The Western Calendar: Religion and Science Intertwined

Matthew F Dowd. Science, Religion, and Society: An Encyclopedia of History, Culture, and Controversy. Editor: Arri Eisen & Gary Laderman. Volume 1. Armonk, NY: M.E. Sharpe, 2006.

Calendars represent an important arena in which religion and science have historically operated fruitfully together. Calendars typically incorporate both scientific material, such as the motions of the sun and moon, and religious concerns, such as the proper celebration of religious festivals. Because temporality is an element essential to many religious practices, properly understanding the functioning of the regular natural processes used to mark time becomes an essential ingredient in the creation of a calendar. We are not talking here about the physical object of a calendar, though that is part of the regulation and promotion of a calendrical system. Our subject is the calendar as a theoretical construct: the periodic natural phenomena used to mark time and the points in time that are set down as being important to the culture that uses the calendar. This analysis will focus only on selective aspects of the calendars of Rome and Christian Europe (the latter of which eventually became the most commonly used calendar in the world), but similar remarks apply to the calendars of many different cultures of many different periods.

Fundamentally, calendars are purely human inventions. They need not follow any particular natural processes; however, because calendars delineate recurrent events, only a limited number of reliable, periodic natural phenomena are useful for a calendar. The most commonly used phenomena are the movements of the sun and moon, and indeed these motions have been the basis of the Western calendars.

The length of the solar year is approximately 365.242 days. Because a calendar year uses a whole number, 365 days, a calendar based on the sun must periodically intercalate, or insert, an extra day to compensate for the extra time (a little less than one-quarter of a day) that accumulates with each passing year, or else the date will begin to drift in relation to the seasons or with respect to the stars. The length of the lunar month (the period between the same lunar phases, such as the full or new moon) is approximately 29.53 days, and lunar calendars typically alternate between months of 30 and 29 days. A lunar year, or twelve lunar months, is about 354 and one-third days, about eleven days short of a full solar year. Thus, to keep in line with a solar year and to deal with the fractional difference between the lunar period and whole numbers of days, an extra month or day, respectively, must occasionally be intercalated. These problems lead to serious difficulties when one tries to combine the motions of the sun and moon within a single calendrical system.

Calendars also represent human choices about important events to be marked down or celebrated. Holidays and festivals are the most obvious religious events that calendars mark. And, as we will see below, natural phenomena are often used to set down the proper date for events, either at the same time each year (using the sun, or sometimes the moon, to date the events) or on moveable dates (which typically use a combination of the motions of the sun and moon).

The Roman Calendar

The origins of the Roman calendar are lost in antiquity. The Romans, however, attributed the origin of the calendar to the first two, semimythical kings of Rome: Romulus and Numa. Romulus was said to have created the initial calendar, and Numa was said to have modified it and to have instituted the college of pontifici, a category of priests whose responsibilities included monitoring and regulating the calendar. Very little reliable knowledge of the precise history of the calendar of the Roman republic is available, but certain general characteristics can be gleaned from ancient sources.

The first thing to note about the Roman calendar is that it was simultaneously a civil and religious calendar. The Romans believed that the proper functioning of society arose from maintaining the proper relationship to the gods, which entailed making the proper sacrifices to propitiate the gods. Only when the divine powers were propitiated could human society function in an orderly manner. This meant that various religious festivals had to be celebrated both in the proper fashion and at the proper time throughout the year. It was the responsibility of those who held priestly offices to make sure this happened, and the calendar was one important means by which they did so.

The calendar also performed the very practical function of setting out what sorts of activities could take place on which days. Each day of the year had one or more labels applied to it, indicating what kind of activity could take place on those days (we can see similarities in the modern calendar in the way holidays or religious worship take place on certain days). The dies fas were days on which legal business could be conducted, while the dies nefas were days on which legal business was not permitted. On the nefas feriae publicae, public festivals, which included propitiatory sacrifices to the gods, took place. Assemblies (political, legislative, and so on) were held on the comitialis, and markets were held on the nundinae. Essential elements—economic, legislative and political, and religious—of the proper functioning of society were embodied in the calendar.

