The other colours pass through the Earth's atmosphere to reach us, but because of the great abundance of blue light wavelengths, our eyes see the sky as blue. Technically, the short wavelengths that scatter across the sky correspond to the colours blue and violet, making the real colour of the sky a bluish purple.
If we judge by the most prominent color, the sky is violet. But the sky appears blue due to the limitations of our eyes. Our sensitivity to light decreases as we reach the shortest wavelengths of the visible spectrum. The violet is there, but our eyes detect it only weakly.
However, technically, violet light is scattered the most, so the sky is actually a blueish violet. Biology is part of the reason we see the sky as blue. Those color-sensitive cones in our eyes react most strongly to blue, green, and red wavelengths; this is why we see colors the way we do.
At noon, when the Sun is overhead it appears white. This is because the light travels a shorter distance through the atmosphere to get to us; it's scattered very little, even the blue light. During the day the sky looks blue because it's the blue light that gets scattered the most.
Fed by nutrients in the sea and powered by the sun, cyanobacteria exploded across the ocean, pumping more and more oxygen into Earth's atmosphere. Slowly, over the next two billion years, oxygen in the atmosphere rose to its present levels, and the sky took on the blue hue on view today.
It turns out our sky is violet, but it appears blue because of the way our eyes work. We don't see individual wavelengths. Instead, the retinas of our eyes have three types of color sensitive cells known as cones.
To an astronaut, the sky looks dark and black instead of blue because there is no atmosphere containing air in the outer space to scatter sunlight. So, there is no scattered light to reach our eyes in outer space, therefore the sky looks dark and black there.
The blueness of the sky is also affected by other factors, such as the moisture and particulate matter in the air. So tropical countries, such as Singapore or India, which are closer to the equator and are more humid or have pollution or dust in the air, have less blue skies than Australia, most of the time.
The blue color of the sky is a result of this scattering process. At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered.
John Tyndall was a keen mountaineer and spent quite a lot of time in the Alps, both climbing and investigating phenomena such as glaciers. This interest in nature can also be seen in many of his other diverse discoveries, including his discovery in the 1860s of why the sky is blue in the day but red at sunset.
The water is in fact not colorless; even pure water is not colorless, but has a slight blue tint to it, best seen when looking through a long column of water. The blueness in water is not caused by the scattering of light, which is responsible for the sky being blue.
The particles and gases in the air scatter visible light all over the atmosphere, and since blue light has a shorter / smaller wave than most of the visible light spectrum, we see the sky as mostly blue during the day. During the evening however, the sky can change colors and become red, orange, yellow, etc.
Chlorophyll is the most common among plant pigments; it absorbs blue and yellow light and reflects green. This is why most plant leaves are green. Plants also have “accessory pigments” that improve the efficiency of light harvest: they absorb light that cannot be absorbed by chlorophyll.
Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. Closer to the horizon, the sky fades to a lighter blue or white.
Space is 'beige'
The researchers used a colour-matching computer programme to convert the cosmic spectrum into a single colour visible to humans. They concluded that the average colour of the universe is beige, not too far off from white.
Normal colour vision ranges from wavelengths of around 380 nanometres (violet) to 750 nanometres (red). Most people can't easily see light shorter than 380 nanometres because the lens of the eye absorbs it.
Practically, we cannot even imagine thinking of the end of space. It is a void where the multiverses lie. Our universe alone is expanding in every direction and covering billions of kilometres within seconds. There is infinite space where such universes roam and there is actually no end.
Far outside our solar system and out past the distant reaches of our galaxy—in the vast nothingness of space—the distance between gas and dust particles grows, limiting their ability to transfer heat. Temperatures in these vacuous regions can plummet to about -455 degrees Fahrenheit (2.7 kelvin). Are you shivering yet?
Space, however, is a vacuum—meaning it's basically empty. Gas molecules in space are too few and far apart to regularly collide with one another. So even when the sun heats them with infrared waves, transferring that heat via conduction isn't possible.
What's different is that the Southern Hemisphere has much less land in it than the Northern Hemisphere, and especially no Sahara Desert. Which means there is much less dust at high altitudes, so there is less light scattering, and the blue colour is clearer.
Rio de Janeiro came top of the list, followed by New Zealand, Australia, Fiji and South Africa. Standards labs like the NPL are interested in colorimetry — the science of measuring colour — because of its many useful applications.
Blue skies
In Dubai, the sky has a yellowish, sandy color on most days. With all the dust, sand and humidity blowing up in the air every morning, sometimes it is hard to remember that the actual color of the sky should be blue.
This is because sound travels through the vibration of particles, and space is a vacuum. On Earth, sound mainly travels to your ears by way of vibrating air molecules, but in near-empty regions of space there are no (or very, very few) particles to vibrate – so no sound.
No, you cannot hear any sounds in near-empty regions of space. Sound travels through the vibration of atoms and molecules in a medium (such as air or water). In space, where there is no air, sound has no way to travel.
The stars aren't visible because they are too faint. The astronauts in their white spacesuits appear quite bright, so they must use short shutter speeds and large f/stops to not overexpose the pictures. With those camera settings, though, the stars don't show up.