What does the night sky tell us
Why is it dark at night?
The earth rotates once every 24 hours. This means that the sun seems to move across the sky during the day. And when the sun is below the horizon, it is dark night. But that doesn't answer the question of why it's dark at night.
The sun only illuminates the facing side of our planet. On the other hand, it's dark in the meantime - only the stars twinkle in the night sky. But if the universe were infinitely large and evenly filled with stars, wouldn't the night sky have to shine as brightly at every point as a star or like our sun? So it would be bright as day even at night. Johannes Kepler pointed out this problem in 1610 and used it as an argument against the then hotly debated idea of an infinite universe.
View into a galaxy cluster
For Kepler, the dark night was clear proof that our cosmos must be finite. Today this problem is mostly referred to as the "Olbers Paradox". Because the astronomer Heinrich Olbers brought the question up for debate again in 1823. Gas between the stars would absorb the light, Olbers suspected. In this way it could be explained in an infinite cosmos why it is dark at night. But he was wrong: the starlight would heat the gas until it shines as brightly as the stars themselves. In fact, until the beginning of the 20th century, astronomers believed that the Milky Way was an “island universe”, surrounded by infinite emptiness. That was how convincing Kepler's argument was.
It was only in the 1920s that it became apparent that the inconspicuous "nebulae" - weakly glowing objects in the night sky - were not gas clouds in the Milky Way, as was assumed at the time. Instead, one looked at distant galaxies made up of billions of stars. The cosmos on the other side of the Milky Way is anything but empty - and when asked why it is dark at night, there must be a different answer.
It is true that the universe is not - as in Kepler's original argument - equally filled with stars. Rather, the stars collect in galaxies and these in turn in galaxy clusters. But averaged over large distances, the distribution of matter in the cosmos is even and the basic principle remains: Our view of the sky always meets the surface of a star in an infinite universe at some point. However, the mean distance from us to a star is quite long: about 102 3 or one hundred thousand billion billion light years. The light from such an average star would therefore need 10²³ years to reach us. However, according to current knowledge, the universe is only about 13.8 billion years old. So we only oversee an area of the cosmos that is ten billion times too small to provide a bright night sky.
And even in the distant future, the night sky will never be as bright as day. Because by the time we have a sufficiently large part of the universe, all the stars in space would have been extinguished long ago: the nuclear fuel is only sufficient for a comparatively short time.
In summary: It is dark at night because light propagates with finite speed - the speed of light - our cosmos has a finite age and does not contain enough matter in the long term to illuminate the whole universe.
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