Did you see yourself in heaven
The sky is our window to the universe. And just like with a window, we look through a 'pane of glass'. Except that it is not made of glass, but of air. The air is invisible to us, but it affects how we see the sky.
Our atmosphere is over 100 km thick. This has an impact on the sunlight that has to penetrate this layer in order to reach us. On its way through the atmosphere, the light is filtered, scattered, reflected or refracted (see below for more details). What remains of the originally white sunlight is mostly blue, or sometimes orange or red or purple or or or ...
The sky from the surface of the moon looks different again. There is no blue here, only black. The moon has no atmosphere. So here you have a direct view of space, the window is open, so to speak. This means that the sky is pitch black even during the day. The stars do not twinkle on the moon, but are simply calm bright points on a black background.
The glitter and sparkle of starlight only emerges when it, like sunlight, has to pass through a layer of air and is influenced in the process. Let us now return to our familiar earth heaven and make a few considerations about its color!
Blue or black? Or what ?
You may have asked yourself, "Why is the sky blue?" Or, if you have seen a beautiful sunrise or sunset, ask: “Why is the sky red?” How is it that the sky is black at night when we can see thousands of other suns!
It is so natural that the sky is blue during the day and black at night that you might think the reason is very clear. But that's not it!
It took thousands of years of observation and deliberation, guesswork, discussion and analysis by the cleverest people until a scientifically founded explanation could be found in the end.
Well .... with all the colors of the rainbow .... why blue?
Couldn't the sky have just been green? Or yellow? When we see a rainbow, we recognize the color green in the sky, as well as blue, purple, orange, yellow, red, and all sorts of mixed colors in between.
The white light that comes from the sun is actually made up of all of these colors of the rainbow. A rainbow is, so to speak, a by-product of rain.
Raindrops turn into tiny prismswhen they are penetrated by sunlight. They bundle the light and split it into its individual colors (see illustration on the right), which we can then see in the rainbow.
But why are there these different colors?
The light that you can see is only a tiny fraction of the light that spreads in the universe and is everywhere, including around you! Light is made up of different wavelengths that have different colors. Together they result in the color white.
Similar to an ocean, light energy travels in waves through the universe. What distinguishes the different types of light are their different wavelengths.
Visible light contains the wavelengths that your eyes can perceive. The longest wavelength we can see looks red to us. The shortest one visible to us appears to be blue or purple.
The wavelength of red light is around 700 nanometers, while blue or violet are around 400 nanometers. A nanometer is one billionth part of a meter! A human hair is roughly 50,000 nanometers thick! So the wavelengths of visible light are very, very tiny ..
Obstacles to the light
There is still something important to say about light: it always travels straight ahead until something gets in its way, and that can be:
- a mirror on which it is then reflected (thrown back)
- a prism in which it is bundled or split
- Molecules in gases (e.g. in the atmosphere) that disperse it.
When the white light of the sun penetrates into the earth's atmosphere, its red, yellow and green components reach us almost unfiltered (mixed together they result in almost white light). Large parts of the blue and violet light, however, are swallowed up by the gas molecules in the atmosphere and released again, but scattered in all possible directions.
Let it be green!
Head over to the join in page and find out how to outsmart your senses and actually give the sky a green tint!
And what happens now with all the 'non-blue' wavelengths? They are still mixed together, not scattered by the gas molecules, and appear white to us. The diffused blue and violet light dominates the sky above us and makes it appear blue.
Parts of the violet light are then still absorbed in the lower atmosphere. In addition, our eyes perceive blue better than purple. So the sky looks blue to us when it is penetrated by sunlight.
Towards the horizon the sky turns a lighter blue to almost white. The sunlight that reaches us from the horizon has to pass through thicker layers of air than the light that comes directly from above. The gas molecules have scattered the blue light back and forth so often and in so many directions that it does reach us.
Why is the sunset red?
As the sun sinks lower and lower in the sky, its light has to pass through more and more atmosphere in order to reach us. While the blue and purple parts are scattered, red and yellow light reaches our eyes almost unhindered.
But larger particles of dust, air pollution and water vapor in the atmosphere reflect and scatter large proportions of the red and yellow light and make the sky in the west shine for us in the most beautiful colors, in red, orange, yellow and all their intermediate levels.
What does the scattering of light do?
How much sunlight hits the Earth's atmosphere and how much of it is reflected back into space? How much light is absorbed by land and water, how much by asphalt roads and sun-tanned surfers? How much light do water and clouds reflect back into space?
Every material and every surface has a very specific reflectivity, also called albedo. The earth has an average albedo of 0.37. This means that 37% of the incident sunlight is thrown back into space. This reflected light makes the earth visible. If the earth did not reflect light, it would be invisible. 63% of the solar energy that hits us is absorbed by the earth and its inhabitants.
