A Lecture On Chronology

On June 4th, 2021, John Coleman of Apocatastasis, an institute for the humanities, invited me to give a 60-90 minute lecture on chronology during his Medieval history class. I happily accepted and we scheduled for July 8th at 2pm EST. This article contains the notes which I used for the lecture.




The word “chronology” (chrono + logy) can be defined as “the science of time”. In this sense, it provides the foundation for time-keeping and the ordering of events through past, present, and future. It provides the necessary knowledge for determining when events occurred. It has long been clear that the writings about history would be chaotic without chronology. Rhyme and reason would be absent from the historical canon.

Despite its importance, chronology has not been given adequate attention in contemporary times. It is often grouped in with a number of disciplines collectively known as the historical auxiliary sciences (also called the auxiliary sciences of history or the ancillary sciences of history). The members of this group can vary depending on which source you’re consulting but there are some which are much more common than others. These popular members include paleography (the study of writings), archeology (the study of material remains), genealogy (the study of familial history), numismatics (the study of coins), and chronology (the study of time). Some other popular members are heraldry, epigraphy, geography, philology, and anthropology. Some of these have wider applications for history than others, but college degrees have been established for all of them, with one exception, that exception being chronology. There are professors for anthropology, for archeology, for paleography, and for geography, but none for chronology.[1]

To emphasize the severity of this neglect, among the historical auxiliary sciences, only one of them have been styled as “the soul of history”. Keeping in mind the studies I just listed off, can anyone here guess which one has been called the soul of history? The correct answer is chronology.

More commonly than being called the soul of history, it has been called the eye of history, for it allows us to see the order of past events. However, in this latter stylization, it is not always portrayed alone, and it is not always without competition. When two eyes are identified, they are always chronology and geography. When only one eye is identified, chronology is most often in competition with geography, and less often with truth. These two eyes of history, geography and chronology, allow us to not only see the order of past events, but the locations in which those events occurred. Hence, geography and chronology are among the most important subjects for historical education. They can also be seen as space (geography) and time (chronology), the natures of which are investigated in the field known as the philosophy of space and time.[2]

Where chronology and geography supply the “when” and “where”, history (especially archeology and paleography) supplies the “who” and “what”. Here we can start to see the give and take between these fields of scholarship.

Types of Chronology

Aside from chronology being the study of time, “chronology” can also be defined as “an order of events”. In this sense, a chronology is synonymous with a timeline. Each of these two definitions has two different types. For chronology as the study of time, the two types are (1) mathematical and (2) historical.[3, pp.240-241] For chronology as an order of events, the two types are (1) relative and (2) absolute.[4, p.62]

Mathematical chronology, also known as theoretical or astronomical chronology, or calendariography, focuses primarily on establishing laws for chronometry, time-measurement. In this category, chronology is largely indebted to metrology, the science of measurement, which itself is indebted to mathematics, from which it derives its name, mathematical chronology. Chronology is also highly indebted to astronomy, the science of celestial phenomena. While chronology has been called “the eye, the light, the life, and soul of history”, astronomy has been called “the eye, the light, the life, and soul of chronology”.[5, p.4] It is in mathematical chronology that the length of days, months, years, and other units of time are determined.

Historical chronology, also known as technical chronology or chronography, focuses mainly on establishing dates for events, such as when a war occurred, or when an object was created. This is by far the most popular type of chronology. It’s within this category that we find relative chronologies and absolute chronologies. A relative chronology is an order of events that occurred at an unknown time before this present moment. For example, this sequence: our solar system formed, our earth formed, and then our moon formed. This sequence positions events in relation to each other but does not precisely indicate when these events occurred in relation to where we are today. Did they occur last Thursday? Or perhaps the Thursday before that? The relative chronology does not answer these questions.

An absolute chronology fixes events to our current day. For example: our solar system formed 13.8 billion years ago, our earth formed 4.543 billion years ago, and our moon 4.53 billion years ago. We could also say: our moon formed 4.53 billion years ago and our earth formed about 130 million years before that. These times are significantly greater than merely a week or two ago. Any chronology that is linked to our current year is an absolute chronology. Any chronology that is unconnected is relative.