The Roman calendar appears to have arisen out of a lunar calendar. According to later Romans authors, the calendar reform of Numa made the year 355 days long, with months of 31 or 29 days, but 28 for February, plus an intercalated month to keep the calendar in step with the seasons. Another indication is the system of referring to dates of the year according to the kalends, ides, and nones. The kalends of a month was the first day of that month, which originally may have corresponded to the new moon. The ides would then fall close to the full moon, which meant the 13th or 15th of the month (depending on the number of days in the previous month). The full system of naming the days can be understood in relation to these two important days of the month. The ides fell on the 13th or 15th of the month; the prior days, counting backward (the Roman calendar employed backward counting when it came to naming the days), would be referred to as the day before ides, the third day of ides, and so on. In the case of the ides, this ran for eight days each month, which gets one back to the 5th or 7th day. The 5th or the 7th day was then known as the nones of that month. Similar backward counting went on for the four or six days of the nones, which got one back to the kalends on the first day. The kalends then ran backward into the previous month (for example, the last day of January would have been referred to as the day before the kalends of February) until one reached the ides of that month, and hence there could be between sixteen and nineteen kalends for a month.

Because of the difficulty of maintaining congruence between solar and lunar elements in the calendar, the Romans faced numerous calendrical difficulties, particularly when the religio-civil officials in charge of its upkeep could not accomplish their task (for example, because of war or political struggles). By the time of the late republic, the calendar was in need of reform. Julius Caesar, who had been in the college of pontifici some years previously and was now dictator of Rome, undertook such a reformation in 46 BCE. He instituted what has since taken his name: the Julian calendar. On the advice of the astronomer Sosigenes, Caesar changed the number of days in each month (and Augustus, some years later, changed them to have the modern values) to make the calendar a solar year: 365 days per year, with a day intercalated every four years to account for the extra part of a day that accumulates each year.

The Roman calendar, then, shows a mix of astronomical and religio-civil concerns. Both the moon and the sun are used to mark time through the use of measurement of physical quantities. But the reasons for doing so are quite outside what we think of as scientific: to keep society functioning properly by helping Romans to observe and preserve the inseparable civil and religious aspects of their culture that they understood to be vital to maintaining their society.

The European Christian Calendar

When the Western Roman Empire began to dissolve in the fifth and sixth centuries CE, being replaced by successive Germanic kingdoms, Christianity had already taken on a prominent cultural role within the region. To continue to make use of the Roman calendar was only natural, especially given that Christianity had become intertwined with the Roman Empire when it was made the legal religion of the empire at the end of the fourth century. But the Roman calendar was in certain ways inappropriate for a Christian community. Three particular concerns led to significant changes in the calendar during late antiquity and the early Middle Ages: the numbering of years, the problem of pagan religious festivals, and the dating of Easter. Each of these led to modifications of the calendar, though only the last required significant astronomical science (the replacement of pagan festivals sometimes was peripherally related to astronomical phenomena).

The Romans had typically referred to a year in one of two ways: either by the rulership of its leaders (for example, the consuls or the emperors) or by reference to the mythical founding of Rome (753 BCE by modern reckoning). Dionysius Exiguus, a monk living in the early sixth century CE, proposed instead numbering the years according to the birth of Christ, so that the first year of the Christian era would begin on the first day of January after the nativity. Due to a mistake in reckoning, he chose a year that was apparently three years too late, placing Jesus’s birth in the year 4 BCE, rather than 1 BCE. Some modern scholars speculate that the birth may have occurred some years prior to that. What is important, however, is not whether Dionysius got it right but that, in a conscious rejection of traditional practice, he changed the calendar to fit a cultural demand, replacing a secular event (the founding of Rome) with a religious event (the birth of Jesus) as the founding event on which the calendar would be based. By doing so, Dionysius was self-consciously incorporating religious belief into the calendar. Though Dionysius’s change was adopted only sporadically and over centuries, it eventually became common across Europe.

As the Christian church spread across the Roman world and northern Europe, it confronted older religious traditions in which festivals and observances celebrated astronomical events or were tenuously tied to celestial events to fix the time of the holiday. It was a common Christian practice to replace these traditional celebrations with Christian festivals. Certain practices of a holiday might be kept or altered, but the reason for the event was replaced with a thoroughly Christian one. One famous example is the replacement of Samhain, a Celtic holiday oriented around the position of the sun, with the Christian holiday of All Saints Day. In this case, ecclesiastics self-consciously and explicitly stated that church officials should try to replace the traditional celebrations with ones of a more Christian tenor, or at least modify existing customs to be in keeping with Christian celebration. Other examples abound, both in the patristic period, when the Roman religion was the object of attack, and in the early medieval period, when the Germanic or Celtic religions were seen as a threat. In all cases, the calendar was a means by which to convey religious beliefs and counteract undesired influences.

