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The cubit — the forearm, elbow to fingertip — is the oldest length unit with a paper trail, and it was never one unit. Egypt used two: the common cubit of 6 palms and the royal cubit of 7 palms, about 52.5 cm, standardized on surviving cubit rods for pyramid-building. Herodotus, describing Babylon’s walls, pauses to explain the difference to his Greek readers: “the royal cubit is three fingers longer than the ordinary” (Histories 1.178).
Mesopotamia had its own standards — Gudea’s famous diorite statues fix a Babylonian cubit of 49.5 cm — and the Hebrew amah grew up between these neighbors. Ezekiel complicates matters further, measuring his visionary Temple in “cubits of a cubit and a handbreadth” (Ezek 43:13): a long cubit, one-sixth larger than standard. Chronicles hints why, saying Solomon’s Temple was built in “cubits following the first measure” (2 Chr 3:3) — the standard itself had shifted over time.
The rabbis inherited this layered history and made it law. The Mishnah fixes the amah at 6 tefachim, but which tefach? Modern authorities split: R. Chaim Naeh’s amah is 48 cm, the Chazon Ish’s 57.6 cm, and Rabbi Benish’s reading of the Rambam lands near 45.8 cm — close to the archaeological estimates. Rav Moshe Feinstein typically followed the larger figures where stringency mattered. So Noah’s ark (Gen 6:15: 300 × 50 × 30 cubits) is 137 meters long by one authority and 173 by another — and both are “correct” within their systems.
The lesson of the cubit is the lesson of all ancient measure: a unit is a social agreement wearing the mask of nature. Try the figures yourself in the Jewish length converter, which computes every opinion side by side.