Astronomy and Art, two worlds seemingly opposite. One scientific and rigorous, the other perceived as light and whimsical. But if these two worlds were united? If art gave itself to the service of science? If fantasy became a playful pretext to transmit knowledge to young children? I recount here my experience, that of a meeting, and of an adventure. How and why an astronomy book for the very young emerged from the collaboration between a painter and an astronomer of the Observatory of Paris.