The Star Gauge (Chinese: 璇玑圖), also known as Xuanji Tu ("Picture of the Turning Sphere") is a Chinese poem written in the 4th century AD. It was written by the poet Su Hui to her husband.  It consists of a 29 by 29 grid of characters, which can be read in different ways to form roughly 3,000 smaller rhyming poems.  The outer border forms a single circular poem, thought to be both the first and the longest of its kind.