What or who are The Sailing Seven that William Butler Yeats refers to in the second stanza of his poem A Cradle Song?

In this delightful poem, which expresses the affection of God and the Cosmos for a small infant, Yeats is refering to the Pleiades, also known as M45, the Seven Sisters, SED, or Subaru. It's an open cluster in the constellation Taurus that actually has nine named stars in it. The Pleiades were nymphs, the seven daughters of the Titan Atlas and the sea nymph Pleione -- their group name is derived from their mother's name. The cluster is visible in the Mediterranean at night during the summer from mid-May to early November, which was the sailing season in antiquity -- thus they are known as "The Sailing Seven".