# Olbers' paradox: why is the night sky dark?

Started 6 months ago | Discussion thread
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 Re: Olbers' paradox: why is the night sky dark? In reply to trisd, 5 months ago

trisd wrote:

Jonny Boyd wrote:

The question is whether the two shell appear equally bright.

Do you see two equally bright shells on these two images?

No, you did not. Every time you started talking about flux and luminance, and wondered what is it I am actually asking, and every time I told you the question is about BRIGHTNESS to which you gave no answer.

As I've told you plenty of times, you need to define what you mean by 'BRIGHTNESS.' That's what we did in the last couple of posts, yet here you are reverting to a word without a clear definition. How is anyone supposed to communicate with you if you won't define words?

We both seemed to be happy to use irradiance (which is related to radiant flux) as a measure of brightness. These images have equal irradiance. The power of the EM radiation is distributed differently though so if you measured the irradiance of individual pixels, you would find variance within one shell – a factor that gets cancelled out once you stack shells to get an image of the entire sky.

Why on earth are you still going on about something that is so clearly established? Both these facts have surely been agreed by everyone since this discussion began. Where the problem lies is in you understanding what happens when you stack all the shells to get the image that our eyes see.

Although since I made a number of points that you haven't addressed, and asked questions which you ignored, perhaps that means that in fact you at last agree with me and the discussion is over. Great! We all now agree on the following facts:

2. irradiance is a good measure of brightness
3. luminance determines flux
4. there are infinite shells in Olbers' paradox
5. stars only appear as point sources to a low resolution detector, but in reality have an angular diameter, even if the detector cannot measure it
6. the number of stars measured by a photoreceptor will depend on their angular diameter, even if this angular diameter is not measurable by the detector
7. because you can fit a greater number of distant stars into the same angular area as closer stars, this cancels out the effect of the inverse square law on brightness
8. since the angular irradiance of every star is the same, then every angle of the sky will appear uniformly bright
9. my calculations are correct
10. the inverse square law by itself is inadequate to explain Olbers' paradox

I'm glad to see that you now agree with each of these statements and have joined with the rest of the world in accepting modern physics!

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