The final issue for the Christian calendar during this period was the dating of Easter, and thereby all the moveable feasts. A moveable feast was a religious celebration or observation that had a different date from year to year. Easter is dated according to a lunar calendar because of the biblical narrative and the sequence of the passion following the commencement of the Jewish Passover festival, the date of which was fixed by the moon. Easter was to be celebrated on the first Sunday after the first full moon after the spring equinox; thus the phases of the moon had to be calculated in order to know the date of Easter, and thereby to work back to the other moveable feasts of the year, such as Lent. There were controversies over how the calculation should be made; for example, Bede, in his History of the English Church and People, recounts the events of the famous synod of Whitby at which rival claimants to ecclesiastical authority debated the proper method of determining the date of Easter.

However, the Roman calendar had long ago lost its lunar character. In order to calculate the dates of Easter, the church adopted a nineteen-year cycle of lunar months, with occasional intercalated days, so that it would be easy to know when the new and full moons would occur. This cycle could then be superimposed on the Julian calendar, and one could calculate ad infinitum when Easter and the moveable feasts should fall. A nineteen-year cycle was chosen because this allowed a close correspondence between the solar and lunar calendars. This led to a new science of calendrical computation known as computus, the texts for which frequently incorporated various other elements of the physical sciences. Thus computistical works were often the vehicle by which more general scientific education could be accomplished.

The correspondence between the nineteen-year lunar cycle and solar calendar was not perfect, and as centuries passed, it also became clear that the solar calendar had gotten off track. Fairly simple observations showed that full moons and eclipses were not occurring at the times that the calendar said they should, and therefore the nineteen-year cycle was in error. Eventually it also became clear that the solstices, the most northern and southern points that the sun reaches, were not occurring at the expected times, showing that the solar calendar was in error. The calendar clearly needed to be fixed.

The Gregorian Reform

During the twelfth and thirteenth centuries, Latin Europe learned that the Arabic world was far more advanced scientifically, both because they had preserved the Greek science that Latin literature hinted at and because Islamic scientists had preserved and improved upon ancient science. Various Latin scholars began to seek out and translate Greek and Arabic scientific works, a process that has since come to be known as the translation movement. The appropriation of this scientific corpus had a significant effect on the Western European calendar, as scholars soon learned that errors in the calendar could be remedied. It would take centuries, however, for the reform of the calendar to be enacted, and additional centuries for the new calendar to be adopted around the world.

Some of the earliest calls for reforms came from the English scholar and ecclesiastic Robert Grosseteste. In his Compotus correctorius, probably written in the 1220s, Grosseteste argued that various phenomena showed that the contemporary calendar was in error, and that the work of the Arabic astronomers could be used to correct the calendar. He was, however, not very explicit on how the fundamental nature of the calendar might be changed to correct these errors. For example, he knew the length of the solar year must be calculated more precisely, but he did not offer practical advice for how this would be accomplished.

The problem of errors in the calendar was not merely a scientific one. The real issue was that errors could lead to the improper celebration of religious festivals like Easter. And this had clear theological implications, especially since the celebration of religious festivals was understood as important to salvation. Science might be the means to correct errors, but the goal in so using it was a religious one.

Despite repeated calls for reform, the issue of correcting the calendar did not spur ecclesiastical officials to take action until late in the sixteenth century. Pope Gregory XIII brought together a commission to resolve the issues of correcting the calendar and officially announced the reform of the calendar in 1582. The lunar cycle was modified to be more precise. A few intercalated days were removed. And to bring the solstices and equinoxes back to their “proper” dates, ten days were removed from the year 1582: October 5 through 14. Thus in 1582, October 15 followed October 4.

The Gregorian reform was not immediately adopted across Europe. In Catholic realms, it carried the weight of the pope’s official backing and was adopted very quickly. Most Protestant regions, however, refused to change their calendars for many years. Germany finally adopted a similar reform in 1700, whereas England waited until 1752 to do so. The rejection of the reform had little to do with the scientific work of Gregory’s commission but was instead due to the authority that tried to impose it: the Roman Catholic Church. Just as religious reasons were at the heart of calls for reform, the unwillingness to adopt this particular reform was fueled by religious and thereby political sentiment, namely, that the Roman Catholic pope had no authority in those places. But the practical considerations of operating under separate calendars proved too difficult, and eventually all of Europe was unified under a single calendrical system. Due to the economic and political clout of Europe in the succeeding centuries, the Gregorian calendar spread across the world and now is used nearly everywhere.

The calendar is one arena in which religious and scientific concerns by necessity run concurrently. Scientific information and analysis are vital to creating a calendar that can serve its purpose: tracking recurrent cycles of time. But in many cases, the parameters of what counts as important for the calendar—the dates that need to be figured, the cycles that need to be tracked—are not based on scientific goals or theories. Rather, the history of the Western calendar shows that religious concerns have been an important factor both in creating the calendar and in conducting scientific investigation regarding it.