The concept of the “gyre” (Line 19) appears in the final stanza of “Sailing to Byzantium” as a reference to Yeats’s complex idea of cyclicality. This kind of cyclicality is four-dimensional, like a pair of cones twisting their way around each other. Unable to rely on this image as an adequate explanation, he explicitly lays out his philosophy in his book on poetics and mysticism, A Vision (1925). The timeline of humanity, he argues, is a “Great Wheel” consisting of 28 points—one for each part of the lunar cycle—surrounding the circle like numbers on a clock. Rather than two hands, the wheel has four, each representing polar concepts such as Mask and Body (secularity) and Will and Creative Mind (religiosity). Together the 28 spokes form an entire cycle as the elements coalesce in different combinations. A full cycle symbolizes existence in two ways: On the microcosmic level, it is the life, death, and rebirth of an individual, while on a macrocosmic level it accounts for 2000 years of human history, which periodically shifts from epochs of terror into ones of beauty.

From Yeats’s vantage, the wheel’s halfway point is a fleeting moment of historical splendor. Between the reign of the ancient Greeks in 500 BC and the Italian Renaissance in 1500 AD was the glory of Byzantium, which began in 500 AD.