New Horizons we can Learn 

We use, when we can see it, the actual horizon, when we fly CAVU. When we have our plane trimmed for level flight the horizon become important.

If we scan the horizon, in small segments, it allows us to pick up other planes. At large distances we can't tell whether they are coming towards us or away from us.

If the image is on the horizon it is flying at the same altitude as our plane. If it ascends above the horizon a few minutes later, in the distance, it is gaining altitude and will pass above our plane.

Same observation with a plane descending below the horizon.

Do not forget, in a private plane, to include in your scan a look below and above the immediate vicinity of your plane. These "close encounters of a devious kind" can kill you.

An airliner cannot see well below or above itself. You must make sure your airspace is clear.

Our eye vision is complicated. If an object remains in the same position for several minutes on your retina, it no longer perceives the presence of the image in the brain. This is why you scan in small segments so a plane on a collision course with you is seen so you can take evasive action.

In practice, this occurs often in driving, when a person is hit by a car from the side at an intersection the remark, "I never saw him", is often heard.

This is when the angle doesn't change between your plane and the plane you eventually collide in your vision. This lack of change causes the object to remain on your retina at one spot,without moving. The retina becomes less sensitive to the stimulus and doesn't send signals to the brain that would stimulate a response. The brain, without stimulation, interprets nothing.

Another example of this lack of change, in the angle of observation, is predicting when a train car will pass over the top of your automobile as you pass under an overhead track. this wil hold true if your speed and the trains remain constant.