# Why is the Sky Dark at Night?

The Minute Physics folks have created another great video, this time explaining why the sky is dark at night. Although at first glance it seems like an easy question to answer, throw in Olbers’ Paradox (the light from an infinite amount of stars should make the night sky completely bright) and it really is quite a complicated matter. In fact, it takes the Minute Physics teams nearly four minutes to explain it all!

## 29 Replies to “Why is the Sky Dark at Night?”

1. Enlightening 🙂 The logic I had previously heard mentioned none of this. Only that obscuring dust causes the dark night sky. One objection to Minute Physics’ explanation though: he confuses redshift with Doppler shift – perhaps for simplicity’s sake – but nevertheless a really misleading term, confusing velocity with spacial expansion.

I have heard of that hypothesis, but it is wrong. One can see that in the model of Olber’s paradox. Dust would heat up and radiate with the same temperature and therefore integrated brightness as the average star surfaces.

2. Obviously, the number of stars in the universe isn’t infinite, or they would fill every cubic centimeter of the universe, even if the universe itself was also infinite.

This, of course would also make the universe very, very hot. Too hot for normal matter to exist, in fact. Just like the early universe as we now understand it, in fact!

1. Juniper says:

“Obviously, the number of stars in the universe isn’t infinite, or they
would fill every cubic centimeter of the universe, even if the universe
itself was also infinite.”
Not really. Let’s say that there is one star every N cubic parsec. That is an almost empty cube with a single star inside, and a volume of N cubic pc. If the universe itself is infinite in size, it would contain infinite of those cubes with a star inside. Thus infinite stars.

1. Chetan Chauhan says:

This is one of the most exciting fields of mathemetics currently , that there are different types of infinity, and some infinities are “bigger” than other.
IMO, it is more likely that the universe is “looped” , i.e when you travel far enough you come back to where you started from.

The local topology of the standard cosmology universe isn’t sufficiently constrained, but the simplest theory is the flat topology.

If eternal inflation is a fact, I don’t think a looped universe is possible. In any case, it is a priori unlikely.

3. Chetan Chauhan says:

Take the case of planets which are traveling along a straight line on stable orbits. The planets themselves are following perfectly laws of motion along a straight line. But as relativity says , the space around the sun itself is curved due to gravity due to which planets loop around the sun in orbits.If you get into advanced math then the “line” between a straight line and a circle tends to get “blurred”. Even in nature everything seems to travel in a loop around something else , i.e electrons around the nucleus all the way upto stars orbiting the galactic center.

3. Guest says:

Oh, and interestingly enough, I just read an article in Sky News about the eventual merger of the Milky Way and Andromeda. The resulting galaxy would be both opaque enough to allow distant stars within the galaxy to be seen, and also feature enough stars to basically fill the night sky when the Earth (or any other observation point within the galaxy) is facing that way. The end result is a very bright sky indeed.

4. tenstripe says:

Brightness is precieved by the amount of photons hitting your eyes. The farther away the less photons, and it dosn’t get any better the farther away it is; it has to get dimmer even with redshift. The known universe is finite if you believe in the big bang (space is infinite). If farther out objects are moving faster away from us they would be released from our gravitational influence and sped up even more. Infinity (space) is big and probably has a lot of emptiness (space) that we see looking out as blackness. Its possible other big bang so called “universes” are out there to far away to be seen as to exist. The larger the objects the less there are. If we can’t see it we won’t believe it.

The farther away the less photons

That doesn’t apply to our standard cosmology, which is uniform.

Are you thinking of distance from a star? Uniformity over cosmological distances makes that uninportant.

Or are you thinking of local observers? You can’t really predict conditions “farther away” in relation to the local frame. Local observers would disagree between their local and your “farther away” conditions.

If farther out objects are moving faster away from us they would be released from our gravitational influence and sped up even more.

As soon as objects are gravitationally unbound they move away with the expansion of the universe. So not sped up in relation to something else.

Its possible other big bang so called “universes” are out there to far away to be seen as to exist.

That depends on how you define “big bang”. Local inflationary bubbles can be predicted in the simplest inflationary cosmologies, but we can’t know if they are too far away to be observed as they could erase our local volume at any time (if they have lower vacuum energy).

1. tenstripe says:

Thankyou for your critique. I was, more answering the question the limits of the human eye as to why the sky appears dark as looking out from Earth. The night sky does not appear uniform to our eyes as we can point light gathering telescopes to focus on the differences so far away. There was more implications though that I may have implied. The singularity of the big bang was finite in size and existed in infinite space. Therefore, there is not infinite stars and light should not be everywhere. Although, I did suggest there could be infinite stars if there were more big bangs. My reasoning is that light is far overshadowed by the distances of increasing space diminishing any photons into obscurity. I will tell you that I have only an introductory college astronomy education and enjoy throwing out my own opinions to be scrutinized by more established space people.

2. TractorEngineer says:

I think it has a lot more to do with how your brain interprets the image than anything cosmological. Space is crystal clear for the most part, that’s why you can see stars and other celestial objects that are incredibly distant. But since your eyes are seeing nothing at all between the gaps of stars and other objects, your brain interprets that nothingness as the color black.

5. Diteris says:

This is the most ridiculous explanation I have ever come across. It is so wrong, I could hardly get off the floor laughing. Had the Minute Physics people not done any High school science at all? Simple explanation…energy propagated through space decreases by the distance. In fact …double the distance, and you end up with a quarter of the energy. In fact the number of photons reaching us from the stars is so small that it is insignificant compared to our rather small but very close sun. I’m no scientist, and would like to have a more qualified answer from someone who knows their physics.