Sunlight has enough energy to warm up the earth despite all reflections and to enable life here. Our climate depends, among other things, on how much solar energy is absorbed by forests and deserts, on sand- and snow-covered plains, on different types of clouds, on the layers of air, etc.
A comparison of the Earth's albedo with other planets can be seen in this graph.
Now the sun has set. Why is it so dark
You could think now: Of course it is dark at night, because then this part of the earth is turned away from the sun and lies in the earth's shadow, since our planet rotates once every 24 hours!
But then what about all these distant sunsthat appear to us as stars in the night sky? Shouldn't they light up the sky? There are 200 billion stars in our own galaxy, the Milky Way, and the entire universe contains more than 100 billion galaxies!
You could surely imagine that these many stars could light up our sky so brightly that it would be as bright at night as it was during the day.
Until well into the 20th century, astronomers believed that all the stars in the universe could not be counted. They said the universe would keep expanding, in other words: it was infinitely large.
If the universe were infinitely large and had an infinite number of stars, it would not matter which direction you looked, there would be stars everywhere. And the night sky would be as bright as day.
Now it becomes paradoxical
Apart from the fact that it is very difficult to imagine, there is another problem with the infinite universe: in fact, no matter which direction you look in, you should see millions of stars everywhere. If that were the case, the starlight would dazzle us in the night.
This problem annoyed astronomers for a long time and was known as'Olbers' paradox' known. Heinrich Wilhelm Matthias Olbers (1758-1840), a German physicist and amateur astronomer, described the problem in 1823.
A paradox is a statement that contradicts itself.
The problem can be compared to a forest. If you are in the middle of the forest and look around, you will see trees everywhere, near and far. The trees appear to be close together and the view is restricted to a relatively small area.
You can see a wall made of trees and cannot look through the forest. The stars should actually overlap in the sky like trees in the middle of a forest.
To explain this paradox, astronomers in the 19th century thought that clouds of dust between stars absorb much of the starlight that would otherwise reach us. But later scientists found that the dust itself would have to glow brightly if it were to absorb all this energy of starlight.
The end of infinity
Today's astronomers have found that the universe is not infinitely large, nor is it infinitely old. A finite universe - one of a very specific size, even if it contains trillions of stars - would not have enough stars to light up the entire universe.
Although the idea of a finite universe explains why the sky is not bright at night, there are other reasons why it does get dark. The universe is finite not only in its size, but also in its age. It has a beginning just like you and me.
The universe was born about 14 billion years ago, likely from a giant explosion called the Big Bang. The universe began as a single point and has expanded ever since.
Because the universe is still expanding now, the distances between stars and galaxies are getting bigger and bigger. Although nothing is faster than light, light also needs a certain amount of time to cover great distances.
So when astronomers look at a galaxy that is a million light years away, they see that galaxy as it once looked a million years ago.
The light that is leaving the galaxy today has to travel a greater distance to us, since the universe has meanwhile expanded further and thus the distance between us and the galaxy has increased. So this means that the amount of light energy that reaches us from a star diminishes during the long journey. And the further away a star is from us, the less bright it will appear to us.
Doppler effect - redshift - blueshift
Because the stars move away from us (actually the space between us expands), the wavelength of their light is also stretched. Longer wavelengths have less energy than short wavelengths. Astronomers say the light is 'redshifted' because red is the longest wavelength of visible light.
The redshift works just like the Doppler effect, which you may already be familiar with from sounds. If you are in your car and an oncoming truck honks its horn as it passes you, you can hear the Doppler effect. At first the horn sounds low.
As the truck approaches quickly, the distance between the transmitter (horn) and the receiver (ear) decreases and the wavelengths become shorter. The tone sounds higher than before.
Once the truck has passed and moves away quickly, the distance increases again, the wavelength of the horn has to cover a greater distance and is stretched. The tone then sounds deeper.
If a star moves away from us, the wavelengths of its light are expanded, less energy reaches us on earth. These two effects reduce the contribution this star makes to the brightness of our night sky. The Olbers Paradox assumes that the universe does not change. While the expansion of the universe alone is not enough to create a dark sky, it does help explain why the sky is so dark.
As is well known, clouds also belong to the sky. Often they disturb us when we want to watch the starry sky. Or they bring bad weather with them when we just don't need it. But it wouldn't be anything without clouds, after all they provide us with rainwater, and water means life. Without it, animals and plants would die of thirst.
Then it's better to have clouds now and then They also have a nice side: they decorate the sky and make it look interesting in the first place. And a sunset in which not only a strip of the sky changes color, but also the clouds glow pink or gold, looks much more romantic.
The clouds have also been given their due place in the astro stuff box. Have a look at the cloud gallery!
One color of the sky is in danger of disappearing, and that of all people! The black of the sky hardly comes into its own anymore, because tonight we turn night into day. Millions of lights shine at night, which increasingly make the stars fade and the black color of the sky disappear.
More on the subject of light pollution can be found on the 'Light pollution' page.
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