Here’s another example of turning a relative chronology into an absolute one. God created the universe, the Great Flood occurred, and then God came to earth in the form of a human. By assigning dates to these events in relation to our present day, the relative chronology transforms into an absolute chronology. God created the universe about 7500 years ago, the Great Flood occurred about 5200 years ago, and He came to earth in the form of a human about 2000 years ago.

A relative chronology can have standard units of time in it as long as they are not related to the present day. Here’s what that looks like: God created the universe, after about 3300 years the Great Flood occurred, and about 3200 years after that He came to earth in the form of a human. Here we can plot out a timeline of three events using standard units of time, but no information is given as to when any of this happened in relation to where we are temporally today. Did the final event in that sequence occur a century ago? a millennium ago? two millennium? There is not enough information here to answer those questions, and so we call this a relative chronology.


With this talk of relating events to our current year, it’s appropriate to discuss what year it currently is, and also some other basic units of time. Years are typically counted out from a specific event, and group of these years can be called an era (or age). The two most popular eras in use today are the Common Era and Hijri Era. The common era (or Anno Domini) was determined by the traditional dating of Christ’s birth, which placed this event some 2,021 years before our present year. Because of this dating, the current year has been given the numerical value of 2021, last year was 2020, and the year before that 2019. This sequence continues all the way back to the year 1, at which point it begins going into the negatives, meaning three decades before the birth of Christ would be about 30 BCE, and about 2051 BP. This is the basis of the Christian calendar known as the Gregorian calendar. In this system, the year zero is skipped. It goes straight from 1 CE to 1 BCE.

These abbreviations are common for today’s datings:
CE = Common Era, the secularized version of AD = Anno Domini
BCE = Before Common Era, the secularized version of BC = Before Christ
BP = Before Present
AH = Anno Hegirae
AM = Anno Mundi
AUC = Anno Urbis Conitae

The Hijri era, known as Anno Hegirae, was determined by the traditional dating of Mohammed’s flight from Mecca to Medina, which places this event some 1,442 years before present. This is the basis of the Islamic calendar.

Thus, the current year is simultaneously 2021 CE and 1442 AH. However, if the era goes unspecified, the year becomes more difficult to determine. For it cannot be 2,021 years out from the start of an era and also be 1,442 years out from the start of the same era. There are other ways of determining the year in this case, such as checking the document for other dates and seeing if any of those fit into a system of chronology with which we are familiar. Another way would be to check how letters and numbers were composed. Were they composed by hand or by press? The answer might be able to help us narrow down what year is being referred to.

We would be blessed if only two era systems were in existence, but this is not the reality we are dealing with. The Gregorian calendar places us in 2021 and the Islamic calendar in 1442, as I’ve already mentioned. But the Assyrian calendar places us in 6771, the Coptic calendar around 1737, and the Korean calendar in 4354. These are only a handful of the calendars in existence and the differences span about 5300 years.

Further complications arise from eras of the same name starting from different points. The most robust example of this can be seen in Anno Mundi datings, those datings that attempt to pinpoint the year in which God created the world. I’ve compiled a list over the years of all the Anno Mundi datings that I’ve come across. They range a span of about 3800 years from 7388 BCE to 3616 BCE. Between those two points on my list are 93 other unique datings. Hence, at face value, a document dated 4000 AM could hypothetically have been written anytime between about 3400 BCE and 400 CE. This is a massive window of time, but still not as large of a window as that between the Islamic and Assyrian eras.[6]

There are three variations of the Tibetan calendar that span about 11 and a half centuries. These place our current year in 994, 1766, and 2147. The Chinese calendar has two variations separated by about 60 years and those place us in 4657 and 4717.

Years, Days, Months

The start of the era is not the only difference between calendars. There are also differences between how long a year is and when it begins. As per how long the year is, the Gregorian year has 365 or 366 days in it, the Ancient Egyptian and Roman years had 360 days in them, and the Islamic year has 354 or 355 days in it.

A solar year is the amount of time it takes for the sun to return to the same spot in the sky. As the year goes on, the sun continues to cross the same imaginary line in the sky with each complete rotation of the earth. This is what we call a day. However, it moves from one extreme point on that line to another and back again. The time it takes the sun to do this is known as a solar year, a seasonal year, or a tropical year. It is 365.24219 mean solar days long, in other words, 365 days, 5 hours, 48 minutes, and about 46 seconds.

