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What Is Obler's Paradox?

Why is the sky dark when it is full of bright stars?

Why is the sky dark when it is full of bright stars?

Obler's Paradox

Let’s jump right into it. If there are so many stars in the night sky, why is it black? Yes, the sun goes down and removes a huge light source, but what about all the stars, galaxies, and nebula out there? Even if the Universe were infinite in size, we should see something everywhere. And yet…darkness reigns with just a few pinpoints of light to break it up. But some might bring up some possible answers to this conundrum, which should be examined to see their veracity. Some point out that stars that are too faint to see at the distance they are located do exist, but when they initially formed, the Universe was a lot smaller, and so they have distributed out fairly well as the Universe has expanded, creating no bias in brightness. Others have suggested that maybe stars are not evenly distributed in the Universe but follow some bias or that there are simply not enough of them out there. And this is almost certainly a true statement, but when you look at a larger and larger scope of the Universe, everything seems to average out rather well, again because of our expanding Universe. And the number of stars needed to light up the Universe isn’t that hard a figure to get to, as it turns out. So…why is the sky dark? (Al 43-4, Chase, Nave)

Herrmann Bondi named the paradox in 1952 after hearing about Heinrich Wilhelm Obler’s efforts in the 1800s to resolve it, but it was around a long time before either people. We have to go all the way to 1576, when Thomas Digges modifies the Copernican system of the Universe. In his new edition of A Prognostication Everlasting, he essentially keeps the Copernican system untouched except for one detail. Stars are not tiny holes in some outer shell of the Universe but are actually objects that are dispersed in a space that is infinitely large in size. Quite the statement from someone without proof in those days, but Digges claimed that the idea arose from Tycho Brahe’s supernova of 1572. That event displayed no parallax of motion, indicating that it was far away. But the heavens never changed according to the contemporary views of the time. Since this was now in question, why couldn’t other parts of the world view be changed also? Digges felt that with enough stars far away, the darkness in the sky could be explained, and the system would be intact (Al 45-8).

Years later, Jean-Phillippe de Cheseaux shows that this cannot be true using geometry. Those outer stars have the same brightness component as the inner ones because of the volume of space they occupy together, acting like a big source of light even at such a distance. Obler, in 1823, felt that interstellar dust and gas could obscure a lot of light from distant stars. As it turns out, the Universe has been around so long that the dust and gas have heated up via collisions, and they actually emit the same light as those distant stars they obscure, so no dice there (Al 50, Chase).

No, the solution lies in the expanding Universe. You see, light can only go so fast, and if the space it moves through expands fast enough, then it will take a long time for the light to reach you, especially when expanding faster than c. And even if the Universe wasn’t expanding currently, you still would have a dark sky at night because of inflation in the early Universe driving space apart faster than c. It didn’t last long, but it did cause portions of space to be forever screened out. And because of a finite Universe, only so much has happened in a span of time. There is no way enough time has passed for stars to get to any of the configurations needed for a bright night sky. Sorry, everyone. The mechanics of the world are still maintained, and now you can look up at the night sky and realize you are witnessing an amazing moment in science (AL 58-9, NASA, Nave).

The sky as it would seem it should be...

The sky as it would seem it should be...

Works Cited

Al-Khalili, Jim. Paradox: The Nine Greatest Enigmas in Physics. Broadway Paperbacks, New York, 2012: 43-8, 50 PAGES. Print.

Chase, Scott I. “Olber’s Paradox.” UCR, 2004. Web. 19 Sept. 2017.

NASA. “Why is the Sky Dark at Night?” NASA. Web. 19 Sept. 2017.

Nave, R. “Olber’s Paradox: Why is the Sky Dark at Night?” Georgia State Univeristy,2000. Web. 19 Sept. 2017.

© 2018 Leonard Kelley


Alexander James Guckenberger from Maryland, United States of America on January 10, 2018:

Of course! I loved learning what this is!!!

Leonard Kelley (author) on January 09, 2018:


Alexander James Guckenberger from Maryland, United States of America on January 09, 2018:

very nice :)