Why is the Sky Dark at Night?

Posted on October 11, 2012  Comments (8)

The answer isn’t quite as simple as it seems. I find the wording in the video a bit confusing.

The point I believe, is that the sky is dark instead of light. But not that the brightness would be huge (so for example, you couldn’t necessarily read my book outside just by starlight). The light would be very faint, it is just that it would be lightish instead of blackish, due to the reasons explained (redshift etc.). At least that is my understanding.

Related: Why is it Colder at Higher Elevations?Why Does the Moon Appear Larger on the Horizon?Why is the Sky Blue?Why Wasn’t the Earth Covered in Ice 4 Billion Years Ago – When the Sun was Dimmer

8 Responses to “Why is the Sky Dark at Night?”

  1. Jeff Loughlin
    October 11th, 2012 @ 8:18 am

    But isn’t the answer much simpler than that? I’m a photographer, not an astrophysicist, but I know from photography that light (from a flash, for example) falls off according to the inverse square law – that is, the amount of light provided by a flash decreases by the square of the distance to the subject. In simpler terms, that means that a more distant subject receives less light from the flash than a closer subject, and so requires a stronger flash pulse to fully illuminate it (that’s why you can’t use a flash to take a picture of something 100 yards away). So expanding that concept to the night sky, the light from the distant stars appears much dimmer than the light from our nearest star, the sun, simply because of the distance. Red shift and cosmic background radiation aside, the night sky is dark simply because the light sources are farther away. I think the video is trying too hard to explain something that’s really much simpler than they make it seem.

  2. curiouscat
    October 11th, 2012 @ 11:25 pm

    No, I don’t think so.

    As I stated the darkness versus lightness is what they are talking about not the brightness of the light. As I said, I think the brightness would be low – similar to what you state about the source of the light. But that doesn’t mean it would be dark v. light. When they look with powerful sensing equipment they see darkness (just like we do with our eyes) not lightness that is feint.

  3. Kenny
    October 12th, 2012 @ 4:03 am

    It is very difficult for a layman to understand the workings of NASA and all other scientists. But yes as far as the personal experience is concerned we all say that the night is dark. But I agree that it is not completely dark but just lightish, enough to see figures moving, if all other details are not visible.

  4. Elsa
    October 12th, 2012 @ 1:28 pm

    Well, the sky (or what we call sky) doesn’t have light, it’s just space. The dark night is the normal state. The day exists only because there is a sun, an independent light source. This sun creates an exception to the normal darkness, so shouldn’t the question be: why is the sky Light at Day?

  5. Hanamiti
    October 12th, 2012 @ 1:48 pm

    Seriously, man? The sky is dark at night because the point on Earth where you are is facing away from the Sun, thus you are in the shadow of Earth cast by the Sun. Some light is bounced at you off the Moon, some is provided by distant stars. What’s hard about that?

  6. Kristina Marchant
    October 18th, 2012 @ 11:26 pm

    I agree with Hanamiti. Why is there confusion about this?

  7. curiouscat
    October 19th, 2012 @ 11:08 pm

    I am sad that people come to a science blog and make comments without understanding the issues and clearly without any interest in learning or watching the video.

    I don’t have a problem trying to learn and not understanding. But just making off the cuff remarks without even knowing what is being discussed is lame.

    Any sensible discussion of the video has to discuss the matters in the video. The rotation of the earth is not the issue being discussed.

    I understand many people don’t have any interest in learning about science but just making comments that don’t relate to the discuss at hand is not useful.

    Jeff watched, and thought about the video.

  8. Nikhil Naik
    October 31st, 2012 @ 9:13 am

    I feel that the video raises more questions than it answers.

    I guess the more intuitive question to ask is why even when there’s a moon, the sky isn’t blue with scattered moonlight (answered above), but the video jumps right to assuming we know “the sky ought to be blanketed with a complete coat of stars” and then proceeds to show why that isn’t the case. For someone who wasn’t thinking the sky ought to be blanketed with a complete coat of stars in the first place, it’s not as interesting to hear someone explain why it’s not.

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