Sorry for the depressing title, but today is the darkest day of the year. The word equinox (which is not what we are in, we're in the solstice, but I did this word just for giggles) entered the English language in the fifteenth century, and came from the twelfth century french word equinoce. This comes from Medieval Latin equinoxium, which came from some older version of Latin and the word aequinoctium, literally meaning "the equality of night and day" and a portmanteau of the Latin words for "night" and "equal". The Latin word aequalis, meaning "uniform or identical" came from the older word aequus, defined as "flat or level", since things that are on level ground are equal. Aequus has uncertain origins past that, but the Latin word for "night" goes all the way back to PIE, and the word newkt, with the current definition. As PIE dissolved into a hodgepodge of otherwise unrelated language families, newkt went into Latin as nox (also the spell to turn your wand-light off in the Harry Potter books). Noctium could be used to describe nox the night, and is also present in today's words nocturnal and equinox (the -nox suffix is from noctium, not nox, just to clarify). It is thus that equinox literally translates into "flat night", if you trace back the word far enough.
Adam Aleksic has been described as the internet's sixth most famous etymologist. He loves writing about himself in the third person, he's a freshman at Harvard University, and he has disturbing interests in linguistics, vexillology, geography, board games, limericks, and law.
The Etymology Nerd