Comments on: October 10th: I know why it’s dark at night. Do you? http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/ An astronomy podcast every day, all year Fri, 10 Sep 2010 12:09:26 -0700 hourly 1 http://wordpress.org/?v=3.0 By: Terry Johnson http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/comment-page-1/#comment-12189 Terry Johnson Mon, 19 Oct 2009 15:17:31 +0000 http://365daysofastronomy.org/?p=2105#comment-12189 Mohan, this is why Olbers' Paradox is such a fascinating question to me. There have been (and some might argue there are still) so many competing solutions that it's difficult to point to any single explanation. You have to take a series of logical steps to explain it away. No one answer will work, which is why we had to attack each of Olbers' conditions. You have a very good point, but I believe you mean *additional* reason. Olbers' question was concerning the distribution of stars -- the density being practically irrelevant given his conditions. The question is: Since we know the night sky isn't bright, what's wrong here? Each of the conditions fails, which tells us much about the universe. Your solution still requires a contradiction to each part of the paradox, which you gave. The visible universe must be of finite size and age. Actually, I guess you can ignore the uniformity of the distribution [I mostly did in my explanation], but until you take care of those other two the density isn't the issue. BTW, in your calculations does it matter whether the excess non-stellar material reflects, absorbs, or re-transmits? I ask because your comments made me think about how Olbers' question might have been modified if he'd known more about the make-up of the universe. Mohan, this is why Olbers’ Paradox is such a fascinating question to me. There have been (and some might argue there are still) so many competing solutions that it’s difficult to point to any single explanation. You have to take a series of logical steps to explain it away. No one answer will work, which is why we had to attack each of Olbers’ conditions.

You have a very good point, but I believe you mean *additional* reason. Olbers’ question was concerning the distribution of stars — the density being practically irrelevant given his conditions. The question is: Since we know the night sky isn’t bright, what’s wrong here? Each of the conditions fails, which tells us much about the universe. Your solution still requires a contradiction to each part of the paradox, which you gave. The visible universe must be of finite size and age. Actually, I guess you can ignore the uniformity of the distribution [I mostly did in my explanation], but until you take care of those other two the density isn’t the issue.

BTW, in your calculations does it matter whether the excess non-stellar material reflects, absorbs, or re-transmits? I ask because your comments made me think about how Olbers’ question might have been modified if he’d known more about the make-up of the universe.

]]> By: Mohan Ramanathan http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/comment-page-1/#comment-12096 Mohan Ramanathan Thu, 15 Oct 2009 06:31:04 +0000 http://365daysofastronomy.org/?p=2105#comment-12096 Sorry guys...I have a better explanation. If the explanation given by you is the only reason, when we get out of earth the sky should be bright all around as we are very close to a very bright source of light. There are two factors to be considered for a bright neighbourhood. Density of matter around to scatter the light and the second is the density of light sources. Since the light is a directional stream, in the absence of scattering you see a beam when you look at the source and darkness otherwise, unlike on earth where the light is scattered. With visible universe around 13.7 billion light years in radius (i.e where the light sources matter) we can compute the volume of the spherical universe and the density of matter to scatter and the matter making up the stars. The density of matter in the observable universe is known to be 6 hydrogen atoms per cubic metre and the density of stars is believed to be 10-9 stars/ cubic light year, both these are not enough to brighten up the neighbourhood space! Sorry guys…I have a better explanation. If the explanation given by you is the only reason, when we get out of earth the sky should be bright all around as we are very close to a very bright source of light. There are two factors to be considered for a bright neighbourhood. Density of matter around to scatter the light and the second is the density of light sources.
Since the light is a directional stream, in the absence of scattering you see a beam when you look at the source and darkness otherwise, unlike on earth where the light is scattered. With visible universe around 13.7 billion light years in radius (i.e where the light sources matter) we can compute the volume of the spherical universe and the density of matter to scatter and the matter making up the stars. The density of matter in the observable universe is known to be 6 hydrogen atoms per cubic metre and the density of stars is believed to be 10-9 stars/ cubic light year, both these are not enough to brighten up the neighbourhood space!

]]> By: Oblers’ Paradox Podcast! « EIU Astro http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/comment-page-1/#comment-12001 Oblers’ Paradox Podcast! « EIU Astro Sun, 11 Oct 2009 07:02:59 +0000 http://365daysofastronomy.org/?p=2105#comment-12001 [...] http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/ [...] [...] http://365daysofastronomy.org/2009/10/10/october-10th-i-know-why-its-dark-at-night-do-you/ [...]

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