天象: tenshô. In the traditional charts for renku, this category is listed simply as "moon." The moon is indeed a special category of experience for deep historical and cultural reasons—the foremost being its association with the harvest.
In our renku of twenty verses, two moon verses are required. But you should also feel free to touch on other celestial objects and phenomena: stars, plants, comets, meteors, black holes, and the like. Because celestial bodies are so dazzling and memorable, the rule is to separate them by at least five intervening verses—challenging in a 20-verse renku but certainly possible.