At night, a dim object is seen best by:

The eye has two kinds of photoreceptor and they are distributed unevenly. Cones, which handle colour and fine detail, are concentrated in the fovea at the centre of vision and need good light to function. Rods, which are far more sensitive in low light but see no colour and resolve little detail, sit in the periphery and are absent from the fovea itself. The consequence at night is counter-intuitive: looking directly at a dim object places its image on the part of the retina least able to detect it, and the object fades or disappears. Off-centre viewing — looking about 10 degrees to one side — places the image on rod-rich retina and the object becomes visible. The technique needs a further refinement, because rods stop responding to an unchanging stimulus after a few seconds, so the eye must be moved periodically to keep the image on fresh receptors. For a remote pilot maintaining visual contact with a small aircraft at night, this is a practical skill rather than a theoretical one.

Rods, which provide low-light sensitivity, lie in the peripheral retina and are absent from the fovea, so looking slightly to the side of a dim object places its image on the receptors able to detect it.

Cones concentrated in the fovea provide colour and detail vision in good light. Rods in the peripheral retina provide low-light sensitivity but are absent from the fovea, making off-centre viewing necessary at night.

Look Beside It, Not At It. The centre of your eye is blind in the dark.

Pair the technique with dark adaptation timing. Off-centre viewing works far better on an eye that has had time to adapt.

• •Staring directly at a dim aircraft and losing it
• •Sweeping the eyes instead of dwelling off-centre
• •Holding a fixed off-centre gaze until the rods adapt out