Is it for the same reason as that when light shines through a rectangular peep-hole, it appears circular in the form of a cone? The mystery was resolved centuries later, when it was concluded that the circular shapes on the ground were camera obscura projections of the sun and thus became crescent-shaped during an eclipse.
Euclid is sometimes reported to have mentioned the camera obscura phenomenon as a demonstration that light travels in straight lines in his very influential Optics (circa 300 BCE).
Light from an external scene passes through the hole and strikes a surface inside, where the scene is reproduced, inverted (thus upside-down) and reversed (left to right), but with color and perspective preserved.
The camera obscura box was developed further into the photographic camera in the first half of the 19th century when camera obscura boxes were used to expose light-sensitive materials to the projected image.
Before the term "camera obscura" was first used in 1604, many other expressions were used including "cubiculum obscurum", "cubiculum tenebricosum", "conclave obscurum" and "locus obscurus".
His disciples developed this into a physics theory of optics.
“Why is it that an eclipse of the sun, if one looks at it through a sieve or through leaves, such as a plane-tree or other broadleaved tree, or if one joins the fingers of one hand over the fingers of the other, the rays are crescent-shaped where they reach the earth?
In his 1088 book Dream Pool Essays the Song Dynasty Chinese scientist Shen Kuo (1031–1095) compared the focal point of a concave burning-mirror and the "collecting" hole of camera obscura phenomena to an oar in a rowlock to explain how the images were inverted: "When a bird flies in the air, its shadow moves along the ground in the same direction.
But if its image is collected (shu)(like a belt being tightened) through a small hole in a window, then the shadow moves in the direction opposite of that of the bird.[...] This is the same principle as the burning-mirror.
There are theories that occurrences of camera obscura effects (through tiny holes in tents or in screens of animal hide) inspired paleolithic cave paintings.
Distortions in the shapes of animals in many paleolithic cave artworks might be inspired by distortions seen when the surface on which an image was projected was not straight or not in the right angle.
Lit objects reflect rays of light in all directions.
A small enough opening in a screen only lets through rays that travel directly from different points in the scene on the other side and together form an image of that scene when they are reflected on a surface into the eye of an observer.
As the pinhole is made smaller, the image gets sharper, but the projected image becomes dimmer.