A line, and a clock

The monk who counted the years

In Rome, in the year we now call AD 525, a monk named Dionysius sat down to extend a table of Easter dates. Dionysius Exiguus (“Dionysius the Little”) came from Scythia Minor, on the Black Sea, and lived from about 470 to about 544. Easter is always a Sunday, but it depends on the moon and the equinox, so every church needed a table saying which Sunday to keep. The table he inherited came from Alexandria and counted years from the reign of the emperor Diocletian (the count begins in AD 284). Dionysius would not carry it on: Diocletian had persecuted Christians, and Dionysius wrote that he did not wish to keep alive the memory of an impious persecutor. He numbered his years instead from the incarnation, the Christian term for the coming of Jesus into the world. By that count the year he was writing in was 525.

It spread because scholars found it useful. The English monk Bede explained it in The Reckoning of Time (725) and used it in his Ecclesiastical History (731); Alcuin brought it to Charlemagne’s court in the late 700s; and over the following centuries it became the standard count in western Europe. Dionysius used no year zero (Roman numerals had no zero), so 1 BC is followed directly by AD 1. Astronomers later added one, calling 1 BC year 0 (Jacques Cassini’s tables of 1740 are the usual reference). The activity below tests what a missing year does to arithmetic.

Dionysius never said which year Jesus was born. The Gospels place the birth in the reign of King Herod, and by the usual reckoning, based on the Jewish historian Josephus (who ties Herod’s death to a lunar eclipse), Herod died in 4 BC; a minority argue for 1 BC. Most scholars therefore place the birth between about 6 and 4 BC. In 1606 the astronomer Johannes Kepler, in his book on the new star of 1604, took up a thesis of the Jesuit Laurentius Suslyga (1605) that the count was about four years late, and he restated the case in 1614; he also linked the Star of Bethlehem to a meeting of Jupiter and Saturn in 7 BC. (James Ussher, whom Course 20 follows, also put the birth in 4 BC.) The count was by then too well established to change, so the calendar of the Christian world begins a few years after the birth it commemorates: Jesus was born “before Christ.”

This course sees something larger in Dionysius’s choice. In the Christian form of a tradition shared with Jews and Muslims, the world has an absolute beginning, creation; a single center, the incarnation, which happens once; and an end, the return of Christ, whose date, according to the Gospels, nobody knows. In this reading, three events that cannot happen twice give time a scale on which every event has one place, the same for everybody, and a reason to keep records: if nothing repeats, a lost event is lost for good. They also tie time to freedom, since an unknown end keeps the future open, and this course argues that freedom of the will needs an open future. It contrasts Plato with the Christians too: for Plato the soul’s rescue lies in another place, the world of the Forms; for the Christians, in another time, the future. This is interpretation, not settled history; the “Another view” box weighs it.

A correction belongs here, because it is often said that the Council of Nicaea, in AD 325, adopted the Julian calendar. It did not. That calendar is Julius Caesar’s: introduced in 46 BC and in force from 1 January 45 BC, with 365 days and an extra day every fourth year. What Nicaea settled was a quarrel about Easter. The council decided that all churches should keep it on the same day, independent of the Jewish calendar, and gave no method of calculation. The method came later: Easter is the first Sunday after the first full moon on or after 21 March, with the moon read from a 19-year table, the computus.

A Julian year of 365.25 days is a little longer than the year of the seasons, about 365.2422 days, so the calendar slips against the sun by about a day every 128 years. By the 1500s the spring equinox, which the Easter rule fixed at 21 March, was falling around 11 March. Pope Gregory XIII’s bull Inter gravissimas, dated 24 February 1582, dropped ten days (in the first countries to adopt it, Thursday 4 October 1582 was followed by Friday 15 October), changed the leap-year rule for century years, and corrected the moon table. Britain and its American colonies followed in 1752, when Wednesday 2 September was followed by Thursday 14 September; Russia in 1918; Greece in 1923. Orthodox churches kept the Julian rule for Easter; since the Julian calendar now runs thirteen days behind ours, their Easter usually falls later, and the two coincide only in some years.

Before the beginning

About AD 397 to 400, in Hippo, a port in what is now Algeria, its bishop Augustine wrote his Confessions. In Book XI he takes up a question that, he says, people put to Christians with a sneer: what was God doing before he made heaven and earth? He reports a joke answer, that God was preparing hell for people who pry into such mysteries, and refuses it (XI.12). His real answer is that God made time along with everything else (in J. G. Pilkington’s translation, “that very time You made”), so there was no “before.” God is outside time, and eternity is not endless time: in the eternal, Augustine writes, “nothing passes away, but the whole is present” (XI.11).

