Engetsu Island — Sunset Through the Natural Arch

Engetsu Island (円月島, 'full moon island') is a small uninhabited island in Shirahama Bay, famous for its natural rock arch formed by wave erosion over millennia. The arch stands approximately 9 meters tall and 25 meters wide, resembling a full moon when viewed from shore. The island's name and fame derive from the spectacular sunset phenomenon — around the spring and autumn equinoxes (late March–early April and late September), the setting sun aligns perfectly with the arch, creating a glowing 'moon' silhouette framed by the rock formation.

The island is best viewed from Engetsu Island Observatory and the adjacent coastal promenade, where benches and viewing platforms provide unobstructed sightlines. The island is located close to shore (150 meters offshore), making it prominent in the seascape without requiring boat access. The arch is visible year-round, but sunset alignments vary seasonally — winter sunsets occur further south, summer sunsets further north, and only during equinox periods does the sun pass directly through the arch. The scene is a favorite among photographers and couples (it's considered romantic), and the island has become Shirahama's iconic symbol, featured on local souvenirs and tourism materials. The natural arch exemplifies the power of geological processes — the island was once solid rock, gradually hollowed by wave action until the arch broke through.