"It's an imaginary planet, but size is still an attribute planets have, even when they don't physically exist. I'm not an artist; I didn't imagine this picture and then decide it should display planetary rotation. I imagined a set of characteristics for a planet, and then I had the magic generate an appropriate image. If I'd imagined a planet the literal size of the picture on the cover of this book, it would be a lump of rock, or maybe a neutron star."