The etymology of 초 is interesting. This should be pronounced 묘 in accordance with the Korean reading of the character 秒. The identically pronounced characters which look similar probably caused this particular reading:
They are all part of the same phonetic series and so by mistake, 秒 also received the same pronunciation.
The reading 초 for 秒 apparently derives from 初 (초, start) as it could refer to year one in the sixty-year cycle. -- https://ko.wiktionary.org/wiki/%EC%B4%88
The Japanese reading, totally unrelated, probably derives from 部 (부: unit for counting books or newspapers, department) from which they get the hiragana character ヘ, the only hiragana derived from only one part of a character in cursive. べうー＞びょう。
The ideograph 秒 means something like chaff or fine grain, but it can also mean a beard, an awn, or a beam (of light) at least in Japanese. Oh, it came from Sun Tzu (of The Art of War book of strategy) who used it as a measure equal to a tenth of a hair's breadth. I hope someone appreciates that -- that took me too long . . .