Field experiments show that a wire may be seen against a sky of high brightness when its diameter subtends only half a second of visual angle. For this the wire must be at least one degree long. Shorter wires must be thicker. Silhouetted squares may be seen at angles of only 18 seconds. Considering length and width, squares are more efficient visual targets than fine lines by about a factor of 3.
No takes yet. Share an insight, caveat, or question.
Hecht et al. (1947) studied this question.