The Origins of the Calendar. How did the modern calendar come to be? The history of the calendar illustrates our evolving understanding of the universe.
The measurement of time is a fundamental function of physics. The calendar is one of our first and most basic tools for measuring time. The first thing that must have struck our early ancestors are the repeating cycles of time evident in living on a planet. The repetitions of day and night are the most obvious. Following the phases of the Moon required a bit more observation and concentration to see the regularly repeating pattern. Weather changes over the seasons required even more observation – and likely complex language and discussion amongst many individuals to understand and later exploit.
In nearly every ancient culture (from the Aztecs to the Egyptians to the Chinese to the Norse and many more), there is a consistent symbol, the ouroboros. Usually depicted as a snake eating it’s own tail, this symbol represents infinite repetition. In many cases, the meaning was expanded far back into deep time, to explain what came before recorded history. So, even early on, our intuitive understanding of infinity was well developed. It is a concept that has bothered civilization ever since.
The concept of cyclical time remains with us, but it competes with a linear concept of time. Our early ancestors also noticed the arrow of time (we now call the arrow entropy). People, plants, and animals grew old and died, buildings decayed. Time had a direction.
Both linear and cyclical concepts are simply interpretations of the nature of time, which is still very elusive. Cyclical and linear concepts combine in the calendar.
The first calendars were almost all based on the cycles of the Moon. Often calendars were created locally, city by city, based on eye witness observation of the phases of the Moon. Each complete cycle became a month. Months had and have varying numbers of days because the phases of the Moon do not line up exactly with the day/nught cycle of the Earth. Much effort was placed in putting toe proper number of days in a month as religious celebrations needed to be held at specific times.
As cities grew to civilizations, civil administrators needed to have consistent calendars. The solar cycle more accurately reflected seasonal changes, anchored by solstices and equinoxes, and the lunar calendars became more complex lunisolar calendars. Some societies still rely on lunar calendars which are easier to calculate. The Muslim calendar is a notable exception. The Prophet Mohammed rejected an earlier Arabic lunisolar calendar in favor of the current lunar calendar.
The Sun and the Moon were not the only astronomical influences on calendars. The stars in the night sky, the constellations, as well as the wandering stars, the planets, also played a part. The constellations carried symbolic meaning, and would change position in the night sky in accordance with the solar year. This lead to the science of astronomy.
Over the centuries as civilization grew to a global society, the need to regulate and synchronize civil functions became increasingly crucial. Our current western calendar began as the Julian calendar of ancient Rome in 45 BC. The names of months are roughly consistent with the Julian months in many western languages. The weeks and names of days are also similarly related.
The Julian calendar didn’t quite fit the solar year either. The Earth does not orbit the Sun in a whole number of days. The calendar was reformed several times more, most notably by Pope Gregory XIII, adding periodic leap days to make up the extra time in the new gregorian calendar.