R. H. Blyth wrote that in autumn the Milky Way is most clearly seen and felt. Sadly that is no longer true in many places. The reason is the pollution of the night sky by uncontrolled artificial lighting. These days, a city dweller is fortunate to see even a few stars at night. We have lost touch with our place among the stars.
Through the hole in the shōji —
The River of Heaven.
To understand that, one must know that a shōji is a door or window that is a light wooden frame covered with white paper. It allows light to penetrate, but of course one cannot see through it unless there is an accident — a hole poked or torn in the shōji.
So in this hokku, Issa is in the darkened interior of a poor house where holes in the shōji paper are not quickly mended. He notices that through the hole, he can see the dark night sky outside; and slanting across it, the faint brightness of the Milky Way, which Japanese call the River of Heaven. Among Native Americans it was commonly known as the Spirit Road or Spirit Path — the path followed by spirits to the afterlife.
Neither Issa nor nor Native Americans knew that the Milky Way is actually what we see when we look toward the center of a galaxy in which our planet is less than a dust mote. We live on our tiny planet about halfway between the center and the outer edge of a cosmic whirlpool composed of untold billions of stars. And our galaxy is just one of many billions of galaxies in the universe.