At its aphelion (most distant location from the Sun) the outermost-known planet Eris is about 100 times as far from the Sun as Earth. What does the Sun look like from Eris' surface? Is there still a proper day or does the Sun look more like the brightest star in a night sky? Would it hurt to look into the Sun from Eris? And would it still be dangerous to look into the Sun with average binoculars?

  • $\begingroup$ I saw your pending edit for the Wiki excerpt for Hill Sphere: "Questions about the region around an astronomical body where its gravitational field dominates that of satellite bodies." I've voted to reject it (for "no improvement") because "dominates" doesn't mean much of anything. The Hill sphere is about long-term stability, no? Can you come up with something that makes it better suited to the site? Check a few questions and answers that use the term and see how it's used perhaps? Thanks! $\endgroup$ – uhoh Feb 20 '20 at 15:36
  • $\begingroup$ The others looked much better, I didn't vote either way on them. fyi if you ping me back I'll delete these temporary comments. I tried to leave a message in chat but your username didn't autocomplete indicating there was a good chance you wouldn't receive a notification. $\endgroup$ – uhoh Feb 20 '20 at 15:40

Light flux decreases as one over distance squared. So if Eris is 100 times further away from the Sun than the Earth is, then the amount of light that reaches Eris is 10 000 less than that on Earth.

Since a difference of five magnitudes is a factor of 100, The Sun would have an apparent magnitude -16.7 seen from Eris' aphelion, as opposed to -26.7 magnitude at Earth's 1 AU. For reference, this is still brighter than a full Moon, which is listed by Wikipedia's Apparent_magnitude; table of notable celestial objects at a magnitude of -12.9.

So you would still tell the difference between night and day, but the day would be rather dim and the Sun would appear point-like.

Solar eclipse glasses reduce the Sun's brightness by a factor of 500 000, so it might still be dangerous to look at the Sun with average binoculars (or even with the naked eye).

  • $\begingroup$ Even without binoculars, the Sun would be tiny but very bright when looked at directly. It could burn retinal tissue, granted only tiny regions. The effect would probably still be noticeable. $\endgroup$ – userLTK Feb 3 '20 at 10:58
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    $\begingroup$ Yes, light flux decreases inversely with distance squared, but the area of the image on the retina decreases at the same rate. So flux per unit image area stays the same with distance until reaching the diffraction limit. From that point on, the flux continues to decrease but the flux per unit image area starts to decrease so at some distance it would be safe to view the sun. I think I once read that even at Pluto's distance it might not be safe. $\endgroup$ – Not_Einstein Feb 3 '20 at 18:57
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    $\begingroup$ @userLTK any damage probably wouldn't be noticed early enough to do any good. Human vision has a huge amount of 'fake it until you make it' processing to hide things like the blind spot where your optic nerve connects to the retina, not having sharp focus over most of your field of view at any time, and not having sharp focus anywhere during some eye movements. As a result your brain is fully capable of patching over minor injuries without your noticing anything until so much of your retina is damaged that the entire house of cards collapses and your vision is ruined. $\endgroup$ – Dan Is Fiddling By Firelight Feb 3 '20 at 19:21
  • $\begingroup$ Even more, one might even say it is more risky, not less, because some of the same effects that occur during a solar eclipse can be considered as operating here: the small circle of Sun disk will act similarly to the thin sliver of disk during an extreme partial eclipse. Viewing the thin bit of exposed disk with a naked eye is more dangerous than even glancing at the Sun in ordinary daylight because it is an unusual condition your eyes aren't built for - in ordinary daylight, your pupils are constricted as much as possible, so the (cont'd) $\endgroup$ – The_Sympathizer Feb 26 '20 at 12:31
  • $\begingroup$ (cont'd) minimum amount of light will be entering your eye, and moreover, the blink and pain reflex will generally prevent you from tolerating it long enough - though of course it's possible to power through that as some nuttys do (with the expected consequences to their eyes). But in a solar eclipse and hence also, likely, Eris astronaut (w/clear spacesuit visor) scenario, both of those are not happening: the ambient conditions are (cont'd) $\endgroup$ – The_Sympathizer Feb 26 '20 at 12:31

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