Night sky

There are no markings on the night sky, though there exist many sky maps to aid stargazers in identifying constellations and other celestial objects. Constellations are prominent because their stars tend to be brighter than other nearby stars in the sky. Different cultures have created different groupings of constellations based on differing interpretations of the more-or-less random patterns of dots in the sky. Constellations were identified without regard to distance to each star, but instead as if they were all dots on a dome.

Orion is among the most prominent and recognizable constellations.[6] The Big Dipper (which has a wide variety of other names) is helpful for navigation in the northern hemisphere because it points to Polaris, the north star.

The pole stars are special because they are approximately in line with the Earth's axis of rotation so they appear to stay in one place while the other stars rotate around them through the course of a night (or a year).

Planets, named for the Greek word for "wanderer," process through the starfield a little each day, executing loops with time scales dependent on the length of the planet's year or orbital period around the Sun. Planets, to the naked eye, appear as points of light in the sky with variable brightness. Planets shine due to sunlight reflecting or scattering from the planets' surface or atmosphere. Thus, the relative Sun-planet-Earth positions determine the planet's brightness. With a telescope or good binoculars, the planets appear as discs demonstrating finite size, and it is possible to observe orbiting moons which cast shadows onto the host planet's surface. Venus is the most prominent planet, often called the "morning star" or "evening star" because it is brighter than the stars and often the only "star" visible near sunrise or sunset, depending on its location in its orbit. Because of its brightness, Venus can sometimes be seen after sunrise. Mercury, Mars, Jupiter and Saturn are also visible to the naked eye in the night sky.

Earth's Moon appears as a grey disc in the sky with cratering visible to the naked eye. It spans, depending on its exact location, 29–33 arcminutes – which is about the size of a thumbnail at arm's length, and is readily identified. Over 29.53 days on average, the moon goes through a full cycle of lunar phases. People can generally identify phases within a few days by looking at the Moon. Unlike stars and most planets, the light reflected from the Moon is bright enough to be seen during the day.

Some of the most spectacular moons come during the full moon phase near sunset or sunrise. The Moon on the horizon benefits from the Moon illusion which makes it appear larger. The Sun's light reflected from the Moon traveling through the atmosphere also appears to color the Moon orange and/or red.

Comets come to the night sky only rarely. Comets are illuminated by the Sun, and their tails extend away from the Sun. A comet with a visible tail is quite unusual – a great comet appears about once a decade. They tend to be visible only shortly before sunrise or after sunset because those are the times they are close enough to the Sun to show a tail.

Clouds obscure the view of other objects in the sky, though varying thicknesses of cloud cover have differing effects. A very thin cirrus cloud in front of the moon might produce a rainbow-colored ring around the moon. Stars and planets are too small or dim to take on this effect and are instead only dimmed (often to the point of invisibility). Thicker cloud cover obscures celestial objects entirely, making the sky black or reflecting city lights back down. Clouds are often close enough to afford some depth perception, though they are hard to see without moonlight or light pollution.

The Milky Way can be seen as a very large streak or arc across the night sky if the visibility conditions are good enough. This panoramic photo was taken at Death Valley.

The Milky Way contains billions of stars, arranged in two strikingly different structures: halo and disc.[7]

On clear dark nights in unpolluted areas, when the Moon appears thin or below the horizon, the Milky Way, a band of what looks like white dust, can be seen.

The Magellanic Clouds of the southern sky are easily mistaken to be Earth-based clouds (hence the name) but are in fact collections of stars found outside the Milky Way known as dwarf galaxies.

Zodiacal light is a glow that appears near the points where the Sun rises and sets, and is caused by sunlight interacting with interplanetary dust.

Gegenschein is a faint bright spot in the night sky centered at the antisolar point, caused by the backscatter of sunlight by interplanetary dust.

Shortly after sunset and before sunrise, artificial satellites often look like stars – similar in brightness and size – but move relatively quickly. Those that fly in low Earth orbit cross the sky in a couple of minutes. Some satellites, including space debris, appear to blink or have a periodic fluctuation in brightness because they are rotating. Satellite flares can appear brighter than Venus, with notable examples including the International Space Station (ISS) and Iridium Satellites.

Meteors (commonly known as shooting stars) streak across the sky very infrequently. During a meteor shower, they may average one a minute at irregular intervals, but otherwise their appearance is a random surprise. The occasional meteor will make a bright, fleeting streak across the sky, and they can be very bright in comparison to the night sky.

Aircraft are also visible at night, distinguishable at a distance from other objects because their navigation lights blink.