Then Augustine turns on himself: but what is time? Everyone speaks of it easily, and nobody can say. (This is the passage Lesson 1 opened with.) The past is no longer and the future is not yet. The present is a puzzle too, because any stretch of time can be divided, a year into days, a day into hours, an hour into moments, until you reach a piece so small it cannot be divided at all, and that, he says, flies from future to past with no length (XI.15). So the present has no duration, and the past and future do not exist. How, then, do we measure time, as we plainly do?

His answer takes time inside the mind. There are three times, he says: a present of things past, which is memory; a present of things present, which is attention; and a present of things future, which is expectation (XI.20). He gives the example of a psalm he knows by heart (XI.28). Before he begins, his expectation is stretched over the whole psalm; as he recites, what he has said moves into memory and what is left shrinks, until all of it is memory. Time, he concludes, is a stretching, distentio in Latin, and he marvels if it is not a stretching of the mind itself (XI.26). In this reading, this is a founding account of the second kind of time in Lesson 1, time as it is lived.

This course holds that Augustine’s inward turn was ignored for a thousand years, until Edmund Husserl recovered it. The second half is right: in his lectures of 1905 on the consciousness of internal time, which Martin Heidegger published in 1928, Husserl called Augustine the first thinker to feel the difficulty deeply and told readers to study Book XI closely. The first half needs care: Boethius, Anselm, and Thomas Aquinas worked out their accounts of God’s timeless eternity in Augustine’s tradition; what was less followed was his analysis of the mind. (The argument that the present has no length is his own.)

Why did Christians care so much? Because a cycle was a rival idea. Some Greek schools, as Lesson 1 told, taught that the world and its events recur. In The City of God, Book XII, Augustine quotes a line of the Psalms in his Latin Bible, “the wicked walk in a circle,” and turns it: the circle is the going-around of false teaching, not of the world (XII.13). The main reason is that Christ died once for our sins and, rising again, “dies no more,” as the apostle Paul says (Romans 6:9). If history repeated, Christ would have to die again and again. “Far be it from us,” Augustine writes (XII.14), and he mocks the picture of the same Plato teaching in the same Academy at Athens, with the same pupils, over numberless ages. In this reading, Christian theology and straight time went together for that reason: a single unrepeatable event at the center needs a time that does not come back.

A clock in the town square

Take the long way around, to Kaifeng, capital of the Song dynasty, in the 1090s. There stood a tower about 12 meters (40 feet) tall, the 水运仪象台 (shuǐyùn yíxiàng tái, “the water-driven armillary-and-globe tower”), built by the official and scholar Su Song (1020–1101) and a team between 1088 and about 1092. It carried an armillary sphere, a celestial globe, and little figures that rang bells at the hours. The power was water, poured at a steady rate into the 36 scoops of a great wheel about 3.4 meters (11 feet) across. The wheel could not simply spin: levers held it and let it move only when a scoop was full, one step at a time. A device that cuts a smooth drive into equal steps is an escapement, the heart of every mechanical clock. Su Song described the machine in his Xin yixiang fayao (about 1092). (Course 2 tells what the tower was for.)

Was it the world’s first mechanical clock with an escapement? Probably not the first, but the best documented. Joseph Needham, Wang Ling, and Derek de Solla Price, in Heavenly Clockwork (1960), traced Chinese water-driven instruments with escapements back to Yi Xing and Liang Lingzan, about 725, and Zhang Sixun in the late 900s. Whether they count as “mechanical clocks” is partly a matter of definition: they ran on water, not falling weights. Whether the idea traveled west is debated. The tower itself did not last. When the Jurchen took Kaifeng in 1127 they dismantled it and carried at least its armillary sphere north to Yanjing, today’s Beijing; the Song court, now in the south, asked Su Song’s son Su Xie to build another, and nobody could follow the instructions. This course adds a reason: clock time was a court monopoly, a secret guarded by the emperor’s astronomers. The record shows less: the knowledge lived in a few people and a few books, and war broke the chain.

In Europe the first mechanical clocks enter the records about two centuries after Su Song’s tower. English church accounts mention large clocks at Dunstable Priory in 1283 and, as a “great horloge,” at Canterbury Cathedral in 1292 (the Latin word, horologium, can also mean a water clock). A clock in Milan in 1336 is the first known to strike the hours. The clock in Salisbury Cathedral, from about 1386, is claimed to be the oldest working one, and it has no dial at all: it strikes the hours on a bell, as the word “clock” itself, from a medieval word for “bell,” suggests. Such clocks ran on a falling weight, regulated by a bar that swung back and forth, the verge and foliot, and could be wrong by a quarter of an hour or more a day.

What the new machine changed was the hour. Until then the day was cut into twelve hours of daylight and twelve of darkness, whatever the season, so an hour stretched and shrank with the year: at London’s latitude a midsummer daytime hour lasted about 80 minutes and a midwinter one about 40 (our calculation). A clock counts equal hours, twenty-four to the day, the same in July as in January and on every street within earshot of the bell. In the late Middle Ages equal hours gradually replaced the old unequal ones, and mechanical clocks encouraged the change.

