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You may think a calendar is simply nothing more than (by its use) a way of knowing when now is. The fact is that calendars are not simple at all and represent the most ambitious attempt by humanity to control and recognise time. They are based on three astronomical certainties: (1) The Earth spins on its axis (a day), (2) The Moon circles the Earth (a month) and (3) The Earth revolves around the Sun (a year).

The Prague Astronomical Clock (or Prague Orloj) is a medieval astronomical clock in Prague, the capital of the Czech Republic.
Picture Credit:
“Prazsky orloj” by George M. Groutas is licensed under CC BY 2.0

For primitive people, there were only two measures of time available: the day (the in-between space between two nights) and the month (the period between new moons). We have moved on a lot since then – as explained below.

The word calendar is taken from the Latin calendae, the term for the first day of the month in the Roman calendar, related to the verb calare (‘to call out’) – taken as meaning the new moon had just been seen.

The Latin calendarium meant ‘account book, register’ (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as calendier and then in Middle English as calender by the 13th century (the spelling calendar is early modern).

What is it?
calendar is a system of organising days, achieved by naming periods – typically days, weeks, months and years. As we know, a date is a single, specific day within such a system. Before today’s calendar, the most common form was the lunisolar calendar – a lunar calendar that occasionally added one intercalary[1] month to remain in sync with the solar year over the long term. The solar calendar is based on the Sun’s movement and is the one with which people are most familiar. It differs from lunar calendars that calculate months using the Moon.

Archaeologists have reconstructed methods of timekeeping that go back to prehistoric times – at least as old as the Neolithic[2]. The natural units for timekeeping used by most historical societies are the day, the solar year[3] and the lunation[4]. Calendars are explicit schemes used for timekeeping. The first historically attested and formulated calendars date to the Bronze Age[5], dependent on the development of writing in the ancient Near East[6]. The Sumerian calendar was the earliest, followed by the Egyptian, Assyrian and Elamite calendars.

A larger number of calendar systems of the ancient Near East appear in the Iron Age[7] archaeological record, based on the Assyrian and Babylonian calendars. This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar.

The Gregorian calendar is one of about 40 active calendars in use in the world today. It is not completely accurate: it runs 26 seconds fast every year. This equates to an error of six days every 10,000 years.

Calendars in antiquity were usually lunisolar[8], depending on the introduction of intercalary months to align the solar and the lunar years. This was mainly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd century Coligny calendar[9]. It is believed to have been prohibited by the Romans during the Roman Empire because at the time, Julius Caesar was making imperative the use of his calendar – the Julian calendar. Nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan 10-month solar year[10].

The Roman calendar was reformed by Julius Caesar in 45 BC. Caesar’s Julian calendar no longer depended on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation.

In 1079, in Persia, a calendar reform led by Khayyam was announced – when the length of the year was measured as 365.24219858156 days[11]. Given that the length of the year is changing in the sixth decimal place over a person’s lifetime, it is outstandingly accurate. For comparison, the length of the year at the end of the 19th century was 365.242196 days, while today it is 365.242190 days.

The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582 and is today in worldwide use as the “de facto” calendar for secular purposes.

Different Calendars

Roman Calendar
The Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year. The calendar was reformed and adjusted countless times. In the Roman Republic (509-27 BCE). It was also known as the Republican calendar and is the earliest calendar system from Rome for which historical evidence exists. It was used until 45 BC, when the Julian calendar replaced it.

Julian Calendar
Julius Caesar reformed the Roman calendar in 46 BC as the Julian calendar. He borrowed from Egyptian and Jewish calendars by creating a solar year of twelve months, each of 30 days, but with five days remaining, which he dealt with by having a leap year every four years, dissociating the calendar month from the lunar month. The Julian calendar adopted 1st January as the start date of each year. This date has remained in place even with the Gregorian calendar.

Gregorian Calendar
The Gregorian calendar was introduced by Pope Gregory XIII in 1582i and is the international standard used almost everywhere in the world for international trade and other civil purposes.

The widely-used solar aspect is a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the solar year. The lunar aspect approximates the position of the moon during the year and is used in the calculation of the date of Easter – the most important day for Christians is the resurrection of Christ and Gregorian is a Christian calendar.

It is generally considered that the spring equinox date was miscalculated on the Julian calendar. Easter was the first Sunday following Full Moon after the Spring equinox. This miscalculation worried Pope Gregory III, who introduced the Gregorian calendar. Apart from Easter, the Julian calendar was miscalculated by 11 minutes. These 11 minutes made the Julian calendar longer than the Gregorian calendar. Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29th February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days).

The Islamic or Hijri Calendar
The Islamic calendar is a Lunar calendar and contains 12 months based on the motion of the Moon. The calendar started in AD 622 during the emigration of Muhammad from Mecca to Medina. It is a Lunar calendar with 12 months in a year of 354 days or 355 days. But starting In 639 AD, Caliph Umar started the Muslim Calendar, counting it from the Lunar month. Each month starts when the lunar crescent is first seen by a human observer’s eye after a new moon. The 12 months are: Muḥarram, Ṣafar, Rabīʿ al-Awwal, Rabīʿ al-Thānī, Jumādā al-Awwal, Jumādā al-Thānī, Rajab, Shaʿbān, Ramaḍān (the month of fasting), Shawwāl, Dhū al-Qaʿdah, and Dhū al-Ḥijjah.

