Night sky

The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. It is a beautiful and fascinating view that has captured human imagination for thousands of years.

Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing. Aurorae light up the skies above the polar circles, and sometimes, powerful events from the Sun can make these colorful lights visible much farther from the poles.

Stars in the night sky

The night sky has played an important role in many cultures and throughout history. Ancient farmers used the night sky as a calendar to help decide when to plant their crops. Many cultures created constellations by connecting stars and told legends and mythology about their deities using these patterns. Today, scientists study the night sky through observational astronomy to learn more about the universe.

Unfortunately, light pollution from cities makes it harder to see the night sky clearly. The brightness from streetlights and other sources creates skyglow that washes out faint stars and other objects. This problem is growing, but amateur astronomers and professionals often travel to dark locations to get the best views of the stars and planets.

Brightness

The Milky Way is brighter in the Southern Hemisphere than in the North. (Photo taken at La Silla Observatory)

Main article: Sky brightness

Even when there is no moonlight or city lights, the night sky is not completely dark. This is because of natural light sources such as airglow and the faint scattering of sunlight and starlight. The brightness of the sky changes throughout the day. During the day, sunlight scattered in the atmosphere makes the sky bright. After sunset, the sky gradually darkens through periods called civil, nautical, and astronomical twilight, until it reaches its darkest point when the Sun is far below the horizon.

Visual presentation

Depending on where you are and the conditions, the stars in the night sky can look like hundreds or thousands of tiny white points of light. Long ago, people thought the stars were all the same distance from Earth because they looked like they were stuck on a big dome. Stars can be blue or red depending on their temperature, but most look white to our eyes.

To see the most stars, it is best to go to a place far from city lights where it is very dark. It takes a little while for our eyes to adjust to the dark so we can see more stars. Using a red flashlight helps us see things like star maps without ruining our night vision.

Constellations

Star maps can help us find patterns in the stars called constellations. Different cultures see different pictures in these star patterns. Some well-known constellations are Orion and the Big Dipper, which can help find the north star, Polaris.

Paranal Observatory nights. The concept of noctcaelador tackles the aesthetic perception of the night sky.

Planets

Planets move slowly across the sky and can look like bright stars. They shine because the Sun's light reflects off them. Venus is often the brightest object in the night sky and can be seen near sunrise or sunset. Other planets like Mercury, Mars, Jupiter, and Saturn can also be seen without special equipment.

The Moon

The Moon looks like a grey circle with craters and changes its shape over about a month. It is bright enough to see even during the daytime. Near sunrise or sunset, the Moon can look orange or red because of Earth’s shadow.

Comets

The Milky Way contains billions of stars, arranged in two strikingly different structures: halo and disc.

Comets are rare visitors to the night sky. When they appear, they often have tails that point away from the Sun. A really bright comet, called a great comet, appears about once every ten years.

Clouds

Clouds can hide stars and planets. Thin clouds might make a colorful ring around the Moon, while thicker clouds hide everything completely.

Other objects

On very dark nights, you might see the Milky Way, our galaxy, as a fuzzy band of light. Sometimes satellites move across the sky and look like stars, but they move quickly. Meteors, or shooting stars, sometimes flash across the sky, especially during meteor showers.

Sky map

Main article: Starchart

The night sky is filled with stars, and special maps called starcharts help us locate them. There are different maps for the Southern Hemisphere and the Northern Hemisphere, showing the brightest stars and groups of stars called asterisms. These maps also highlight important features of our galaxy, like dark areas and groups of stars, as well as maps showing groups of galaxies in the night sky.

Future and past

Main articles: Fixed star § "Fixed stars" not fixed, Variable star, and Astronomical chronology

This series of photo illustrations shows the predicted merger between the Milky Way and the neighboring Andromeda Galaxy.

The night sky is always changing. Stars move slowly across the sky and can get brighter or dimmer over time. Sometimes, very bright stars explode in events called supernovas.

In the very far future, over many billions of years, our Milky Way galaxy will slowly merge with the nearby Andromeda Galaxy, creating one large galaxy. This will change how the night sky looks from Earth.