Eras and cycles

1) Eras

Across the pages dedicated to different calendars, we have already come across a number of eras tied to each system.

But the notion of an era deserves more than passing mentions, so on this page I suggest a deeper look at the concept, along with a survey of the best-known eras (the eras invented by human societies over the centuries are countless, and claiming to list them all would be rather presumptuous).

As always, I welcome additional information. Feel free to write to me for that reason... or any other.

Note: unless otherwise stated, all dates on this page are given in the Julian calendar.

Since the aim here is to provide an inventory of eras and cycles, we will naturally revisit elements already encountered elsewhere on the site.

What is an era?

The French Academy dictionary says:

“

ÈRE n. f. 16th century, here. Borrowed from Late Latin aera (“number”), then “starting point of a period”.
1. Archaic. A fixed point from which one begins counting years. The era of the Muslim calendar is the Hegira.
2. A long period counted from a noteworthy event that became the beginning of a new chronology. The Christian era begins at the presumed date of Christ’s birth; the Muslim era is counted from the Prophet’s departure from Mecca to Medina (AD 622). Year 50 of our era, after Jesus Christ. The beginning of the Republican era was set by the Convention on 22 September 1792, the date of the proclamation of the Republic. Specialized, for the Ancients: a period calculated from zodiac constellations. The Age of Taurus. The Age of Pisces.
3. By analogy. A historical period marked by a salient feature or event. To open, inaugurate an era of peace. The industrial era. The atomic era. An era of freedom, of prosperity. A new era seemed to be opening. By extension: it was, in his life, an era of happiness.

4. GEOLOGY. Each major division in the history of the Earth. The Primary, Secondary, Tertiary and Quaternary eras.

And under CHRONOLOGY, we read:

“

CHRONOLOGIE (with “ch” pronounced as “k”) n. f. 16th century. Borrowed from Greek khronologia, from khronos (“time”) and logos (“discourse”).
1. The science concerned with dating historical events. There are several systems of chronology.

2. A set of historical facts presented in the order of their succession. The chronology of the Second World War. A chronological error.

Since we are dealing with calendars, the definition of era that best fits, among those listed by the Academy dictionary, is definition 2: a long period counted from a notable event that established itself as the starting point of a new chronology. Chronology being the science concerned with dating historical events.

Let us also note that the starting point of an era has a name: epoch (in English, epoch), in the sense of definition 1 in the same dictionary: “Classical. A historical moment marked by a major event. The epoch of the founding of Rome. The birth of Jesus Christ is the epoch at which the Christian era begins.”

As for the origin of the word, the Academy’s explanation leaves us somewhat unsatisfied. Let us go a little further:

Aera, the origin of ère, may be the plural of aes, meaning bronze. Bronze is an alloy of copper and tin. In short, bronze. According to this view, priests in charge of the calendar in Rome drove bronze nails into a temple wall to indicate that a new count of years had begun.

Another possible origin: in one of his many “Pinakidia” pieces, Edgar Allan Poe writes: “On ancient monuments one often finds the letters A. E. R. A., meaning Annus erat Regni Augusti. The ignorance of copyists probably turned these letters into the simple word area. Might this not be a better derivation than the Latin AES?” It is a fair question.

We can, and we shall, divide eras into two broad families: “event-based” eras, tied to real historical events (the start of a king’s reign, a battle date, proclamation of a new state, etc.), and “World” eras, rooted in philosophical or religious reflections referring to a possible date for the creation of the world.

Naturally, eras of the first type are far more numerous. The main suppliers of event-based eras are likely civilizations that chose to change era at every new king or emperor (Sumerians, Japanese, etc.). Next come expansionist peoples who “stamped” conquered territories with a new era marking the date of conquest (ancient Greece or Rome, etc.). Finally, there are countries where calendars multiplied (India, Thailand, etc.).

“World” eras

Apart from the special Maya case (see below), “World” eras originate in the Bible (taken here in the broad sense of the term).

We immediately run into a problem.
There are several Bible translations, and two are especially relevant here: the Septuagint and the Vulgate. Yet biblical event dates do not match from one version to the other. For example, the Flood is dated to 2262 BC in the Septuagint and 1656 BC in the Vulgate. In addition, each early chronologist interpreted the dates in their own way.

a) The Jewish Creation Era, or Anno Mundi

The Talmud considers that the 400th year after the destruction of the Second Temple is year 4,231 since the creation of the world. As that destruction took place in AD 70, the Jewish date of creation would fall in 3761 BC. The specific retained date is 6 October 3761 BC at 23:11:20. Quite precise. This era is based on the Vulgate tradition.

b) Other biblical eras

• The Alexandrian era appears in AD 221. Julius Africanus calculated it and arrived at 5501 BC. Based on the Septuagint.
• The Antioch era, calculated by Panodorus, begins in 5492 BC. Based on the Septuagint.
• The Byzantine (or Constantinopolitan) era begins on 01/09/5508 BC. Based on the Septuagint. It was used by the Greek Church and Russia until Peter the Great.
• Other eras were also calculated later: Joseph Justus Scaliger (1540-1609), 3951; Father Pezron (1639-1707), 5874; James Ussher (1581-1656), 4005.