A day within the Islamic calendar is defined as beginning at sunset. Since the Islamic Lunar year is 11 to 12 days shorter than the solar Gregorian year, the Islamic New Year doesn’t come on the same day of the Gregorian calendar every year. The first day of the week corresponds with the Sunday of the planetary week. Each month begins approximately at the time of the new moon. The months are alternately 30 and 29 days long except for the 12th, Dhū al-Ḥijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the Moon.

Hebrew Calendar
The Hebrew calendar (aka Jewish calendar) is a lunisolar calendar used today for Jewish religious observance and as an official calendar of the state of Israel. It determines the dates for Jewish holidays and appropriate public reading of Torah portions, yahrzeits (dates to commemorate the death of a relative), and daily Psalm readings, among many ceremonial uses. In Israel, it is used for religious purposes, provides a time frame for agriculture, and is an official calendar for civil holidays, alongside the Gregorian calendar. The present Hebrew calendar is the result of a process of development, including a Babylonian influence.

AD or BCE?
The calendar epoch used by the Gregorian calendar comes from the medieval convention established by Dionysius Exiguus (a 6th century monk born in Scythia Minor) and associated with the Julian calendar. The year number is variously given as AD (for Anno Domini) or CE (for Common Era or Christian Era).

Naming the Months
The Romans used their gods, emperors and kings as names of the months on the calendar, explained as follows:

  • January was initially named after the Roman god of Gates of Janus, the god of beginnings and endings.
  • February was Februus, the god of purification.
  • March was Mars, the famous Roman god of war.
  • April was Aprilis which means to open in Latin. It meant the blossoming of trees and flowers, and it was a special month to honour Venus, the Goddess of love. Aprilis is also derived from the Greek Goddess of love, Aphrodite.
  • May was Maia Maiestas, the Goddess of springtime.
  • June was named after Juno, the Goddess and wife of Jupiter and protector and mother of Mars.
  • July is named after Emperor Julius Caesar. August was Augustus, the first Roman emperor.
  • The remaining months derive their names from Romulus, the first King and conqueror of Rome. September is Septem, the 7th month of Romulus. October is Octo, the 8th month of Romulus. November is Novem, the 9th month of Romulus. December is decom, the 10th month of Romulus.

Pope Gregory VIII

A painting of a person sitting in a chair

Description automatically generated with low confidenceThe calendar invented by Julius Caesar was to endure for 1,600 years, but it was finally discarded when Pope Gregory XIII came up with a more accurate version in 1582.
Picture Credit: Bartolomeo Passarotti: Portrait of Pope Gregeor XIII. Public Domain.
File URL:

Pope Gregory VIII was born Alberto di Morra in about 1105 AD in Benevento, Italy. He was head of the Catholic Church and ruler of the Papal States for two months in 1187. Becoming Pope after a long diplomatic career as Apostolic Chancellor, he was notable in his brief reign for reconciling the Papacy with the estranged Holy Roman Empire and for initiating the Third Crusade.

Sourced/Excerpted from and for Further Reading

  1. Intercalary: (adjective), meaning interpolated; interposed. inserted or interpolated in the calendar, as an extra day or month, or having such an inserted day, month, etc., as a particular year.
  2. The Neolithic, also called New Stone Age, is the final stage of cultural evolution or technological development among prehistoric humans. The Neolithic followed the Paleolithic Period, or age of chipped-stone tools, and preceded the Bronze Age, or early period of metal tools.
  3. A solar year is the time between successive spring or autumn equinoxes, or winter or summer solstices, roughly 365 days, 5 hours, 48 minutes, and 46 seconds.
  4. A Lunation is the period of time averaging 29 days, 12 hours, 44 minutes, and 2.8 seconds elapsing between two successive new moons
  5. The Bronze Age is a historic period, approximately 3300 BCE to 1200 BC characterised by the use of bronze, in some areas proto-writing, and other early features of urban civilisation.
  6. The ancient Near East was the home of early civilisations within a region roughly corresponding to the modern Middle East: Mesopotamia, ancient Egypt, ancient Iran, Anatolia/Asia Minor and the Armenian Highlands, the Levant, Cyprus and the Arabian Peninsula. Sorce: Wikipedia
  7. The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age and the Bronze Age. The concept has been mostly applied to Europe and the Ancient Near East, but also, by analogy, to other parts of the Old World. Source: Wikipedia
  8. Lunisolas calendars are lunar calendars with – in contrast to them – additional intercalation rules being used to bring them into a rough agreement with the solar year and thus with the seasons.
  9. The Coligny calendar is a 2nd century Celtic lunisolar calendar found in 1897 in Coligny, France. It is a big bronze tablet that was originally found broken into 73 pieces. It is a lunisolar calendar with a five-year cycle of 62 months. It has been used to reconstruct the ancient Celtic calendar. The letters on the calendar are Latin and the language is Gaulish. The calendar features “weeks” that consist of 5 days. Each month has six weeks and either 29 or 30 days. There are twelve such months in a year, or 354 days. A calendar cycle consisted of five years of this type, sixty regular months plus two intercalary months. For the calendar to remain in synch with the lunar phases, the five-year cycle must have been 1,831 days long. This would have made the calendar drift out of synch with the seasons by almost a day every year. Roman sources suggest that the Celtic calendar had a thirty-year cycle. Source:
  10. Source: “Religion in the Etruscan period” in Roman religion in Encyclopædia Britannica
  11. Source: “Khayyam Biography”

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