Max has asked about two issues that I really ought to get into: the stars Shen and Shang, and my use of the pronoun "they" in the beginning of the translation, only to switch to "I" later on. First, stars:
人生不相见,动如参与商。
These human lives do not see each other, moving
like the stars Shen and Shang.
Shen and Shang are individual stars, the first lying within the constellation of Orion (specifically corresponding to one of the stars of his belt) and the second corresponding to Antares. As such, each rises only when the other sets, and they do not seem to move relative to each other -- natural symbols for two people who share an intimate connection and yet must be apart, as in Du Fu's poem. As far as I know, however, these two stars have no metaphorical connection of this kind at all in the Greco-Roman-based Western astronomy Americans learn in school. By contrast, as early as the Zuo Comentary (compiled c. 400 BCE but contained information from much earlier sources) there have been Chinese written references to the association between the stars Shen and Shang and stories of separation. The Zuo Commentary itself cites the story of two brothers, sons of the Legendary Emperor Ku (the great-grandson of the Yellow Emperor), who were so at odds that their father enfoeffed each with different lands. E'bo (阏伯), the eldest, was given land that became known as Shangqiu (商丘), and was called "the Shang star" after he died. His younger brother Shishen (实沈) was then naturally called "the Shen star" after his death -- and the legend, so they say, was born.
The Greco-Roman and Chinese astronomical traditions independently developed many of the same predictive techniques and even fit some of their stellar groupings into vaguely similar conceptual frameworks, but of course despite their similarities remain extremely different. In the classical Chinese astronomy of the Tang Dynasty, for example, the sky was divided into a total of "Twenty-eight Mansions" (二十八宿), plus "Three Enclosures" (三垣). So one would say that the star Shen from the poem is located in the Shen Mansion of the White Tiger of the West (东方白虎参宿), and the Shang star is located in the Xin Mansion of the Azure Dragon of the East (西方青龙心宿, which in earlier times -- as early as the Shang Dynasty, supposedly -- was 苍龙; just as in earlier times 方 was 宫).
All of this background probably only scratches the surface of what Du Fu, a well educated man of the Tang, would have known. He would have been aware of other famous references to the stars, such as to the story of the Cowherd and the Weaving Girl, and perhaps had in mind some resonance with another text I've never even heard of. Indeed, his use of the Shen-Shang reference is illustrative of the sophistication of his interaction with the Chinese canon. Just as when he plays with the well-known "To-night, oh, what a night", this choice of metaphor provides an unexpected new lens through which to view the rest of the action of the poem. The stars are cosmically determined to never meet -- their separation is a definite, natural feature of the world. Human lives, says Du Fu, do not see one another in precisely this fashion. That is, they simply couldn't.
