As the sun set in the west, the old couple walked hand in hand through the park.
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history
From Ma Zhiyuan's (马致远) Yuan-dynasty 散曲《天净沙·秋思》: 夕阳西下,断肠人在天涯 - 'The setting sun sinks in the west; the heartbroken traveler stands at the edge of the world.' One of the most beloved lines in Chinese poetry.
usage
Beyond the literal sunset, often metaphorical - used for declining industries (夕阳产业), aging populations, or someone in their twilight years.