Title from a poem by Du Fu. I believe the constellations in question are the Great Dipper and the morning star, that never appear in the sky at the same time.
Background to the Rainywillow settei can be found here and here. Professor Grant is a canon character, the antiques-mad Englishman with his love of horidashimono- bargain 'finds' from temple bazaars and small junk stalls. He's the one who gets scolded by the spirit of a teapot for not using the exact kind of tea (a very rare and precious one, as it happens) that the tea pot is used to. I think Hatsu sensei rather likes him, with his wide-eyed wonder and his innocent romanticism, but then Hatsu likes most of her characters, even the ones that make us roll our eyes.