1. Diteris says:

My apologies if I have offended anyone! In any case thanks for the references … Nevertheless, the sky is dark at night and the explanations seem inadequate to explain it. Indeed if we had eyes the size of the large telescopes, the sky would be very bright, enough light gathered to see in the dark (literally). However the human eye can only see roughly 2000 stars, due to light gathering limitations. This seems to indicate to me that the sky is dark because of our own biological limitations, not on red shifts, expanding universe…etc. However I did like the final explanation in your second reference, and forgive me for quoting, but I think it is relevant, and I do concur that it is the best explanation.
‘ “Too little energy”:
Originally proposed by American cosmologist Edward Harrison in 1964. It
was derived after computing the amount of energy required to create a
bright sky, and finding out that it implies an overwhelming large
number: the observable universe would need 10 trillion times more energy
than it currently shows. This means that even if all matter in the
universe were transformed into energy according to Einstein’s famous
formula, the night sky would be barely brighter than it really is. This
argument is truly one of the few heavy weight solutions to the riddle.’

2. RikyUnreal says:

I really like the table in that article.
As you can see the viability of each proposed solutions varies and this means that the “problem of the dark sky” is more complicated than you might think.
In The Minute Physics video, they mainly give an explanation using both “Cosmic age too short” and “Redshift” solutions, and I’m agree with you that it’s only a part of the story.
But I firmly believe that it’s not a ridiculous summary.
Regards.

3. Diteris says:

It is a very interesting question. The table is a bit dismissive of some of the explanations offered. Dismiss Edgar Alan Poe? – I would hesitate :). The shell formula confuses me totally – perhaps because it arrives at a conclusion that does not accord with observation, or I’m just a bit dumb. I imagine that the sky would be much brighter if we could see the total energy spectrum, but we only have a very tiny part of the spectrum available to our eyes. Perhaps we are a little like the fleas on a dog, and imagine that one dog is the entire universe, unless we are prepared to leap into the unknown….
Thanks for the discussion, much appreciated.

Simple explanation…energy propagated through space decreases by the distance

That doesn’t apply to our standard cosmology, which is uniform. You may make the same assumptions as tenstripe, so see the rest of my responses to various erroneous assumptions there.

The video describe what is the problem and what happens in a uniform universe, which is the conditions posed in the model of Olber’s paradox. Other cosmologies, like mistaking universal expansion for local “explosion”, would make other predictions such as the erroneous one you propose. Those had to be abandoned several centuries ago (as the age of Olber’s paradox hints at).

Note that Harrison’s prediction is based precisely on uniformity. Which is why it works, exactly like the more detailed prediction of the standard cosmology described in the video.

5. Diteris says:

Perhaps cosmological theories have a long way to go before we have a clear idea of which assumptions are correct and which are in error. Disagreement and debate is a wonderful and inspiring aspect of scientific enquiry.
Thanks

1. Olaf2 says:

I bet you yourself can’t explain it to ordinary people and kids in a 3 minute time frame. I predict that the kids would run away after 30 seconds as boring.

1. Diteris says:

Not sure what your point is … your prediction about my ability to hold the attention of children regarding the wonders of science is based on almost no information about me, and perhaps you’re underestimating the intelligence of ‘ordinary people and kids’.

2. Olaf2 says:

This clip does a wonder to explain physics in only a 3 minute period easy understandable to uneducated people and kids. And it covers every possible angle that is possible to be explained in a 3 minute time frame.

And yet you are complain about it. So I challenge you to explain it in a 3 minute time frame to the uneducated and kids.

3. Diteris says:

Children, let us examine why the sky is dark at night. One of you … take a very bright torch and walk away from us. and go back as far away as you can so that you not obstructed by anything. How far away do you have to go before you will no longer see the light? Ikm..2km…3km…hmmmm. Do you think you will get so far away that you will not be able to see the light…yes? Ok, children, the stars are a very long way away. In fact the closest star, apart from our sun is so far away that its light is at least 500 billion times less bright than our sun, and that is why the sky is mostly dark…everything is a very long way away.
Sorry that’s the short answer….but it’s less than three minutes. The long answer discusses cosmological theories and constructs that may or may not be correct…and indeed could be boring for some….

4. HelplessApe says:

Diteris, light acts differently in space than within an atmosphere. If you’re going to critisize what minute physics is trying to do, then you should probably get your facts straight too.

5. TractorEngineer says:

I can give it to you in a one-liner (or close to it): when you’re seeing nothing, your brain interprets it as the color black.

6. Olaf2 says:

But the problem is why do you see nothing? Infinite space should mean it is as bright as hell.

7. TractorEngineer says:

But light doesn’t keep its intensity all the way down to your eyes, it obeys the inverse square law. Space appears black for the same reason that the inside of your broom closet with the door shut appears black: you can’t see anything, so your brain interprets that color as black. It could just as easily appear a washed-out white, green, or red. It’s how the brain interprets the image.

8. Olaf2 says:

I know, but space is infinite. So infinite light-sources.

The 3 minute clip is addressing the many different questions in detail for people that do not enough understand science. Your one-liner only says the end-result, but the end result is that you have lack of photon’s in the visible range. The inverse square law is only part of the sollution.

6. TractorEngineer says:

I don’t believe a word of this. I think the answer lies in how your brain interprets the image. The color black is how your brain interprets seeing nothing.