Why in churches? The classic answer is Lewis Mumford’s, in Technics and Civilization (1934): “The clock, not the steam-engine, is the key-machine of the modern industrial age.” He traced it to the monasteries. The Rule of Benedict (sixth century) set seven prayers by day and one by night, and bells called the monks to them; the monasteries, Mumford wrote, helped give human enterprise “the regular collective beat and rhythm of the machine.” This course finds the idea plausible: a community with a fixed timetable of prayer has every reason to want a machine that rings the hour. But Mumford noted too that the clock tower’s bells “almost defined urban existence,” and later historians pressed that point. Jacques Le Goff argued in 1960 that towns and merchants were building a measured, worldly time beside the Church’s time, and Gerhard Dohrn-van Rossum, in History of the Hour (1992), gave more weight to town councils. The records fit both: the first clocks we know of were in cathedrals and a priory, and in time clock towers stood at the centers of towns.

China had public time too: the Drum Tower of Beijing, first built in 1272 and rebuilt in 1420, and the Bell Tower beside it announced the hours to the capital until 1924. What differed, this course argues, was who the machine was for. Su Song’s tower served the court’s astronomers, while a European clock hung where anyone could hear it, which this course ties to Christianity’s claim to be a religion for all peoples. When Matteo Ricci gave the Wanli Emperor clocks in 1601, the court prized them as marvels; the Palace Museum in Beijing still holds more than a thousand clocks and watches, Chinese and foreign.

The clock gets a job

The next step was the pendulum. A story says that as a student in Pisa, in the early 1580s, Galileo watched a lamp swinging in the cathedral and noticed that wide and narrow swings took the same time. It comes from his pupil Vincenzo Viviani, who wrote it down in the 1650s, some seventy years afterward; Galileo’s own documented work on the pendulum begins about 1602, so the scene is better treated as a story than as a record. The practical clock was Christiaan Huygens’s, invented on Christmas Day 1656 and patented in 1657 (Course 19 tells how he found that a pendulum is steady only for small swings). It cut a clock’s error on land from about a quarter of an hour a day to about 15 seconds.

The great test of a good clock was at sea, where finding longitude needs the time somewhere else. The earth turns 360 degrees in 24 hours, which is 15 degrees an hour, or one degree in four minutes, so a clock that keeps the time of the home port, set against local noon, tells how far east or west the ship has come. In October 1707 four warships were wrecked on the Isles of Scilly and between 1,400 and 2,000 men died (several navigational errors were to blame, not longitude alone), and in 1714 Parliament passed the Longitude Act. It set up a Board of Longitude and offered £10,000 for a method good to one degree, £15,000 for two thirds of a degree, and £20,000 for half a degree, tested on a voyage from Britain to the West Indies, about six weeks. Newton, who advised the committee, doubted that any clock could be accurate enough. Half a degree is two minutes of time, and two minutes over six weeks is a little under three seconds a day.

John Harrison (1693–1776) got there. His first sea clock, H1, dates from 1735; his fourth, H4, a watch finished in 1759, made the case. In November 1761 his son William carried it to Jamaica aboard HMS Deptford. The watch lost a steady 2.67 seconds a day, 3 minutes and 36.5 seconds over 81 days. With that known loss allowed for, it was about 5 seconds out at Kingston, a longitude error of 1.25 minutes of arc, close to one nautical mile. A second trial, to Barbados in 1764, left it within 39 seconds, an error of less than ten miles. The Board put the success down to luck, asked for more tests, and never awarded the prize as such; in 1773, when Harrison was 80, Parliament paid him £8,750 after the king’s intervention. The watch was not perfect. It was steady, and a steady loss can be measured before sailing and subtracted afterward.

Meanwhile the clock was turning into a social machine. In 1748 Benjamin Franklin put the new attitude into one sentence of his Advice to a Young Tradesman: “Remember that time is money.” (An English newspaper had printed it in 1719.) In 1967 the historian E. P. Thompson argued, in “Time, Work-Discipline, and Industrial Capitalism,” that something real had changed with the phrase. Before the industrial age, work in much of the world followed the task and the season; in the mill and the factory, people had to be at the machine together, the employer bought hours, and the clock, the bell, and the fine for lateness became part of work. Historians still debate how far the clock caused these changes and how far it only marked them.

This course draws a further point. When time is a quantity that runs whether or not you use it, sleeping begins to feel like wasting it, and productivity, output per hour, becomes a number that exists only because there is a clock to read. It puts the point as a joke: we go to bed because it is time, not because we are tired. What is documented is that the hour became equal, public, and countable, and that by the 1700s people were saying it could be bought and wasted.