Sanji literally saw the circle Saturn came out of, with his eyes. He still has way better CoO than Zoro, but that's beside the point. Sanji drew a cake in full ass detail diagram after seeing it once. He'd have recognized the circle. He's not got the peanut sized brain of of Luffy and Zoro or the short term memory loss of Zoro and his 'oh that's a Lunarian,' ass.