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Why is the sky dark at night?

Why is the sky black at night?
Deacon Tuck from Tyne and Wear (Age 5-14)

If all the stars in the universe give off light why isn’t it always light?
Tom from Essex (Age: 5-14)

If there are as many stars as it is recognised there are, and each one is in a relatively finite position and emits a discrete amount of light energy, let’s say, just 1 photon per second, why is the sky dark at night?
Martin Hedges from Greater London (Age 55+)


4 Responses

  1. Hi Martin,

    I am not sure what your question is about but it is interesting. The stars do emit a lot of photons and our eyes catch these photons and we can focus the image of the individual stars on to our retina. That is why we see all the little bright dots in the sky. Each one that we see is delivering a lot more than one photon per second to our retina.

    The black part of the sky is black because there are very few photons coming from those bits of the sky. There are probably less than one photon per sec coming from the black bits. Indeed the black bit of space that we see from above the atmosphere is very black.

    Interestingly, some photons will come from this black bit. Some of these photons are in the Microwave end of the spectrum so we can’t see these. Other photns that we might be able to see(see the answer to Deacon’s question) can be scattered by dust and they will appear to come from the black bit but this level of scattering will be very low in outer space. There will be more of this scattering once the light is passing through our atmosphere(this is the same process that makes the day time sky blue and the setting sun red). However this is a very low level of light and so to our eyes, the bit in between stars still looks very black.

    You can see this scattering and reflection difference when you compare the night sky in the city with the night sky in the country. Out in the country, it appears like there are more little stars. This is because we get less light reflected back from the black bits from sources such as our own street lights etc.

    Hope this helps.

  2. ESA estimates10 to the power 22-24 stars in the universe. (I just googled it)

    You say of the visible stars:- ‘Each one that we see is delivering a lot more than one photon per second to our retina.

    My point is…Aside from a very few stars with gravitational issues, the majority of this 10 to the 24 stars are regarded as emitting in the visible spectrum.

    Thus I ask my question again… with ten to the 24 power stars each emitting billions of photons a second, why is the sky black with points of light, rather than an overall colour (probably blue, based on the refractive properties of the atmosphere)

  3. The question is interesting. One way to rephrase your own analysis is to catagorise the 10 to the power 24 stars in star category level. Some of them are not detectable to our eyes simply because of the minicule collection area of our eyes and hence appear black. There isn’t sufficient scattered light in between the brighter stars for our detectors to register them.

    If we had an artificial detector system with sufficient dynamic range we would indeed find that there are photons coming from the space inbetween the bright stars and it may indeed be biased towards the blue end of the spectrum.

  4. coz dark night give us change to rest our body up ,,,

    if there isn’t dark we can’t rest our body up ….

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