If you go far enough, you will get to the point where the earth and the sky meet, and can poke your head out in the world beyond. This well-known print is obviously pseudo-medieval, from the 19th Century or later; lordtangent, who has provided the following scan, has told me that it first appeared in one of the works of the romantic astronomer Camille Flammarion (apparently L'atmosphère: météorologie populaire, 1888). Flammarion's works are full of wonderful pictures of real - or supposedly real - meteorological and astronomical phenomena; and it has been plausibly suggested that some of Van Gogh's paintings of starry skies may have been inspired by them.
The original illustration.
Flammarion's (somewhat misleading) depiction of the Brocken Spectre. Lots of photographs of such phenomenona here:
Ulloa's Circle, as observed in the Ecuadorian Andes.
"Ulloa, being in company with six fellow-travellers upon the Pambamarca at daybreak one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the sun, when it rose, dissipated them, leaving only in their stead light vapors, which it was almost impossible to distinguish. Suddenly, in the opposite direction to where the sun was rising, “each of the travellers beheld, at about seventy feet from where he was standing, his own image reflected in the air as in a mirror. The image was in the centre of three rainbows of different colors, and surrounded at a certain distance by a fourth bow with only one color. … All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person they enveloped as with a glory."
These two prints came from this site, which is full of splendid pictures of strange atmospheric phenomena:
The canals that were supposed to exist on Mars.
Snow at the South Pole of Mars!
A comet. I saw one a few years back but, alas, it was nothing like this.
Now we get to Van Gogh and his starry sky.
By way of comparison, the Whirlpool Galaxy (Messier 51) in Canes Venatici.
It was discovered by Charles Messier himself in 1774, but its spiral structure was first observed by Lord Rosse in 1845. He had the benefit of being able make use of the largest telescope in the world on his Irish estate:
A chronology of how the term whirlpool came to be applied to celestial objects:
Another drawing of it by Rosse. It was called the Whirlpool Nebula in the old usage.
Form which the print was made that would be published by Flammarion, and thus come to the knowledge of Van Gogh! His painting may look crazy (and actually is a bit crazy), but is really exceedingly sober when compared to modern photographs of galaxies like this.
There are plenty of books by Flammarion at the Internet Archive, here is one in English translation: