Aircraft detection is different depending on day or night. During the day, the further the object falls from the fovea (the center of the eye), the larger the image must be in order to be noticed. At night, detection is sometimes superior if the target is on the peripheral retina (i.e. off-center), rather than the Fovea.
The brightness of an object itself is one factor in determining whether or not an object is detectable. A neon-orange paint job has greater luminance than a dull grey paint job. Reflectivity is another way to think about luminance.
While luminance is based on the object, contrast is the difference between the object and the background environment. A white Cessna might be easy to spot flying over evergreen forests. The same aircraft would be nearly invisible flying over ice or snow.
The eye requires at least 30 minutes to adjust to darkness. Even a brief exposure to white light can destroy your night vision. This will make it harder to acquire and track targets at night. Conversely, eyes need time to adjust to bright light too. Using appropriate vision aids (sunglasses for days, red lights for night) will help preserve your adaptation to the flight environment.
Against a stationary irregular background, an aircraft needs only a few minutes of arc-per-second motion to be detectable. Against a featureless background, like a cloudless sky, an object's perceived motion must be 10 times faster for visual acquisition.
An aircraft that darts in and out of clouds presents a special challenge to the viewer. When not continuously exposed to view, the pilot has to judge speed and direction in order to track an object. Small, slow moving targets that present little contrast against its background can be easily lost during intermittent observation.