system
Given the following post, respond as if you're commenting on Reddit:
human
How far from a mirror do you have to be to see yourself blink?

I understand that there is a delay in the time for the light to travel to the mirror and come back at any distance, so technically you always see a tiny portion of your blink in the mirror, but how far would you have to be so that you clearly see the blink and comprehend that you're seeing yourself blink.
gpt
Let's say that you want to see the entire blink, from the moment your eye closes until when it opens again. Therefore, your eye must re-open before the light from the moment your eye first closed has had time to travel to the mirror and back. If the mirror is a distance *R* from your eyes, then it takes a time 2*R*/*c* for light to travel from your eye to the mirror and back. If *T* is the duration of your blink (the amount of time between the initial close-eyes event that begins the blink and the final open-eyes event that enables you so see the light coming back to you), then we must have:

*T* < 2*R*/*c*

Solving for *R*, this expression becomes:

*R* > *cT*/2

The average blink lasts *T* = 0.4 seconds, so you would have to stand at least about 60,000 km (37,000 miles) away from the mirror to see the entire blink. For reference, the distance from New York to Paris is about 5800 km. At this distance, you would need a powerful telescope and no air in the way to actually see yourself in the mirror.

UPDATE: Depending on which study you reference, average human blink duration can run anywhere from 0.1 seconds to 0.6 seconds. Blinking faster will change these numbers a bit, but not by much. A 0.1 second blink would require at least a distance of 15,000 km from the mirror.