Insomniacs After School, Vol. 6 (manga)

Nakami and Isaki are on their astronomy club's summer trip, but when Isaki’s older sister (and their chaperone!) ditches them, they’re left all alone. Free from their daily obligations and lost in the bliss of summer, they feel time seeming to slow down. After the trip takes a surprising turn, they reach their final destination: the starry sky of the Mawaki ruins.