Picture the moon's month as a circle that and fills. New moon is the bottom of that circle — the point where the old month has run out and the next one has not yet shown itself. Two things sit at that same point, and English lets the phrase name either of them: the moment the cycle turns over, and the first of light that appears when it does. That is why the same two words can mean a moon you can just see, and a moon you cannot see at all.
From the word’s history
Wiktionary records new moon as coming from Middle English newe moone, newe mone — simply new plus moon. The literal sense, a moon that is new, matches the everyday meaning: the moon at the point where its round of shapes starts over.
詞源資料記載，new moon 來自中古英語 newe moone、newe mone，就是 new 加上 moon 兩個字。字面上的「新的月亮」正好對上日常用法：一輪月相重新開始時的那個月亮。（天文學那個「完全看不見」的定義是後來的專門用法，不在這條詞源裡。）
Wiktionary, entry “new moon” (etymology_text supplied in the evidence packet; kaikki source_ref 3f6f69a190036ae4:804207008)
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