Youkai Astronomy (妖怪の天文, Yōkai no Tenmon) is a unique form of astronomy in Touhou Project used by the youkai of Gensokyo and it's highly presumed that Yukari Yakumo named these constellations based on Rinnosuke's observation.

It's first explored in Chapter 22 of Curiosities of Lotus Asia with Reimu, Marisa, and Rinnosuke, received an off-hand mention by Aya in Double Spoiler, then it became the focus of Chapter 25 of Touhou Ibarakasen where Nitori and the Kappa established a planetarium to formally teach the humans about youkai astronomy. The subject hasn't really been relevant since then, aside from Okina's tabard.

• Despite the name, the music album Magical Astronomy has nothing to do with youkai astronomy as this music album actually predates the Curiosities of Lotus Asia chapter by a few months.
• There's a theory that Orion (Ibuki-dōji) is the Sumiyoshi Sanjin's deification, the three Shinto gods that the Watatsuki sisters briefly mention in Chapter 8 of Cage in Lunatic Runagate.