We are still far from exhausting biblical eras. In A New Analysis of Chronology and Geography, History and Prophecy, chronology expert Dr William Hales wrote in 1830 that nearly 300 dates of various origins existed between “the creation of the world” and the birth of Christ. He himself listed 120 proposed dates by various authorities.

c) Freemasons

They use an era whose starting point is said to be 4000 BC (Gregorian). Its origin is not entirely clear. It may come from old traditions, one of which said:

“

For more than 4,000 years,
all the prophets announced it to us;
for more than 4,000 years,

we waited for this blessed time.

d) Maya eras

Which date corresponds to the Maya Long Count 0.0.0.0.0, and what does that date represent?

• As explained on the page devoted to the Maya calendar, this date would represent either the “creation of the world” or the birth of a deity (which deity? your pick).
• As for its absolute correlation, it varies according to scholars. Here are some published results. Correspondences in both Julian and Gregorian calendars are all BC.

The Goodman-Martinez-Thomson correlation is the one with the broadest support.

Reminder: year AD 1 follows directly after 1 BC, with no year zero in between. Astronomers’ numbering, by contrast (negative years for BC dates), includes a year zero corresponding to 1 BC. So, for example, in the Goodman-Martinez-Thomson correlation, 11 August 3114 BC = 11 August -3113.

Event-based eras

The hyperlinks below point to other pages on this site where these eras are discussed in connection with their respective calendars.

Olympiad era: introduced in Greece by the historian Timaeus, who lived in the 4th and 3rd centuries BC. Before him, years were named after Greek office-holders. Timaeus established four-year periods from the first year of the first Olympiad. Years were counted by giving the number of elapsed Olympiads and the year number within the current Olympiad.

Era of the founding of Rome: established by Terentius Varro Reatinus (Varro). Roman historians using it wrote “Ab Urbe Condita” (from the founding of the City).

Another era was also used (the Capitoline era), lagging one year behind the first (752 BC), aligned with the list of consuls. Consuls entered office on 15 March, while the civil year began on 1 January.

Era of Nabonassar: used only by historians, this era was established by the Greek astronomer Ptolemy in the 2nd century. It corresponds to the accession of Nabonassar, king of Babylon.

2) Cycles

Let us begin by defining what we are talking about.

The French Academy dictionary says:

“

CYCLE. n. m. A circle, period or revolution of a certain number of years, at the end of which astronomical phenomena should reappear in the same order as before. The solar cycle is twenty-eight years. The lunar cycle is nineteen years.

That suits us perfectly. We will not address here cycles that are not defined in a “number of years”. In our study of the week, we already touched on day-based cycles.

As with eras, claiming to inventory every cycle would be pretentious. So we will stick to the most important ones, especially those that were used at one time or another in calendar design.

We will proceed simply: one summary table ordered by increasing cycle length, with references to additional notes when needed.

As we shall see, there are two broad types of cycles:

• “Basic” cycles that match the strict definition: after a certain number of years, astronomical phenomena repeat.
• “Complex” cycles built by combining other cycles to produce a longer cycle.

Greek cycles

Let us read an excerpt from De die natali (On the Birthday), written by the Latin grammarian and chronologist Censorinus in the 3rd century AD. The French translation used here is by J. Mangeart, available in full on Wikisource or in scanned form on Gallica:

“

...There are also several other great years, such as the Metonic year, made up of nineteen solar years, by Meton of Athens: hence it is also called enneadecaeteris; seven months are intercalated, and the total is 6,940 days. One also distinguishes the year of the Pythagorean Philolaus, made up of fifty-nine solar years with the intercalation of twenty-one months; then that of Democritus, made up of eighty-two years thanks to twenty-eight intercalary months; and then that of Hipparchus, made up of three hundred and four years with one hundred and twelve intercalated months. This difference in the length of great years comes from astrologers disagreeing both on the fraction to be added to the three hundred and sixty-five days of the solar year and on what should be subtracted from the thirty days of the lunar month (...) There is also the year Aristotle called supreme rather than great, formed by the revolutions of the Sun, Moon and five wandering stars when all these heavenly bodies return to the point from which they departed. This year has a great winter... that is, a flood; then a great summer..., that is, a conflagration of the world. For the world seems in turn to be flooded or set ablaze at each of these epochs. According to Aristarchus, this year consists of 2,484 solar years. Aretes of Dyrrachium makes it 5,552 years; Heraclitus and Linus, 10,800; Dion, 10,884; Orpheus, 120,000; Cassander, 3,600,000. Others finally regarded this year as infinite, never to begin again. But among all these spans of time, the one most commonly used among the Greeks is the pentaeteris, or period of four years, which they call an Olympiad; and they are now in the second year of their 254th Olympiad. The great year of the Romans is the same as the period they called a lustrum. Its institution dates back to Servius Tullius and applies to each five-year period, at the end of which the census of citizens was held. But this was changed later; for from the first lustrum closed by King Servius to the one closed by Emperor Vespasian, consul for the fifth time, and by Caesar in his third consulship, nearly six hundred and fifty years are counted, and yet there were only seventy-five lustrums in that span; and since then there has no longer been any question of it. However, the great year was not lost from view; instead, it began to be observed more carefully through the Capitoline Games. These games were first celebrated by Domitian, in his twelfth consulship and under that of Ser. Cornelius Dolabella.

Olympiad cycle: cycle or era? Strictly speaking, by definition, it is closer to an era, since no astronomical phenomenon is involved.

Octaeteris cycle: failing to recover a treatise titled Octaeteris written by the Cnidian Eudoxus (c. 400-355 BC), a major scholar of the 4th century BC who, after returning from Egypt with astronomical knowledge, proposed a reform of the Greek calendar, the creation of the octaeteris is usually attributed to Cleostratus of Tenedos.

This cycle aimed to reconcile lunations and the tropical year through a system of intercalations. For operational details, see the Greek calendar page. In short: over eight years (hence the cycle name), a 13th month is intercalated in years 2, 5 and 8; still, it was not perfect.

Metonic cycle (lunar, enneadecaeteris): without question, this is the cycle most frequently encountered in many calendars.

It is also the one whose origin is least clear. It is found well before the 5th century BC among Babylonians and Chinese. Even so, it is indeed Meton who introduced it in Athens in 433 BC.

As already noted on the page devoted to the Greek calendar, Meton of Athens, son of Pausanias (?), was a geometer in the age of Pericles (5th century BC). According to tradition, once the Athenians learned of his discovery during Olympic games, they had his cycle engraved in gold letters on the columns of the Temple of Minerva. It is said this was the origin of the golden number used in ecclesiastical computus to denote a year’s rank in a 19-year cycle. Sadly, this is legend. For more on the legend and on Meton himself, see here.

The discovery of the cycle would have resulted from astronomical observations made by Meton in his observatory on Lycabettus Hill. None of his writings has survived.

The purpose of this cycle is the same as that of the octaeteris: reconciling, in the shortest possible time, the tropical year and lunation. It is a 19-year cycle of 6,940 days, corresponding to 235 lunations, i.e. 12 common years of 12 months and 7 embolismic years of 13 months ((12x19)+7 = 235). In this cycle, 110 months are hollow and 125 are full. It includes four kinds of years: for a 12-month year, the mean duration of 12 lunations is 354.367056 days. The Greeks therefore established two kinds of years: regular (354 days) and abundant (355 days). Likewise, 13-month years are either regular embolismic years (384 days) or defective embolismic years (383 days), the mean duration of 13 lunations being 383.897644 days.

Even though more precise, it was still not perfect. At the end of the cycle, it lagged by 10 hours with respect to the Sun and by 8 hours with respect to the Moon.

This Metonic cycle was improved by Callippus (370-310 BC). A little-known figure, he seems to have worked with Aristotle. He continued Eudoxus’ work and devised the 76-year Callippic cycle: the Metonic cycle multiplied by four, with one day removed every 76 years.

Philolaus’ cycle: originally from Croton (southern Italy), he is counted among Pythagoras’ direct pupils. After the Cylonians burned their school, Philolaus went to Thebes, where Plato was among his listeners. He is known for being the first to set the heavenly bodies in motion: the Earth was no longer at the center of the world, which was occupied by a central fire hidden from us by a “counter-Earth”. The Sun became a mirror circling that central fire in one year and reflecting its light to us. The principle of celestial motion was thus established, and the terrestrial year could be explained relative to the Sun.

His cycle has 59 years of 365 days and 6 hours. The number of lunations is 729, with 21 intercalations.

Oenopides’ cycle, for its part, predates Philolaus’ and is also said to cover 59 years, but with 22 intercalations.

Hipparchus’ cycle: Hipparchus of Nicaea (or Hipparchus of Rhodes, first quarter of the 2nd century BC) was unquestionably the greatest astronomer of antiquity. Among his many discoveries, the most important is the precession of the equinoxes. Hipparchus also discovered that Earth’s orbit is not exactly a circle centred on the Sun; he measured Earth-Moon distance at between 59 and 67 Earth radii (the real mean value is 60), and produced the first high-quality star catalog, listing 850 objects.

