Originally Posted by
IO540
The Mark 1 eyeball is the best non-technological option of avoiding another aircraft. It is however pretty useless because an aircraft on a genuine collision trajectory will be a stationary point in your field of view (assuming straight line trajectories for both).
Gliders maneuver around so should be more visible, which is probably why there has not yet been a glider-GA midair, whereas GA-GA is about 1 per year.
It's true that a glider thermalling is circling, and by looking in the right places (under Cu) it's often possible to see them from a long way away, especially if the sun is in the right place to reflect of their wings during part of the turn.
However they also fly along wave bars, often at 80 knots IAS or more, and also along cloud streets if they find a line of energy or are flying from one thermal to the next.