Ok, this is one of those thoughts I have in my brain and that I can't quite get rid of.
It breaks down into a couple of questions. For the purposes of this, I'm aware that what eyes see is the reflection of light bouncing off objects, but I'm curious the impact on the visibility of both objects and other lights.

A. If eyes emitted any light, could they still see anything at all?
B. If eyes emitted, for example, red light, could they see everything except red items? What about red lights? Does this change if the light is green or violet?
B.1. If they can't red things would they just be invisible?
B.2. If they can't see red lights, would it matter if the red light they're seeing is brighter or dimmer, and would it still be an invisible/blank space?
C. I'm not sure how infrared interacts here but I know animals that sense infrared do emit it, is there a reason that's different, if it's different.

The internet is mostly not super helpful with this, since eyes don't emit light, just reflect it and look glowy, but yeah, anyway... thanks for entertaining my weird fixation.

I am writing this short story. It is part of the overall book that I am writing, but it is also a story that can be enjoyed completely on its own. In that story, a planet-sized ship approaches our Solar System, and, ultimately maybe, Earth. For dimensions, let's say it is equal to 1.5 of Earth's diameter. According to Google, that's 25,512 19,134 kilometers. The Planet-Ship is probably less dense than Earth, as it is largely occupied by biomass and weird alien electronics. You may think of it as a round Borg cube, from Star Trek.

My "Round Borg Cube" is completely black and spherical, with a smooth surface without any visible features.

What I wanna know is...

• Assuming that the object is on a very slow path towards Earth, at what proximity will its effects be known?
• At which point will scientists observe its effects, view it, or detect it with instruments?
• If that is at all possible, what would be a threshold in which the "Round Borgs" would have to interrupt their movement in order to remain undetected?
• At which distance will it be visible to the naked eye (if at all)?
• And if they chose to get dangerously close to Earth, what would be the impact on our environment?

I understand that is a lot to ask, but I just can't trust GPT for that kind of stuff, even if their answers sound plausible. Perhaps someone with astronomical knowledge as well as an interest in science fiction will find my questions enticing. I don't expect precise answers because I am not providing precise information. So feel free to speculate on that scenario. In any case, I am grateful for any answer I can get.

Thanks!