October 2000
Almost everyone admires the lovely blue sky displayed on a crisp, cool autumn day. As early as 1500, Leonardo da Vinci tried to explain the sky’s color. His notebooks show he monitored sunlight passing through wood smoke, suggesting he understood the basic phenomenon of light scattering. Although the complete physical explanation for the blue sky is complex because of length limitations we will focus on the primary mechanism.
In the 1870’s the English scientist, Lord Rayleigh, weighed in with part of the explanation. But to understand his rationale, we need to know two things about how we perceive light. First, all colors of visible sunlight from the short-wave violet to blue, green, yellow, orange, and the long -wave red, are emitted by the Sun, yet not in equal amounts. The predominant color in the visible spectrum is blue. Second, our eyes detect green light better than the other colors and they perceive blue light pretty well. Mixed together, all the colors of visible light appear white to us.
Imagine a cork at rest on the surface of a tranquil pond. A stone is then dropped into the pond, sending out waves that cause the cork move up and down. The cork receives wave energy and then oscillates up and down, and in a similar manner light waves can interact with air molecules. However, the air particles do not keep the energy from light waves that fall upon them, but quickly re-radiate that light energy in random directions. Hence the initial light that was coming from a specific direction is now scattered (re-radiated) in all directions.
This scattering depends upon particle or molecule size and the wavelength of the incident light. Scattering is proportional to one divided by wavelength to the fourth power. The long waves of sunlight (red) are less effectively scattered than the shorter ones (blue) by the small air particles in our atmosphere. Since red light has a wavelength (700nm) about 1.7 times greater than blue light (400nm) Rayleigh's findings show that blue light has about 9 times greater chance of scattering than red light. This scattered blue light goes out in all directions through the atmosphere and comes to us from throughout the sky during the day.
However, when the sun is near the horizon sunlight must pass through a thicker amount of atmosphere than when it is overhead. As the light travels the longer distance through the atmosphere most of the blue light gets scattered out and the light that remains has proportionally more orange, leading to the beautiful sunrises and sunsets we are often blessed to see. These scattering effects are not however constant. Lord Rayleigh’s explanation ignores the effect of water vapor, dust particles, ozone, chemical pollutants, and eye response. All of these mechanisms can enhance or diminish the beauty or color of a sunrise or sunset.
In space or on the Moon there is no atmosphere to scatter light. The light from the sun travels a straight line without scattering and all the colors stay together. Looking toward the sun we thus see a brilliant white light while looking away we would see only the darkness of empty space. Since there is virtually nothing in space to scatter or re-radiate the light to our eye, we see no part of the light and the sky appears to be black.
FAQs
In space or on the Moon there is no atmosphere to scatter light. The light from the sun travels a straight line without scattering and all the colors stay together. Looking toward the sun we thus see a brilliant white light while looking away we would see only the darkness of empty space.
Why is the sky blue on Earth but black in space? ›
Since you see blue light from everywhere overhead, the sky looks blue. In space, there is no air. Because there is nothing for the light to bounce off, it just goes straight. None of the light gets scattered, and the "sky" looks dark and black.
Why is the sky blue on Earth but not on the Moon? ›
The Moon's atmosphere is so thin that it virtually doesn't have one. When the air is too thin for gas molecules to collide with each other, we call it an 'exosphere' instead. Because of the lack of an atmosphere, sunlight isn't scattered, so whether it's daytime or nighttime on the Moon, the sky appears black.
What color is the sky scientifically? ›
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. However, the cone cells in our eyes that detect colour aren't very sensitive to violet, so we see the sky as blue.
Why is Earth blue from space? ›
When sunlight reaches the water; the water absorbs, lights of all colors in the white light and reflects only blue light. Thus, the earth from space appears blue. If the water absorbs all colors and reflects only yellow, then it would appear yellow.
What is the actual color of space? ›
If we add up all the light coming from galaxies (and the stars within them), and from all the clouds of gas and dust in the Universe, we'd end up with a colour very close to white, but actually a little bit 'beige'.
Can you see color in space? ›
All the universe's “living” stars with active fusion cores emit those same colors and no others. None fail to provoke human visual systems into perceiving red, green, and blue — light's primary colors, which appear white when combined. That's why the universe's overall color is white or beige.
Is the sky really blue, yes or no? ›
Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
What color is the sky not to the human eye? ›
As far as wavelengths go, Earth's sky really is a bluish violet. But because of our eyes we see it as pale blue.
Why is there light on Earth but not in space? ›
Because space is a near-perfect vacuum meaning it has exceedingly few particles there's virtually nothing in the space between stars and planets to scatter light to our eyes. And with no light reaching the eyes, they see black.
If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth's atmosphere, outer space dims even further, fading to an inky pitch-black. And yet even there, space isn't absolutely black.
What color does an astronaut see the sky as? ›
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.
Why is space black but the sky is blue? ›
Since there is virtually nothing in space to scatter or re-radiate the light to our eye, we see no part of the light and the sky appears to be black.
What if Earth was 100% water? ›
Every living thing on earth would die, since water is a large component of all known living cells, every plant, animal, fungus, etc would all dessicate completely, and crumble to dust. With all water vapor gone from the atmosphere, the sky might change color, and become a reddish tint due to the high oxygen content.
What is the real color of Earth? ›
As per NASA's Report From space, Earth looks like a blue marble with white swirls. Some parts are brown, yellow, green and white. The blue part is water. Water covers most of Earth.
Why is it cold in space if the sun is in space? ›
With fewer particles to interact with in the (almost) vacuum of space, there is not enough matter to heat via radiation. However, this doesn't mean that you can send a spaceship close to the Sun and expect to experience very cold temperatures.
Is the Universe dark or light? ›
Like the jelly beans in this jar, the Universe is mostly dark: about 96 percent consists of dark energy (about 69%) and dark matter (about 26%). Only about 5 % (the same proportion as the lighter colored jelly beans) of the Universe— including the stars, planets and us—is made of familiar atomic matter.
