Space appears blue to the human eye for a few key reasons. The main factors that contribute to space’s blue hue are the scattered sunlight in Earth’s atmosphere, the chemical makeup of Earth’s atmosphere, and the way human color vision works. Understanding these factors helps explain why space generally looks blue from Earth’s surface.
Scattered Sunlight
The sunlight that reaches Earth’s atmosphere contains the full spectrum of visible light wavelengths. This includes all colors from violet to red. As this white sunlight passes through the atmosphere, some of the wavelengths are scattered by gas molecules and fine particles. Shorter wavelengths, like violet and blue light, are scattered more than longer wavelengths. The scattered blue light spreads out and colors the sky blue during the day. At sunrise and sunset, more red light reaches our eyes because the sunlight travels a longer path through the atmosphere and the blue light has been removed.
Earth’s Atmosphere
The chemical makeup of Earth’s atmosphere also affects which colors are scattered. Nitrogen and oxygen account for most of our atmosphere and preferentially scatter blue and violet light. The bluer components of sunlight pass through the atmosphere and out into space. Looking away from the sun, we see the blue hues of the scattered light. Other planets and moons have different atmospheric compositions and hence different sky colors. The orange skies of Mars and brown haze of Titan are due to different chemical mixtures that scatter colors besides blue most effectively.
Human Color Vision
The receptors in our eyes also contribute to the blue color perception of space. The human eye has receptors sensitive to red, green, and blue light. At low light levels, our blue receptors remain more sensitive than the red and green ones. When looking at the faint light of the night sky, the blue hues dominate over other colors because our blue receptors remain active and send stronger signals to the brain. An alien species with different color vision may not see space as intrinsically blue.
Why Does Space Look Dark at Night?
If sunlight is scattered to fill the entire sky with blue light, why does space look dark at night instead of blue? There are a couple key reasons:
Less Scattering at Night
At night, the sunlight entering Earth’s atmosphere is coming from below the horizon. Without the sun directly overhead, less light is scattered towards our eyes. The longer path through the atmosphere also removes more of the blue components. This leaves darker skies with fewer blues.
Our Eyes Adjust
Our eyes and brain automatically adjust for the difference in brightness between day and night skies. Rod cells in our retinas are more sensitive in dim light. By adjusting and increasing their sensitivity at night, the rods compensate for the darkness and we lose perception of the faint hues. The blue tones present at night are thousandths of times dimmer than daylight. Our visual systems simply filter out these very dim blues.
Is All of Space Blue?
While space generally appears blue from Earth’s surface, not all of outer space looks blue. Here are some examples where space takes on different hues:
Near the Sun
Looking near the sun, space appears black because the bright sunlight overwhelms the scattered blue light from the atmosphere. The black of space is easier to see briefly at sunset and sunrise before the sun appears over the horizon.
Near the Moon
Space also looks dark near the moon. Just like the sun, moonlight outshines the light scattered through the atmosphere. The lunar sky appears black right up to the edge of the moon.
Night Sky Objects
Many celestial objects like stars, planets, nebulae, and galaxies show their true colors against the blackness of space. Their emitted or reflected light reaches us directly without scattering, revealing reds, oranges, purples, and more.
From Space
Being above Earth’s atmosphere in space, astronauts do not have the intervening air to scatter sunlight. The sky appears black even during the day. Only the thin blue ribbon of Earth’s atmosphere provides color.
Blue Hues Around the World
While space universally appears blue from Earth’s surface, the exact shades and intensities vary around the world. Here are some factors that lead to differences:
Latitude
The sun’s path through the sky changes with latitude, impacting the scattering of light. At the equator, an overhead sun scatters light throughout the sky. Towards the poles, the lower sun creates more dramatic oranges and reds at sunset as blue light is removed.
Altitude
Higher elevations have less air mass above, leading to less blue light scattering downwards. Locations like mountain peaks therefore have darker skies during the day. The thinner atmosphere also allows more reds and purples to remain at sunrise and sunset.
Atmospheric Aerosols
Dust, smoke, pollution and other aerosols can scatter additional light and modify the color of the sky. More particles lead to paler and whiter skies as more sunlight is scattered out. Cleaner air results in darker blue skies.
Geographic Location
Localized weather patterns, temperature, humidity and other geographic factors lead to variation in sky colors around the world. Unique hues at a particular place and time arise from that region’s conditions.
Spectacular Blue Views from Space
Looking back at Earth from space provides spectacular views of our blue planet. Some of the most striking images highlighting Earth’s blue hues include:
Apollo 17 “Blue Marble” – 1972
This iconic shot of the whole Earth fully sunlit shows Africa, Antarctica, and the Arabian Peninsula.
ISS Solar Transit – 2012
This composite shows the International Space Station silhouetted against the blue hues of Earth’s atmosphere.
Himawari-8 – 2015
This Japanese weather satellite provides a complete view of the Pacific Ocean side of Earth.
Measuring the Blue Light from Space
Scientists use spectrometers to break down and measure the wavelengths of light coming from space. This data can quantify the components making space appear blue to us.
Spectrometer Measurements from Earth’s Surface
| Wavelength (nm) | Intensity |
| 450 (blue) | 100% |
| 500 (green) | 60% |
| 650 (red) | 10% |
Measurements made looking away from the sun confirm blue light around 450nm dominates.
Spectrometer Measurements from Low Earth Orbit
| Wavelength (nm) | Intensity |
| 450 (blue) | 60% |
| 500 (green) | 30% |
| 650 (red) | 10% |
From space, blue components are reduced without atmospheric scattering.
The Biological Draw to Blue
Humans seem to have an inherent preference for the color blue, both scientifically and culturally. This may stem from biological factors related to our blue planet.
Positive Impacts on Mood and Performance
Studies show blue light improves alertness, reaction times, and mood. Blue environments enhance creative performance. Exposure to blue can even impact hormone levels and heart rate.
Associations with Water and Sky
Blue has strong cultural ties to the water and sky – critical resources for life. Blue often invokes feelings of tranquility and security associated with these.
Rarity in Nature
Few plants and animals display brilliant blues. The scarcity of blue in the common environment may contribute to its desirability and high value.
Conclusion
The blue appearance of space stems largely from sunlight scattering through Earth’s atmosphere. But sky color varies globally based on location and local conditions. Spectrometer measurements confirm blue wavelengths dominate signals from space. Blue also seems embedded in human culture and psyche. Yet while space appears ubiquitously blue from Earth’s surface, the rest of the cosmos shows its true diverse colors against the blackness of space.