rays. Because of this, more than 50% of Earth has daylight at once.

Scientists use a measurement called lux to show brightness. This helps us understand how strong the light is. Bright sunlight at noon can reach 120,000 lux. This much light might even cause eye pain. On a cloudy day, the light is much lower. A typical overcast day at midday is 1,000 to 2,000 lux. Sunrise or sunset on a clear day is 400 lux.

Sometimes, the world can get very dark during the day. A solar eclipse can make things dark. Smoke, dust, or volcanic ash can also block the light. In New England, there was even a "Dark Day." Thick storm clouds can make light less than 5 lux. At the most extreme, it can be less than 1 lux. This can make shadows from street lights visible.

We can compare daylight to the light at night. Nighttime is much, much darker than the day. Moonlight on a clear night is less than 1 lux. A full moon is only 0.25 lux. Starlight is even dimmer than that. On a moonless night, starlight is about 0.0002 lux. Even the planet Venus is only 0.0001 lux. Daylight is always the brightest part of our world.

Daylight is the total combination of all direct and indirect sunlight during the daytime. It is more than just the rays hitting us straight from the sun. Daylight includes direct sunlight and diffuse sky radiation. Diffuse sky radiation is light that is scattered throughout the sky. It also includes light reflected by Earth and terrestrial objects. These objects include landforms and buildings. Even though moonlight is reflected sunlight, it is not considered daylight. Daylight is defined by the Sun being above the local horizon.

How daylight reaches us depends on the Earth's atmosphere. The atmosphere performs a process called refraction. Refraction is when the air bends the light rays. This bending allows light to reach parts of the Earth even when the Sun is below the horizon. Because of this refraction, more than 50% of the Earth receives daylight at any given time. This means daylight is a global phenomenon that covers more than half our planet constantly.

Scientists measure the brightness of this light using a unit called lux. Outdoor illuminance varies greatly depending on the weather and the time of day. At its brightest, direct sunlight at noon can reach 120,000 lux. This level of light is so strong it may cause eye pain. Bright sunlight is slightly less, measured at 111,000 lux. Another specific measurement is the AM 1.5 global solar spectrum. This sunlight measures 109,880 lux or 1,000.4 W/m2.

Other conditions create much lower levels of light. If you are in the shade at midday, you might see 20,000 lux. This is light illuminated by the entire clear blue sky. On a typical overcast day at midday, the light drops to 1,000 to 2,000 lux. A clear day during sunrise or sunset provides about 400 lux of ambient illumination. Even more extreme conditions exist. Thick storm clouds can drop the light to less than 200 lux. In the most extreme cases of thick clouds, it may be less than 5 lux. This can be so dark that shadows from distant street lights become visible.

Sometimes, unusual circumstances can make the day much darker than normal. A solar eclipse can cause a sudden drop in light. High levels of atmospheric particulates can also block the sun. Particulates are tiny pieces of matter floating in the air. These include dust, volcanic ash, and smoke. There was even a historical event called New England's Dark Day caused by smoke. In these extreme cases, the light can fall below 1 lux.

We can understand the power of daylight by comparing it to nighttime illuminance. Nighttime is significantly dimmer than any daylight condition. Moonlight on a clear night sky is less than 1 lux. A full Moon on a clear night is only 0.25 lux. A quarter Moon provides only 0.01 lux. Starlight and airglow on a moonless night are even lower, at 0.0002 lux. If you exclude airglow, starlight is 0.00014 lux. Even the brightest planet, Venus, is only 0.0001 lux on a clear night.

Daylight is a complex system of light and atmosphere. It connects the movement of our planet to the physics of light. The way light bends and scatters determines how we see our world. It affects everything from how much energy we receive to how we see shadows. Understanding these levels of lux helps scientists study the atmosphere and the sun. It also helps us understand the relationship between the Earth and other objects in our solar system.