Long ago, I asked how far Earth would orbit the sun if one year lasts 372 days instead of 365 and how much it'd affect the overall brightness. What I got was that Earth with a 372-day calendar would be orbiting the sun from a distance of 93,930,000 miles, resulting in a two-percent dimness of sunlight. Not noticeable to our eyes, but I suspect that the plants and animals would be more sensitive to that.

In an alternate Earth, there is a moon orbiting the planet, but it is not as large as ours. However, it orbits so closely that, to our eyes, it looks bigger--one-and-a-half degrees wide, three times bigger than our view of the moon. There is another source of light--a series of rings 15 degrees wide, originating from a brown dwarf star that orbits the sun from a distance of 30 AUs.

With these two sources of light, how bright would day and night be on this alternate Earth?

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    $\begingroup$ This is math or physics, not worldbuilding. $\endgroup$
    – Daniel B
    Nov 5, 2020 at 5:07
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    $\begingroup$ Are you sure that the rings can exist in that configuration? I would say momentum conservation would flatten them in a disc $\endgroup$
    – L.Dutch
    Nov 5, 2020 at 6:16
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    $\begingroup$ More details please. What's the albedo of your moon (or is it identical to Earth's) and what's the albedo of the rings? I'm assuming that you're assuming "best case" (as the time of year and the time of night will effect the amount of incident light). Finally, 30AU is the distance from our sun to Neptune. Are you suggesting that the rings are so large that they'd fill 15° in the sky? At that distance? I'm not sure if it's possible. $\endgroup$
    – JBH
    Nov 5, 2020 at 7:34
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    $\begingroup$ @a4android which one? Remember: 30AU distant from the sun but capable of spanning 15° in the sky (John's not talking about Saturn, he's talking about the asteroid belt on steroids). $\endgroup$
    – JBH
    Nov 5, 2020 at 14:24
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    $\begingroup$ @JBH That's my thinking too. Even if the ring system had something hotter than a brown dwarf, and I do mean really hotter, it would visually superb. The amount of illumination on the planet would still be negligible. $\endgroup$
    – a4android
    Nov 6, 2020 at 23:53


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