An observer on the Earth's surface can see a tiny area of the sky that resembles a dome (often referred to as a "sky bowl") that appears flatter during the day than at night. The sky appears blue during the day because air molecules scatter shorter wavelengths of sunlight more than longer wavelengths (redder light). At night, when there are no clouds, stars can be seen within the bowl of the sky. Gas molecules in the atmosphere also emit radiation, but it is mostly at infrared frequencies and so does not contribute significantly to the color of the night sky.
At twilight, when the Sun is just below the horizon, its rays strike the atmosphere directly above the point on the ground where you are standing; therefore, it is said to be "rising." As twilight turns into full darkness, the Moon begins to rise. If it is a clear night with no clouds, then you will be able to see it against the blackness of space. However, if it is a moonless night, then you will have to use other means to mark the passage of time until the first star appears in the east.
The Milky Way galaxy shines with over 100 billion stars including our own Sun. It is an enormous spiral structure made up of hundreds of billions of stars which are packed into a small volume of space. Our galaxy is one of many such structures in the Universe. It is believed that all large galaxies evolved from smaller ones by accumulating matter through mergers or interactions with other galaxies.
Blue As viewed from orbit, the daylight sky is truly blue. If you look attentively at any color-accurate snapshot of the Earth taken from orbit, you will see an unmistakable blue tinge to everything on the day side. The sky has a blue hue to it.
The photo above was taken by one of our astronauts aboard the International Space Station. It shows part of Brazil, with its intense green colors punctuated by white clouds and a thick layer of brownish haze over most of South America.
The photo below was taken by astronaut Tim Peake during his last mission in February 2016. It shows large parts of Europe, including France, Spain, and Italy. You can see many colors in the landscape, ranging from bright greens to deep blues to soft pinks. But despite the diversity of colors, they all seem to blend together into a single uniform dark blue.
The photo below was taken by astronaut Ron Garan in May 2009. It shows part of Africa with its vast deserts and barren land surfaces painted in various shades of red, orange, yellow, and gold.
The photo below was taken by astronaut Michael Fossum in April 2001. It shows part of North America with its dense forest cover and wide open spaces painted in vibrant colors. The photo was taken from the International Space Station where it provides a unique view of how green our planet is!
The daylight sky as seen from space is not a solid, uniform blob of blue for two reasons: (1) there are white clouds in the sky, which can be seen from space just as well as from the earth's surface, and (2) the sky is not opaque. NASA made the photograph available in the public domain. Anyone can use it for any purpose whatsoever without having to ask permission first.
During daytime, the earth's surface is illuminated by sunlight. The light from the sun reaches all parts of the earth simultaneously, but only some of it is reflected back towards space. The rest is absorbed by objects on the ground, such as plants and buildings. This is why night skies are dark - they're not actually black, instead they're filled with bright stars because much of the incoming light is refracted or bent around objects on the ground. At night, when no land masses block out part of the sky, every part of Earth's surface is exposed to sunlight at some time during the day.
From space, the entire globe is always lit up by sunlight, even during the day. But due to cloud cover and other factors, not everyone gets to see all of this light at once. Where clouds pass over large bodies of water, their shadows will move across the ocean creating a cloudy shadow zone that blocks out portions of the sky from view.
The sky is blue because of a process known as Raleigh scattering. This scattering is the scattering of electromagnetic energy (of which light is a kind) by particles with significantly smaller wavelengths. These shorter wavelengths correlate to blue colours, which is why we perceive blue when we gaze at the sky. The scattering occurs because water droplets in the air reflect and refract light from objects below them. The color of this reflected light depends on the type of particle that scatters it. If the particle is small enough, it will scatter blue light and leave red light behind. Scattered red light will not reach the eye because there are no red receptors in our vision systems. Therefore, all we see is blue light.
Raleigh scattering is what makes the sky appear blue. It also explains why clouds look white from far away or near distance: All clouds have particles with sizes less than the wavelength of visible light (about 0.4 microns), so they can scatter blue light and leave red light behind. When clouds are close up, you can see how some clouds contain mostly red particles while others are more blue, but even then, only a fraction of the cloud is scattered red, so most of it remains white.
You might wonder: Why don't all clouds look like they do from far away? That's because clouds are not uniform. There are large areas of different types of clouds, so from a distance they look like a mixture of white and gray.
Blue A bright, cloudless midday sky is blue because molecules in the air scatter more blue than red light from the sun.
Green The color of grass and trees. At a distance, green indicates land, not water.
Gray Colorless, like clouds at a distance.
Hazy White, with no definite color.
Orange You can see for miles in an orange sunset.
Pink Very dark, almost black.
Purple Dark blue-black.
Red Very bright in the daytime, at night it's pale pink or purple.
Silver Almost white, but with a glimmering brightness.
Yellow Very bright in the sunlight, at night it's blue-white.
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