The Gregorian calendar follows a 4 year system that has 3 years of 365 days followed by a forth year with 366 days. This system isn’t perfect and so eventually this empirical calendar will fall out of sync with the solar year. The Islamic year, consisting of 354 or 355 days, falls further and further behind the Gregorian year because the lower number of days. This means that the Islamic calendar stacks up years faster than the Gregorian one, and after about 33 solar years, the Islamic calendar will have gone through an entire calendar year more than the Gregorian one. Over the course of 330 solar years, it has gone through an extra decade, and over the course of 3300 solar years, an extra century in comparison with its Gregorian counterpart.

The difference in days between these two calendars is due to the various types of calendars. These types include solar, lunar, lunisolar, and empirical calendars. Solar calendars are based on the solar year. The Gregorian calendar is an example of a solar calendar. Lunar calendars are based on the lunar year. The Islamic calendar is an example of a lunar calendar. Lunisolar calendars make use of the sun and the moon, and include the Chinese and Hebrew calendars as examples. An empirical calendar is a set amount of days.

Intercalation is the reason why the number of days in a year can be different in the same calendar. This is when a day is or days are added into the calendar in addition to the normally included number of days. The Gregorian calendar intercalates a day every four years. This fourth year is commonly known as a leap year. Months are groups of days and can also be intercalated. Knowing when days or months have been intercalated into calendars is useful for chronology because it helps us determine when exactly certain events occurred.

When a specific calendar’s year begins is also useful to know because one calendar’s year can be half-way through while another calendar’s year is only just beginning. The Gregorian new year begins on the first of January but the Chinese new year begins on a day between the twenty-first of January and the twenty-first of February, depending on the year. Additionally, some Roman new years began in March or September, and the Jewish new year begins in September or October.

Like years, the start of a day has varied. The British, French, German and Americans have all begun their day at midnight. The Greeks and Persians begin theirs at dawn. Astronomers across the world begin their day at noon, and the Austrians, Chinese, Florentines, and Jews have begun theirs at dusk.

As previously mentioned, the situation is not entirely hopeless. There are ways of calculating the difference between the multitude of calendars, given enough information is provided. I think the best work on this topic that I’ve read is Calendrical Calculations: Ultimate Edition, written by Reingold & Dershowitz, published in 2018. This book contains algorithms for modeling calendars on a computer as well as a lot of technical and historical information about those calendars.

So far, I have talked about chronology’s place in the academic world and in historical studies, I briefly covered the types of chronology, and I spoke about some aspects of calendars and units of time.

History of Chronology

Humans have made use of chronology since time immemorial. The study of chronology is vital for the survival of our species. Aside from the fact that we know our ancestors had some concept of time (because we are still here today), the earliest potential evidence we have for people engaging in chronological studies comes to us in the form of a flat bone with circles and crescents engraved into it. This bone was found in France and was dated to around 30,000 years before present. If the symbols depict phases of the moon, this makes the bone our oldest known physical evidence of timekeeping.[7]

During the 5th millennium BCE, that is, the 4000’s, European populations constructed megalithic sites across their continent that helped them with tracking time. In the mid-4th millennium BCE, Egyptians kept time with the help of obelisks. In the mid-3rd millennium BCE, Sumerians had a calendrical system consisting of days, months, years, and intercalations. Around 2100 BCE, Egyptians priests began dividing the days into 24 sections, a system familiar to all of us today. The oldest known water clock was found in Egypt and was date to around 1600. About a hundred years after that, Egyptians were using water clocks, sundials, and merkhets.[7]

Writing developed alongside timekeeping and it gave way to recorded history. The birth of writing is generally placed during the 4000’s BCE. Our earliest known examples of historical narratives typical date to the 500’s BCE and later. In the 400’s is where the fathers of history, Herodotus and Thucydides, are placed. These two Greek historians were the first to get major recognition for their methods of collecting historical information and organizing it into a historical narrative. It was also during their century that Greek scholars began compiling lists of Olympic victors, an important task for determining how many Olympiads had occurred since the first one.[8] Lastly, the 5th century BCE is when Meton became famous for pointing out that the cycles of the moon repeat with the start of the year every 19 years, although it was discovered by the Babylonians earlier in that same century. This cycle has long been known as the Metonic cycle and it played a key role in ancient Greek calendars.

