Watching the sunset, one recalls the poetic image "the white sun leans against the mountains and is gone".
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history
From Wang Zhihuan's (王之涣, Tang dynasty) 《登鹳雀楼》: 白日依山尽，黄河入海流。欲穷千里目，更上一层楼 — 'The white sun leans against the mountains and sinks; the Yellow River flows on into the sea. If you would exhaust the thousand-li view, climb one more storey up.' Twenty characters that every Chinese child memorizes. The second couplet 欲穷千里目，更上一层楼 is one of the most quoted proverbs in the language.
usage
依 here = 'to lean against'. 尽 = 'to finish' — the sun 'runs out' behind the mountain. Parallels 黄河入海流 in the next line: sun vs river, ending vs continuing.
