Someone once asked me how to visually tell the difference between seeing a star in the night sky versus seeing a planet and then stubbornly refused to accept the explanation.
Consider the human eye for which at Retinal Cell Density, we find mention of retinal cell densities of 125,500 cones/mm² and 150,000 rods/mm² . These two numbers actually seem to vary in complex ways across the retinal area but we can still get some idea of the order of magnitude of retinal cell size with our simplistic perspective. (Pun intended.)
If we next look at Subtended Arcs, we find the apparent size of several astronomical objects given in dimensions of arc seconds subtended in the sky. Saturn is shown as subtending 14.50 to 21.37 arc seconds which number varies with the distance between Saturn and Earth as the two planets orbit the sun.
By comparison, the star Betelgeuse appears simply as 0.047 arc seconds which number remains essentially constant because the distance to that far away star varies by very little percentage over the course of one year.
The skeptic to whom I refer above refused to accept that the subtended arc in the sky was different for the star versus for the planet. Both objects looked the same in the night sky so clearly they had to be the same. It was like trying to argue a flat earther out of their flatness fallacy.
Ignorance won the moment and I had to give up.
However, let's now make a very simplistic sketch of a single retinal cell receiving light from one object or the other.
If the path taken by a light ray from the star is caused to vary by atmospheric effects, the ray may now and then cease to illuminate the target, your retina. However, if the multiplicity of rays from the planet is varied to the same degree, other rays traveling adjacently to that first one will probably still provide target illumination so that the target won't know the difference.
For the star, there will be perception of variable illumination or "twinkling" but for the planet, there will seem to be essentially unvarying illumination with no twinkling.
