What does the Sun look like from Eris at its aphelion? 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?
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).