Upsilon υ

Upsilon is the twentieth Greek letter, from Phoenician waw; its value is 400. Upsilon has a genuinely attested moral symbolism as the 'Pythagorean letter' (littera Pythagorica): its forking shape was read as an emblem of the crossroads of life, the point at which the young must choose between the paths of virtue and of vice.

The Greek letters doubled as numerals in the Milesian (alphabetic) system, in which each letter carried a fixed value; the practice of summing a word's letters into a number, called isopsephy, is the Greek counterpart of Hebrew gematria and appears in inscriptions, the Greek Magical Papyri and early Christian texts. Upsilon's Phoenician ancestry places it in a borrowing whose exact point of contact, somewhere among Al Mina, Cyprus, Crete and Euboea in the Aegean world, is still disputed, though the borrowing itself, conventionally dated to around the 8th century BCE, is agreed to be the ancestor of every European alphabet still in use, including the Latin one covered elsewhere in this atlas.

The Greeks' own contribution was to give vowels dedicated letters of their own, something the consonant-only Phoenician script never did. Its isopsephy value of 400 belongs to the Milesian alphabetic numeral system, which also needed three further, otherwise obsolete letters, digamma (also called stigma), koppa and sampi, to cover every value from 1 up to 900; none of the three survives among the 24 letters in ordinary use today.

Upsilon has one of the more obscure modern technical profiles among the Greek letters covered in this atlas, appearing mainly in a single area of particle physics: the upsilon meson, a short-lived particle composed of a bottom quark bound to its own antiquark, was discovered in 1977 by a research team at Fermilab led by Leon Lederman, who named it after the letter following the earlier convention that had already produced the tau lepton, discussed elsewhere in this atlas. The upsilon meson's discovery was significant at the time as indirect evidence for the existence of the bottom quark, one of the six fundamental quarks recognised in the Standard Model of particle physics.

Outside this specialised context, upsilon sees little general use compared with better-known letters such as pi, sigma or lambda, and it carries no comparable idiom, symbol or popular reference in English. Astronomers, following the Bayer star-naming convention of 1603, use Upsilon to mark a constellation's twentieth-brightest star, continuing the same ranked sequence used for the rest of the Greek alphabet, and this remains, for most readers, the letter's only familiar modern role. In the Milesian numeral system, discussed elsewhere in this atlas, upsilon carries the value four hundred, a use that predates the meson by roughly two and a half thousand years and shares nothing with it beyond the borrowed name.