He established that the tropical year is shorter than 365 days + 1/4 and proposed subtracting one day over four Callippic periods (304 years). The resulting mean year (365 d 5 h 55 min) exceeds the modern tropical year by only six minutes. As for the lunation, it differs from the present value by... one second. Bravo, Mr Hipparchus.

back to table

Other cycles

Solar cycle, dominical cycle: this is the time span needed for the same weekdays to fall on the same calendar dates because of leap years. It is a 28-year cycle. In the liturgical calendar it is used in the form of the “dominical letter”. Back to table

The Roman indiction cycle: it appeared under Diocletian. Initially, it was a 15-year period at the end of which land tax was reassessed. Under Constantine, indiction became a chronological period, designating both the 15-year block and a year’s rank within it. Not far from an era, really. Later, indiction became one of the elements of ecclesiastical computus. Back to table

Apis cycle: a demotic papyrus (Carlsberg 9) describes a cycle of 25 Egyptian years. 309 lunar months are as long as 25 Egyptian years, i.e. 9,125 days, after which the Moon’s phases return to the same days of the year. Back to table

The Phoenix cycle: the Apis cycle would drift by one day every 500 years, so the calendar would need correction. Was that correction ever actually made?

The duration of this cycle varies by source: 250 years for Tacitus; 1,460 years for R. Stuart Poole; 1,500 years for Lipsius. It is said the cycle was “initialized” five times in Egypt: under Sesostris (866 BC), under Amasis (566 BC), under Ptolemy II (266 BC), one or two years before Tiberius’ death (AD 34), and during Constantine’s reign (AD 334). If that is true, the cycle would be 300 years and the calendar-correction hypothesis would collapse.

That would be a pity, as it would end a striking parallel between the cycle and the mythical bird of the same name: in the myth, before dying, the phoenix built a nest to which it gave the power to generate a new phoenix. In other versions, a worm appeared from its body and, under the Sun’s heat, transformed into a new Phoenix. Others believe the new Bennu (another name for the bird) rose from its ashes after burning at Heliopolis. It was said to rise again periodically, every five hundred years, after a blazing death on an aromatic pyre lit by the last ray of the setting sun. As a result, Heliopolis became the center for calendar regulation. Back to table

The Sothic cycle: because the civil year in ancient Egypt had only 365 days, the heliacal rising of Sirius shifted one day later every four years; full agreement was restored only after 1,461 vague years. This cycle was called the Sothic (from Sothis, Sirius in Egypt) or canicular period. After that span, the vague year realigned with the solar year. Back to table

Hippolytus’ cycle was intended to compute the date of Easter. He recommended using a 16-year cycle, which is in fact two octaeterides combined. Rome erected him a statue and engraved this cycle’s canons on its base. It feels very much like Meton’s time all over again.

He then observed that after 16 years, the full moon had shifted back by one day. He concluded that adding seven 16-year cycles (84 years) would bring the full moon back to the same weekday. That was wrong, but the 84-year cycle had been born. Back to table

The Kalpa cycle: strictly speaking, this takes us beyond calendars, since the cycle has no direct impact on them. But I cannot resist saying a few words about it.

Hinduism, Buddhism and Jainism all use kalpa to refer to the universal “great year”, divided into time periods with the same four-part structure.

The god Brahma lives for 100 years, but not human years. Far from it: each day (day + night) of those 100 years lasts... 4,320,000,000 x 2 (day + night) of our human years. Even with reduced working hours, that is a long stretch. Brahma’s life therefore lasts 311 trillion human years.

And Brahma has work to do. At each of his days, he creates the universe, then reabsorbs it (Pralaya) into himself during his divine sleep. At the end of his life, a new god begins a new creation cycle.

Brahma’s day is divided into two kalpas (one day, one night). And now stay with me, because this is where it gets complicated.

• A kalpa consists of a period of 1,728,000 human years (Adi Sandhi), followed by 14 manvantaras + Sandhi Kala.
• A Sandhi Kala lasts 1,728,000 years, during which a cosmic catastrophe occurs (a Flood-type event, for example).
• A manvantara consists of 71 mahayugas, each comprising four yugas:
  • Kali Yuga, lasting 432,000 years. This is where we are currently.
  • Dwapara Yuga, lasting 864,000 years.
  • Treta Yuga, lasting 1,296,000 years.
  • Satya Yuga, lasting 1,728,000 years.

If you have followed me correctly, one kalpa therefore consists of 1,000 mahayugas of 4,320,000 years each, i.e. 4,320,000,000 years.

After looking at these staggering numbers, we have earned the right to close this page on eras and cycles. See you in the next study.