Expert: Courtney Seligman - 10/5/2009

Question
Hi. I know all about how Saturn is generally considered the last of the "naked eye" visible Planets but that Uranus can bee seen in certain conditions and so forth. But I heard Neptune could be viewed through binoculars and it got me thinking. Theoretically do you think in a clear night sky in a dark place without allot of lights around somebody with particularly good vision like better than 20/20 could possibly dimly spot Neptune if they knew where to look? What about say the Galilean moons of Jupiter? Of course that generally wouldn't make it a "naked eye" object since Uranus isn't even considered one... But I'm just wondering if it's even conceivable that under the best conditions it could happen? What are your thoughts?

Answer
I've heard apocryphal statements about some people being able to see exceptionally faint objects in extremely dark sky conditions, but the faintest reliable observations I've heard of are half a magnitude to a magnitude brighter than Neptune ever gets. So it might be possible to see it, but I doubt it.

As far as Uranus goes, although not generally considered a naked-eye planet, it is usually visible. It's just not bright enough to be noticeable. All the other planets are not only visible, but first magnitude or brighter under normal conditions. So they 'stand out' from the background of fainter stars, whereas Uranus looks just like any other 5th or 6th magnitude star, and not be noticed without a detailed comparison of every star in its region to a star atlas.

The Galilean satellites, however, are bright enough to be visible without any difficulty, save one -- they are so close to Jupiter that the glare from the planet makes it look larger than the distance between it and its moons. So you need binoculars to see them, not so much because of their faintness, but to put them far enough from the planet to be seen as separate objects.