Sky
The sky is what you see when you look upward from the Earth. It includes the air around us and the space beyond. Some people think of the sky as the area from the ground up to space, while others call it the celestial sphere in astronomy. The celestial sphere is an imagined sphere around Earth where the Sun, Moon, planets, and stars seem to move. It is divided into regions called constellations.
From the ground, the sky looks like a dome. In the daytime it often looks blue because air molecules scatter sunlight, and the shorter blue wavelengths scatter more than red. At night the sky becomes dark, and many stars become visible.
During the day the Sun is in the sky, and sometimes the Moon is visible too. At night you can see the Moon, planets, and many stars. The sky can also show other sights like clouds, rainbows, and the aurora. Lightning and rain are visible in the sky as well. Many living things—birds, bats, insects—and human-made things like airplanes and kites fly in the sky. In big cities, smog during the day and light pollution at night can dim the view.
The word sky comes from old languages, including Old Norse. Heaven was once used to describe the observable expanse above the Earth, and the word slowly shifted to its current religious meaning in English over time.
Light in the daytime sky mostly comes from sunlight scattering in the air. This scattering, called Rayleigh scattering, makes the sky look blue. Clouds scatter light and can appear white. The colors you see at sunrise or sunset—reds, oranges, purples and yellows—happen because sunlight travels through more air at those times, so more of the blue light is scattered away.
The Moon has no atmosphere, so its sky is black even when the Sun is shining on Earth. Colors near the horizon can change a lot because light must pass through a lot of air.
Sometimes you can see a green flash just after sunset or before sunrise. It is a short, green spot or streak above the Sun and is most often seen over a clear horizon, like the ocean. Earth’s shadow and the pink Belt of Venus can appear near the horizon during twilight.
Twilight is split into three stages, based on how far the Sun is below the horizon: civil twilight (sun less than 6° below the horizon), nautical twilight (sun 6° to 12° below), and astronomical twilight (sun 12° to 18° below). When the Sun is more than 18° below the horizon, the sky is usually at its darkest.
Brightening sources in the sky include airglow, light reflected or scattered by the atmosphere, starlight, and artificial city lights. The night sky is what people usually call when they talk about star gazing, but it is still affected by the Moon and city lighting.
Weather can be read in the sky. A clear sky often means fair weather, while changing clouds can signal rain. High, thin clouds in the evening can foreshadow a warm front; morning fog can indicate a stable air layer.
Flight is the act of moving through the sky. This can be done by airplanes, rockets, and other craft, or by natural flyers like birds and insects. The study of flight is part of aerospace engineering, which looks at air travel (aeronautics) and space travel (astronautics), as well as the flight of projectiles (ballistics).
People from many cultures have stories about the sky. Ancient myths told of sun gods, sky gods, and the stars, and people have long used constellations to tell legends and guide their journeys.
In short, the sky is the wide space above us, full of light, color, and movement—from the blue daytime atmosphere to the starry night, with weather patterns, Flight, and human stories all connected to what we see when we look up.
This page was last edited on 2 February 2026, at 03:48 (CET).