How long is a moment?

Published on September 1, 2026

Tags:historyunitscuriosities

A tortoise walks among melting pocket watches draped over rocks on a shoreline at sunset

“Wait a moment” is not a figure of speech that used to be vague. For a long stretch of European history, a moment was a unit — a specific fraction of an hour, written down in textbooks, taught to students, and subdivided further into units smaller still.

It is worth knowing how long it was, because the answer is stranger than a number.

Bede, in a monastery, in 725

Around 725, a monk in Northumbria in his early forties sat down to write De temporum ratione — “On the Reckoning of Time.” Bede’s book contains the earliest surviving Western description of a neat scheme: an hour divides into 4 puncta, or 10 minuta, or 15 partes, or 40 momenta. Bede is careful to say these divisions are not natural but agreed by convention — he is transmitting a scheme, not inventing one.

A folio from a twelfth-century manuscript of Bede's De temporum ratione, showing pen drawings of allegorical hybrid creatures with Latin labels. A folio from a twelfth-century copy of Bede’s De temporum ratione (Bavarian State Library, Clm 18158). This page carries allegorical figures rather than the reckoning tables — the manuscript ranges widely. Wikimedia Commons, public domain.

So a moment is a fortieth of an hour, which makes it ninety seconds. You will see that figure everywhere.

It is a modern average, and it was never once true.

Bede’s hours were seasonal hours: the daylight span, whatever it happened to be that day, divided into twelve. A December hour in Northumbria is much shorter than a June one, so a fortieth of it is shorter too. The moment stretched and shrank through the year, and differed by latitude. Ninety seconds is what you get by averaging away the entire point of the system.

That is the first surprise. The second is what happened next.

Units nobody could possibly measure

Around 1010, another English monk — Byrhtferth of Ramsey — wrote a science textbook that divided the moment further. His unit was the atom, from the Greek átomos, “indivisible”: one 564th of a moment.

That is roughly 160 milliseconds.

Nothing in England in 1010 could measure 160 milliseconds. Nothing could measure a moment. The finest instrument available was a water clock, and the practical markers of the day for almost everyone were the church bells and the position of the sun. Roger Bacon later subdivided things further still, into ounces and atoms, with equally no prospect of observing any of it.

These units were not measurements. They were arithmetic — the output of dividing a day into tidy fractions because the fractions were tidy, in a tradition where computing the date of Easter was a serious intellectual discipline. The numbers came first and the instruments arrived, in some cases, a thousand years later.

That is a genuinely odd way round, and it is the opposite of how we do it now. We measure first, then name what we measured.

The same word, nine orders of magnitude apart

The pattern of naming intervals we cannot reach did not stop with the monks.

In 1926 at Berkeley, the chemist Gilbert Newton Lewis — who named the photon that same year, in a letter to Nature — took an old slang word for lightning and gave it a precise physical meaning. A jiffy, he proposed, is the time light takes to travel one centimetre: about 33 picoseconds.

It was never absorbed into any official system of units, and remains informal to this day, redefined field by field. Meanwhile the word went its own way. When Linux arrived in 1991, its kernel counted time in ticks of the hardware timer, and those ticks were named jiffies too — about 10 milliseconds on the machines of the day. The tick rate is a build-time setting, so a jiffy is 10, 4 or 1 millisecond depending on how the kernel was compiled.

So “jiffy” means either 33 picoseconds or 10 milliseconds depending on who is speaking: the same word covering a span of roughly nine orders of magnitude. Neither is a standard. The 10ms figure is a hacker convention; the 33-picosecond one is a chemist’s proposal nobody took up. It has also been defined as a mains power cycle, a Commodore 64 timer tick, and several other things.

If you want a word that means “a short time”, it turns out you can have as many as you like.

The shake, and two shakes of a lamb’s tail

At Los Alamos in the 1940s, physicists working on fission needed shorthand for the interval between one neutron causing a fission and the next generation doing the same. It kept landing near 10 nanoseconds, so they named it a shake — from “two shakes of a lamb’s tail.”

A complete chain reaction runs its course in roughly 50 to 100 shakes: under a microsecond.

The joke is doing real work there. A unit named for how quickly a lamb can move its tail was adopted because the alternative was saying “ten to the minus eight seconds” several hundred times a day. It is informal, it has never been an SI unit, and it is still in use.

Do not confuse it with the jiffy, which is a different length entirely — a shake is about three metres of light-travel, against the jiffy’s one centimetre.

Bells, and units that tell you something else

Not every time unit answers “what time is it.”

A ship’s bell sounds every half hour and accumulates through a four-hour watch: one bell, two bells, up to eight bells, at which point the watch ends and the count starts again. It is frequently said that eight bells means eight o’clock. It does not. It tells you where you are inside a watch, which is what a sailor actually needed to know — measured, for centuries, by a half-hour sand glass turned by hand.

Rome had a stranger one. The nundine was the market cycle: eight days, though the Romans called it “the ninth day” because they counted inclusively from the previous market. The cycle is written into surviving stone calendars as the letters A to H down the margin. It had probably already fallen out of use by the time Constantine made the seven-day week official in 321 — scholars put the end of the eight-day market anywhere from the late first century to the early fifth.

A fragment of the Fasti Praenestini, a painted Roman stone calendar, showing columns of incised letters and abbreviations.
A section of the Fasti Praenestini, the Roman calendar compiled by Verrius Flaccus. The nundinal letters A–H run down the margin, marking the eight-day market cycle. Photo: Wikimedia Commons, CC BY 2.5.

The floor

At the other extreme is the shortest interval anyone has bothered to define. In 1899 the physicist Max Planck noticed that three constants — the speed of light, the gravitational constant, and his own action constant — combine to give a unique interval, about 5.4 × 10⁻⁴⁴ seconds. The universe is something like 8 × 10⁶⁰ of them old.

Below the Planck time our physics stops describing anything. That is often reported as “the smallest possible unit of time”, which overstates it: it is the scale at which quantum gravity is expected to take over, not a proven minimum. Nobody has measured one, and nobody expects to.

Which puts it in exactly the same category as Byrhtferth’s atom — a number arrived at by reasoning, describing an interval no instrument can reach. We are still doing the thing the monks did. We are just better at the arithmetic.

What survived

Look at which units lasted, and there is a pattern.

The moment did not survive. Neither did the atom, the punctum, or the nundine. What survived is the second, the minute and the hour — the sexagesimal inheritance from Babylon — and they survived for an unglamorous reason. When mechanical clocks arrived in the late thirteenth century, they divided an equal hour, and they inherited the astronomers’ sexagesimal scheme — Ptolemy’s, by way of Babylon — to do it. The moment was a subdivision of the seasonal hour, a computist’s tool for a quantity the new clocks had abolished. It had nowhere to go.

Everything since has followed the same rule. Hundredths of a second became meaningful when cameras could resolve them, not when someone thought of them. A unit becomes real when something can measure it.

Which is worth remembering the next time you look at a stopwatch showing four decimal places. TiCaNo Stopwatch will happily give you ten-thousandths of a second, and they are arithmetically honest — but the thumb pressing the button is a medieval instrument by comparison, good to somewhere around two hundred milliseconds. Roughly, as it happens, one Byrhtferth atom.

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