Instead of a moon, that earth had another world in its sky. The other world, called Selene, had its own people who lived there (though how they got there is still a mystery to this day.) The people there had their own ways of doing things, their own myths, their own styles of clothing and architecture. Instead of the wool of sheep, they used silk gathered from particularly docile moon-spiders. Everyone knew, pretty much, that Selene was an island in the sky, but until the Middle Ages there was essentially no contact with them. It was around that time that the Selenites invented rockets, and their larger firework shows were sometimes visible from Earth. Soon their rockets grew powerful enough to leave the weaker gravitational pull of Selene and enter Earth's atmosphere. They sent gifts of their strange moon-seeds, moon-spider-silks, moon-wine. This so inspired Earth's inventors that flight technology developed very rapidly; and Leonardo DaVinci, as a young man, built the first balloon-plane capable of leaving Earth and flying through the void to Selene. (In that space, strange creature swam and were hunted by the Selenites.) For a hundred years, balloon-plane pilots made flights back and forth to Selene, bringing Selenite goods to Earth and Christianity and gold to the Selenites.
However, Emperor Ming saw the Earth people as a threat, and began shooting every balloon-plane he saw out of the sky with his rockets. This blockade continued until the 1800s. Around the time of the American civil war, an earth admiral took his sky-ironclads to Selene and ended the blockade forever. From that time, there began to be a great exchange of trade between Selene and the Earth. At the turn of the twentieth century, earth artists were drawing inspiration from the glowing paints and oval canvases that characterised Selenite art. The Selenites depicted earth inventions, like steam power and electricity, as manifestations of strange gods.
Tensions between the two worlds grew, however, and in the 1940s the Selenite emperor rained meteors down on the earth. The war between worlds largely took place in space, where the Selenites were gradually driven back. Finally their great space-battleship Luna and two of their cities were destroyed by a devastating secret weapon, and their emperor surrendered. The Selenites accepted the earth admiral as their new ruler, and in return earth helped the Selenites to rebuild.
My grandfathers fought in that war. My father was an airship builder, whose work was threatened by competition from Selenite industry. When I was a young man, I travelled to Selene for a few years and spoke with the moon people in their own language. Selene and Earth have grown to resemble each other in a lot of ways, and yet... when the Selenites draw Earth styles, they get mixed up between centuries and countries and don't really care about it. They adopt some of our stories, but mix them up with their own myths about space beasts and moon-ghosts. Earth stories, too, have begun to resemble moon-myths. The story of the war and the devastating secret weapon left strange echoes on the products of their culture...