Image credit: Samer Daboul via pexels.com
In our previous blog post, we discussed the Gregorian calendar and countries that use it or variations of the Gregorian calendar for their country’s date system. Next time, we’ll discuss countries that follow their own system and might or might not include Gregorian dates on their official records.
Before we do that, however, we’re going to briefly discuss the backgrounds of the calendar systems to explain a bit more about how different calendars can vary so much, not just in start dates as we’ve already seen but in the number of days, weeks, and months.
Solar calendars are based on the rotations of the sun; the dates indicate the position of the Earth as it rotates around the sun. Solar calendars emphasize the progression through the seasons. Roughly 5,000 years, the Egyptians were the first to create a solar calendar using the star Sirius, the Dog Star, as a fixed point in the sky. It coincided with the annual flooding of the Nile and led to the observation that it took 365 days before Sirius appeared at sunrise.
While other elements of that ancient calendar have changed—weeks were ten days long, months consisted of three weeks, and seasons lasted four months, plus some bonus religious days to make up the shortfall—the premise of 365 days a year (plus a 366th day every four years) continues in today’s modern Gregorian calendar and other solar calendars. Each month has the same number of days every year except in leap years.
Lunar calendars, however, are vastly different, older, and easier to track. In a lunar calendar, each month is based on the full cycle of the Moon’s phases. The Moon orbits the Earth every 29.5 days, which is the length of the lunar month or lunar cycle. During that lunar cycle, the Moon shifts through its eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waxing crescent.
Each month in a lunar calendar starts with either a New Moon or the first visible crescent moon after a New Moon, depending on whose lunar calendar it is. The New Moon is the invisible phase; the Moon blends in with the night sky because we see the side that’s not illuminated by the Sun. It’s the opposite of a full Moon. Many lunar calendars begin with the earliest visible waxing crescent, the tiny sliver of the moon visible to the naked eye, while others use the last visible crescent of a waning Moon.
Due to the regularity and visibility of the Moon and its phases, lunar calendars have been around much longer than solar calendars – tens of thousands of years.
Since the lunar month, or lunation, is 29.5 days, the lunar calendar is only 354 days, shorter by 11-12 days compared to the 365-366 day solar calendar. Some lunar calendars add a thirteenth month to the lunar year every 2-3 years to align it with the seasons, but others maintain 12 months a year. Consequently, their seasonal holidays occur earlier each year.
Even in cultures that don’t follow the lunar calendar system, however, religious holidays are often determined by the Moon’s phases, such as Easter in the Gregorian calendar and Lunar New Year in the Chinese lunisolar calendar. What’s a lunisolar calendar?
A lunisolar calendar tries to maintain synchronicity with both the solar year and the phases of the Moon; monthly lunar cycles are combined with the solar year. Lunisolar calendars were used in many early civilizations of the Middle East and Greece.
Since we’ve already observed that there are unequal days in their respective months and years, many lunisolar calendars adjust the number of months to avoid the seasons drifting. Every second or third year will include a thirteenth month, called an intercalary, or leap month.
The lunar calendar has been a critical element throughout human history. However, it doesn’t coincide with the seasonal year the way solar calendars do, which is a big drawback. Intercalation, or inserting days or months into a calculator to align lunar and solar calendars, has also been around for centuries. It has been applied in different ways by different civilizations, which is yet another reason for the difference in calendar implementations across the globe. Leap year days are a popular example.
There’s also fascinating stuff about the 19-year Metonic Cycle and the Golden Number G that affects lunis-solar calendars, but this is intended to be a high-level overview, so we won’t go into that.
The most popular solar calendar in use today is the Gregorian calendar, which was introduced in 1582 as a variation on the Julian calendar. The Islamic calendar is a purely lunar calendar whose months correspond with the lunar phase cycle. Lunisolar calendars are used in the Jewish calendar, the national calendar of India, and the Chinese calendar.
In addition, some calendars include the designations BC, AD, CE, and BCE, some of which you may see on academic records. The Gregorian calendar centers on the birth of Jesus. Dates before his birth are noted as BC, or Before Christ, while AD refers to Anno Domini, meaning after his birth.
Since the Gregorian calendar is so widely accepted, even among non-Christians, secular versions were also been created. BCE stands for Before Common Era instead of BC, and CE refers to Common Era for AD. Several countries that follow their own system or a variation on the Gregorian calendar will list their own calendar dates as well as CE years.
I hope you enjoyed this brief history/astronomy/humanities lesson!
Resources:
Ancient and Religious Calendar Systems. Britannica. https://www.britannica.com/science/calendar/Ancient-and-religious-calendar-systems#ref313439
Calendar Conversion Tools. Harvard University. https://gpks.hsites.harvard.edu/useful-websites/calendar_conversion_tools
Calendar Converter. Fourmilab. https://www.fourmilab.ch/documents/calendar/
Calendar Types: Lunar, Solar & Metonic Cycles. Study.com. https://study.com/academy/lesson/video/the-lunar-calendar-metonic-cycle.html
Intercalation. Britannica. https://www.britannica.com/science/intercalation
Introduction to Calendars. Astronomical Applications Department, US Navy. https://aa.usno.navy.mil/faq/calendars
Lunar vs. Solar Calendars. Night Sky Tourist. https://nightskytourist.com/lunar-vs-solar-calendars/
Lunar Time vs. Solar Time. Tamid: The Downtown Synagogue. https://tamidnyc.org/20-lunar-time-vs-solar-time/
Moon Phases. NASA. https://science.nasa.gov/moon/moon-phases/
NOAA Solar Calculator. Global Monitoring Laboratory. https://gml.noaa.gov/grad/solcalc/
The Science and Shared History behind the Gregorian and Chinese Calendars. National Public Radio. https://www.npr.org/2024/02/12/1198909292/lunar-new-year-chinese-lunisolar-calendar-history
Solar Calendar. Britannica. https://www.britannica.com/science/solar-calendar
Solar Calendar – We Owe Our Notion of Time to the Sun. Solar Facts and Advice. https://www.solar-facts-and-advice.com/solar-calendar.html
What Is a New Moon? When Is the Next New Moon? Almanac.com. https://www.almanac.com/what-new-moon
What is the Common Era? EarthSky. https://earthsky.org/human-world/definition-common-era-bce-ce-bc-ad/
What is the Difference between the Lunar Calendar & the Solar Calendar? Sciencing. https://www.sciencing.com/difference-between-lunar-calendar-solar-calendar-22648/
Why the Lunar New Year Isn’t Really a Lunar New Year. Rocky Mountain Public Broadcasting Service. https://www.rmpbs.org/blogs/science-environment/lunar-new-year-science
The World’s Oldest Solar Calendar May Have been Unearthed in Turkey. National Public Radio. https://www.npr.org/2024/08/11/g-s1-16502/worlds-oldest-solar-calendar-turkey-gobekli-tepe-comet-strike