The stretch of light between sunrise and sunset. Look at that stretch and you can see two things at once: the bright part of the day, and one whole turn of the day that will come round again tomorrow.
可以想成太陽升起到落下的那一段白天。盯著這段光，其實可以看見兩件事：一件是「白天這段時間」，一件是「一天轉完的一圈」。diurnal 的兩個意思就像從這裡往兩邊走——一邊講白天在活動的動物和開著的花，一邊講每二十四小時重來一次的循環。
From the word’s history
From Latin diurnalis, from dies (day). Journal comes from the same root, which is why a journal was first a day-by-day record.
來自拉丁文 diurnālis，字根是 diēs（一天、白天）。有趣的是 journal 也是同一條線：journal 一開始就是「每日的記事」。把 diurnal 和 journal 放在一起記，這個「日」的畫面就很好抓。
Wiktionary, "diurnal"
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