核心畫面
You can think of a tropic as a turning line. Through the year the overhead sun slides north, reaches one line, and comes back; then it slides south, reaches the other line, and comes back again. Those two turning places are the two tropics. The wide hot band caught between them is 'the tropics', and anything belonging to that band can be called tropic.
tropic 可以想成一條「太陽轉彎的線」。一年之中，正午太陽直射的位置慢慢往北移，移到某一條線就停住，然後折回南邊…更多收起
tropic 可以想成一條「太陽轉彎的線」。一年之中，正午太陽直射的位置慢慢往北移，移到某一條線就停住，然後折回南邊；往南也一樣，到另一條線就轉回來。這兩個轉彎的位置，就是那兩條 tropic，中文叫回歸線。太陽永遠在這兩條線之間來回，所以中間那一大片曬得最久，也最熱——英文用複數的 the tropics 來指它。屬於那片地帶的東西，舊一點的寫法就直接用形容詞 tropic。From the word’s history
From Late Latin tropicus, from Ancient Greek tropikos, from trope meaning 'a turn; the solstice'. The name records the moment the sun turns back.
來自晚期拉丁文 tropicus，再往前是希臘文 tropikós，字根 tropē 的意思是「轉；至點」。也就是說，這條線的名字記下的是太陽折返的那一刻，而不是那裡有多熱。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:1074726445) for 'tropic': Late Latin tropicus < Ancient Greek τροπικός < τροπή