'Thinking on the vastness of heaven and earth, alone in sorrow, tears falling' from Climbing Youzhou Tower sums up the loneliness of all ages.
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history
From Chen Zi'ang's 《登幽州台歌》 (early Tang, 696 CE), a four-line poem written from Youzhou Tower (near modern Beijing) while serving in a failing military campaign: 前不见古人，后不见来者。念天地之悠悠，独怆然而涕下 (I look before me - no ancients; behind me - no ones to come. Pondering the vastness of heaven and earth, alone I weep, grief-stricken). A founding text of Tang poetic voice: a single speaker measuring himself against infinite time.
usage
Always paired with 念天地之悠悠, the preceding line supplies the cause, this line the effect. 怆然 = sorrowful and moved; 涕 in classical Chinese = tears (not modern 'mucus').
