Why is it dark at night?

You're asking a very profound question, and it's been asked before. It goes by the name of Olbers' paradox.

The resolution of the paradox is really the basis of modern cosmology. There are two key facts:

1) The universe is not infinitely old. It's only 14 billion years old, so there is only a finite distance over which we can see stars.

2) Because the universe is expanding, the light from distant stars is redshifted, which reduces its energy. Therefore, we get less light from distant stars than you would otherwise expect.

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Look up Olbers' paradox on the web for more information.

The basic argument for the paradox is as follows: Imagine you divide the universe into spherical shells (like onion skins) of uniform thickness, centered on the earth. On the average, the number of stars in each shell is proportional to the volume of that shell, which is proportional to the square of the radius of the shell. On the other hand, the light from distant stars is reduced by the inverse square law.. Each shell contains more stars, but their light is reduced by the same amount. Therefore, each shell should contribute the same amount of light. If the number of shells is infinite, every point on the sky should have the same surface brightness as that of stars -- that is, the entire sky should blaze like the sun.

(Example: Imagine one shell at some distance, and another shell ten times farther away, each of the same thickness. The far shell has 100 times greater volume, so it should contain 100 times more stars. Because of the inverse square law, each star in the far shell will appear 100 times dimmer than each star in the near shell. Therefore, the total light we see from the far shell should equal that from the near shell.)

(One other detail: If there are an infinite number of shells, and each produces the same amount of light, why don't we see an infinite brightness? Because the light from some distant stars is blocked by closer stars, and we end up in a kind of thermal equilibrium.)

Mef's reasoning is invalid. Each shell should produce an equal amount of light to an observer on Earth, and there would be an infinite number of shells in a simple model of the universe (i.e., infinite uniform flat space, filled with stars).

This paradox was made famous by Heinrich Wilhelm Olbers in 1823 (although it was noted by earlier astronomers). Olbers, of course, knew nothing about the expanding universe and the big bang.

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You said that it seems the question has an obvious answer. I don't know whether the "obvious" answer you had in mind is right or wrong.

Most people here are saying that it's obvious that we get less light from distant stars. That doesn't resolve the paradox at all, but simply indicates that they don't understand it. Anyone who doesn't understand the paradox can't understand the resolution either. When Olbers and others pondered this question, they were not being stupid; they were pointing out that the fact that the night sky is dark says something very important about the nature of the universe, but they weren't sure exactly what that was.

The resolution of the paradox is far from obvious. It could not be explained correctly until the twentieth century, when we learned about the expansion of the universe and the big bang.

There are really no paradoxes in science. When we see a paradox, it means that we don't understand something. There have been other famous paradoxes, and they always lead to a better understanding. Here are some:

1) Darwin was bothered by the fact that the time-scale for evolution appeared longer than Lord Kelvin's estimate for the age of the sun. Resolution: Kelvin's estimate was wrong, because he thought the sun's energy came from gravitational contraction. He didn't know anything about nuclear reactions, which were discovered many decades later.

2) Nineteenth century astronomers were puzzled when they found that observed changes in the orbit of Mercury (the precession of its perihelion) differed very slightly from their calculations. Resolution: Decades later, Einstein's theory of general relativity showed that Newton's law of gravity, a fundamental fact of physics for over 200 years, is not correct.

Paradoxes are good for science, because they spur people on to new discoveries.

the sun is considered as a star ,as well but The Sun is roughly 93 million miles away from Earth. Light travels at 186,282 miles per second. After doing the math, it takes the light from the Sun about 8.3 minutes to reach Earth PLUS the sun is near from the earth and it is so Strong lighting that's why the day is brighter than at night ,,

AND Mmm no! .. the light from the far away starts DOES NOT reach us the same way the sun does , WHY.. ? i'll tell you why,, because from what i have studied at school is that the moon derives light from the sun during the day so as the starts as well so of course it won't gives us the heat and the lighting as the sun does!!,, and the distance between the earth and the nearest start to the earth is 4 light years away (Regardless of the SUN) ,the sun is a star too ,!!

and the reason of why do we still see the stars at night ::

First of all, the thing is that sun is also a star as i mentioned before . And we see it at day! The other stars that are far away maybe big or small but they can't give us as much light as our sun((and i already gave you the reason why)). So the sun is the brightest. It isn't that the stars are only on the dark side of the earth. They are in the bright side but we just can't see them because the light of sun is the brightest which doesn't let us see the light of the other stars. But we can't see the sun in the night time so we can see the other stars. It is kind of same with our moon except that the moon reflects the light of the sun.,, and why does the starts exist at daytime so they can derive some lights from the sun to allow them to shine at night ,, i guess..

What you are asking is about Olber's paradox. He reasoned that if the universe were infinitely large and infinitely old, then the sky would be as bright as the sun in all directions and at all times. Therefore, he reasoned, the universe cannot be infinitely old and infinitely large. Astronomical evidence shows this to be true.

However, there is another important point. As the volume of the universe has increased, space itself has stretched. This means that the wavelength of photons from the very hot big bang has been stretched through time. At the time of the formation of atoms, about 400 thousand years after the big bang, the universe looked as bright as the sun is today with a surface temperature of 3,000 degrees C. The current temperature of the background radiation of the universe is under 3 degrees C. This means that the initial visible photons have become microwave photons as their wavelengths increased. We cannot see microwave radiation, even though it bathes us. However it can be seen using a TV set set to a channel that isn't operating. The noise is the cosmic background radiation.

It is dark at night because we are on the side of Earth facing away from the Sun. The amount of light reaching us from the stars is insignificant in comparison to the light from the Sun. The other stars are many light years from us while the Sun is only 93 million miles away. Just look up during the day and see if you can see stars other than the Sun. They are so faint that they are almost never visible in the light of the Sun.

If the universe is eternal w/o a beginning (christians say the same thing about god) then the sky would not be dark. But since it is dark we can assume that light has not penetrated the entire universe in its short 13.x billion year history. A simple mathematical calculation would show that the sky would never be dark if the universe had no beginning.

It's what is called 'the cone of light'.

Because stars are spheres, the light emitted from them travels out in a disipating sphere of light. The farther you are away, the more the light separates. From any one point on the light source, you can draw a cone from that point. As the cone expands, so does the light disipate, or separate.

The Earth exists within a relatively confined cone of light, as compared to the stars. Which are much farther away than the Sun. Meaning, their cone of light has disipated, or been spread out on a far greater scale. Therefore, less light is recieved by our tiny little planet.

Imagine that the light emitted by stars in done in a uniform manner and travels in straight lines direct out from the center of the star....this means that the "light density" so to speak as it travels outward will decrease as a function of the square of the distance from the star....these distances are really quite far, so the amount of light that lands on us from the star is virtually zero (relative to the initial amount emitted from the star).

the night is dark because while the earth revolves around the sun it also spins and so while its night over here its day in australia, new zealand, etc. and another thing that helps with that is earths 23 1/2 degree tilt.

Which does a better job of lighting up your bedroom - your bedroom's ceiling light, or the light in the living room? Even though you can see the living room's light, even though the globe is the same brightness, it doesn't light up your bedroom as well. Why?

It's distance. The proximity of the sun makes more of its light reach us and thus it appears brighter. We can still see the more distant stars, but not as well.

because the stars are billions of light years away the sun is not that far away from us