It sounds like a stupid question only a dumb little kid would ask. And almost certainly most would answer - "Duh!? Because the sun is away at night!" While that is a simple answer it is only partly correct. It doesn't really entirely answer the question which by the way is nothing but simple. Like if you look at the picture below you would notice that sky is dark even if the sun is shining (yes, that small shiny spot is the sun).
At this point some would explain- "Well, that is because there was no atmosphere from where it was taken for the sun to illuminate." So, now we say that it is the illuminated atmosphere of the Earth that make the daytime bright but not at night. Again, that is a correct but still it will not explain entirely why the night sky is dark.
How about the stars, if there are enough would it not light the sky at night?
It is estimated that if you hold up a grain of sand, the patch of sky it can cover would contain 10,000 galaxies with billions of stars per galaxy. So if there‘s all those stars, why is it so dark at night?
There is an idea called the "dark night sky paradox" or also known as Olbers' paradox after the astronomer Heinrich Olbers. It argues that the dark night sky would conflict that the universe is infinite in space and time. If there are infinite number of stars in this infinite space and infinitely old universe, wouldn't there be a star in every patch of the sky to shine for the day and night?
Here is the mainstream explanation. What we know is that the universe is not infinitely old. It started with a Big Bang 13.7 billion years ago and and stars have existed only for part of that time. We also know that light from these stars travel at a constant speed and that everything in this universe since the start is moving away from each other. This expansion of the universe causes light of stars to redshift(see redshift, Doppler effect) and some would redshift beyond the visible spectrum of the human eye.
In addition, this means that our light from stars within out observable universe, which is smaller that the total universe and that the light from stars further away from us than 13.7 billion light years away, will not have had enough time to reach the Earth.
If all this rather became even more confusing, then watch the minutephysics video below which explains it all.
At this point some would explain- "Well, that is because there was no atmosphere from where it was taken for the sun to illuminate." So, now we say that it is the illuminated atmosphere of the Earth that make the daytime bright but not at night. Again, that is a correct but still it will not explain entirely why the night sky is dark.
How about the stars, if there are enough would it not light the sky at night?
It is estimated that if you hold up a grain of sand, the patch of sky it can cover would contain 10,000 galaxies with billions of stars per galaxy. So if there‘s all those stars, why is it so dark at night?
There is an idea called the "dark night sky paradox" or also known as Olbers' paradox after the astronomer Heinrich Olbers. It argues that the dark night sky would conflict that the universe is infinite in space and time. If there are infinite number of stars in this infinite space and infinitely old universe, wouldn't there be a star in every patch of the sky to shine for the day and night?
Here is the mainstream explanation. What we know is that the universe is not infinitely old. It started with a Big Bang 13.7 billion years ago and and stars have existed only for part of that time. We also know that light from these stars travel at a constant speed and that everything in this universe since the start is moving away from each other. This expansion of the universe causes light of stars to redshift(see redshift, Doppler effect) and some would redshift beyond the visible spectrum of the human eye.
In addition, this means that our light from stars within out observable universe, which is smaller that the total universe and that the light from stars further away from us than 13.7 billion light years away, will not have had enough time to reach the Earth.
If all this rather became even more confusing, then watch the minutephysics video below which explains it all.
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