The 300’s BCE gave way to Berossus, a Babylonian historian, and Manetho, an Egyptian historian. Both of their works were not products of rigorous study aimed at portraying the truth, but products aimed at proving that the antiquity of their ancestors was greater than that of their masters. This biased approach to history is typically discouraged today.

The Romans got progressively more interested in timekeeping during the 200’s and 100’s BCE. Varro, in the first century BCE, obtained the popularly used date for the founding of Rome by consulting an astrologer, 753 BCE. In China during the C1st BCE, Sima Qian wrote his Records of the Grand Historian, a work which earned him the title “the father of history”. It was during this century that Cicero supposedly initiated the trend of titling people as “the father of history”. Also during this time was when the Antikythera mechanism is believed to have been created, the oldest known analog computer. It was used for predicting eclipses and horoscopes.

The next major development for chronology occurred in the 300’s CE when Eusebius wrote his Chronicon and his Church History. Jerome followed Eusebius’ example and was rivaled him for popularity. The both of them were writing apologetic history, not scientific history. The works of these two authors served as models for all future Western historians until the time of Scaliger in the late 1500s, early 1600s.

The most notable event that has been placed between Eusebius and Scaliger was the calculation of Christ’s birth in the 6th century. This was done by Dionysius Exiguus but there is some debate about when it was first used for dating events. Supposedly it was used for dating in France and Italy during the 600’s, but some argue that its use for dating was introduced in the 700’s by Venerable Bede. The original calculation is also largely considered incorrect today. Scholars typically place Christ’s birth between 12 BC and 9 AD, but I have seen his birth placed as early as 33 BCE and as late as 1152 CE.

Dionysius was especially concerned with calculating the Easter date, which marked the resurrection of Jesus. He published an Easter table that the majority of medieval scholars used for determining when Easter occurred. Bede was also notably concerned with Easter and his work on calculating its date earned him the spot of top authority until the late 16th century when the Gregorian calendar was introduced. Today, we have hundreds of manuscripts that have been dated in and between the 600’s and 900’s that are devoted to computing the Easter date.[9, pp.32-33]


[1] – Sorensen, Stephen. “Auxiliary Sciences of History”. https://ctruth.today/2020/10/12/auxiliary-sciences-of-history/. Accessed 9 Jun. 2021.

[2] – Sorensen, Stephen. “The Eyes of History”. https://ctruth.today/2020/10/04/the-eyes-of-history/. Accessed 9 Jun. 2021.

[3] – “Part I: The Auxiliary Sciences. II. Chronology.” The Catholic Historical Review, vol. 2, no. 2, 1916, pp. 240–243. JSTORwww.jstor.org/stable/25011427. Accessed 3 Oct. 2020.

[4] – Bickerman, Elias Joseph. “Chronology of the Ancient World” (England, 1968). Accessed 30 Jan 2021.

[5] – Kennedy, John. “An Examination of the Reverend Mr. Jackson’s Chronological Antiquities” (1753). https://books.google.com/books?id=EaVbAAAAQAAJ&pg=PA4&lpg=PA4&dq=john+jackson+%22chronology+is+the+eye%22&source=bl&ots=rgdL8wiVO1&sig=ACfU3U2wrL33TkyfEnYkPmxHW-hsAfYvaQ&hl=en&sa=X&ved=2ahUKEwjBpKL88fHrAhUQH80KHaUSAEkQ6AEwAHoECAIQAQ#v=onepage&q=john%20jackson%20%22chronology%20is%20the%20eye%22&f=false. Accessed 19 Sept. 2020.

[6] – Sorensen, Stephen. “Anno Mundi Reference List”. https://ctruth.today/2019/01/28/anno-mundi-reference-list/. Accessed 9 Jun. 2021.

[7] – Sorensen, Stephen. “The History of Timekeeping”. https://ctruth.today/2021/01/10/the-history-of-timekeeping/. Accessed 15 Jun. 2021.

[8] – Sorensen, Stephen. “Chronology”. https://ctruth.today/2019/05/27/chronology/. Accessed 15 Jun. 2021.

[9] – Sorensen, Stephen. “Anno Domini”. https://ctruth.today/2021/01/09/anno-domini/. Accessed 16 Jun. 2021.

[10] – Nothaft, Philipp. Dating the Passion: The Life of Jesus and the Emergence of Scientific Chronology (200–1600) (Time, Astronomy, and Calendars). Brill, 2011. Accessed 20 Jun. 2021.

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