Eli5:why we use the solar calendar instead of the lunar calendar?
In: Planetary Science
The lunar calendar doesn’t tell us much. What changes from one full moon to the next? Not much. Without looking it up, what even is the current phase of the moon? Most don’t know because it doesn’t affect their day to day life.
But the solar calendar matches with a full round of all the seasons, which are extremely important for society; the spring as plants start to grow, the heat and possible droughts of summer, into the harvest season of fall, preparing for the harsh cold and lack of food in winter, waiting for the warmth of spring again. And even by the equator, there are rain seasons, storm seasons, dry seasons, that sort of thing. It’s important for survival to spend the present preparing for each coming season. This is all determined by where we are in the solar cycle.
Lots of cultures use one or both. The exact calendar that most of the world uses is descended from a Roman calendar that was both lunar and solar.
In general, the benefit of the solar calendar is that you can keep the months and the seasons in sync. For example , “March” is always going to be that transition point between winter and spring. There is drift over time but it can be adjusted with things like leap years. A purely lunar calendar will shift over time so the months cross over multiple seasons as the years go on.
There are also observational calendars that use a fixed reference point and utilize adjustment days to make it start the same time each year. As an example the Baha’i calendar always starts on the first day of spring (the equinox after the shortest day). To make it line up, they will have “extra” days at the end of the last month to make sure the first day of the new year is always the same.
Solar calendars are more useful for almost everything you’d want a calendar for:
* Length of daytime and nighttime depends on what day of the year it is, not what day of the month it is.
* Temperature and weather can be at least somewhat predicted by what day of the year it is, not by what day of the month it is.
* Seasonal cycles relevant to agriculture, animal husbandry, and astronomy depend on what day of the year it is, not what day of the month it is.
The only major advantage of lunar calendars is ease of use:
* Someone who’s bad at math may be able to count to 29½ but not to 365¼.
* If you lose count of what day of the month it is, and there’s nobody available to ask, you can tell (at least roughly) just by looking up at the sky. If you lose count of what day of the year it is, and there’s nobody available to ask, you’ll need a henge or at least a sextant, and the knowledge of how to use it.
So the question is a little bit like asking “Why do we use metal tools instead of stone ones?” Because we are advanced enough to be able to do so, that’s why.
Solar calendars are superior entirely because they’re built around the relative motion and position of the Sun in the sky, which is important because, up until the industrial revolution, quite literally *everything* in human society inevitably derived it’s energy from Sunlight. Sure humans didn’t quite *know* that at the time, but they could obviously see the correlation between the Sun taking up certain positions in the sky and the crops growing again.
Lunar calendars don’t map cleanly to anything useful in human society, at least not over long time frames. The real value is just that, as a clock, the moon is a lot easier to read than the sun simply because looking at the moon won’t obliterate your corneas. In addition, while lunar calendars suck for timekeeping over hundreds of solar years…they’re not terrible for shorter time frames because you still end up getting 12 lunar cycles per year.
Lunar calendars are sometimes better for hunting. Solar calendars are better for farming. We’ve relied much more on agriculture than hunting for many centuries.
Lunar calendars track the brightness of moonlight, basically. Solar calendars track the change of entire seasons and are more useful for planting crops and timing winter migrations. And lastly, the lunar calendar does not precisely line up with the solar calendars. There are cultures that still celebrate holidays every 12th lunar month, but those holidays get earlier every year and can drift between seasons enough to be VERY different from year to year.
All of these comments are super misinformed about the lunar calendars. They’re basing the flaws of a lunar calendar on the solar calendar without regard to how lunar calendars actually work, and they’re trying to explain the benefits of a solar calendar as the reason for the switch when it’s not even remotely the case.
To actually answer the question, we use a solar calendar because that’s what the Romans used. The Romans used one because they had such a large empire that they needed to standardize their calendar; rather than use 20 different (exaggerated) lunar based calendars, everyone would adopt a uniform solar calendar, because the benefits of everyone being on the same time outweighed the benefits of the local calendar that worked for their lifestyle. A lunar calendar from middle Europe would be useless in Cairo, Egypt, because the climates, hunting and growing seasons are drastically different.
The Roman Empire was so influential that their calendar spread to everyone they had trade relations with, because it was beneficial to keep track of Roman time when most of your money came from Rome. Then, through Roman adoption of Christianity, it further spread as the standard calendar of the Church, and later on entire kingdoms became based on the Church thus the kingdom’s calendars as well. Church power displaced the pockets that still used a lunar calendar, leading to widespread adoption of the Roman calendar for practical purposes like trade, with the lunar calendars becoming reserved for traditional religious ceremonies like Ramadan, Yule, or Passover.
The stated benefits of the solar calendar are a product of the longevity of the calendar itself. Over time all calendars have been improved to more closely match what they’re trying to keep track of. In the case of the Roman calendar, it started out as a 10 month calendar (March to December) with an indeterminate amount of days between the end of December (harvest) and the beginning of March (planting) based on how cold the winter was. This was changed adding January and February at some point, then Caesar standardized the calendar to 365 days with a leap year every 4 years in 40BC, then this was changed in the 1500s to the current system of every 4 years unless divisible by 100 and not divisible by 400. (1600 and 2000 were leap years, but 17 18 and 1900 were not)
Because the year is divided by seasons, with a full year being a full cycle of seasons, and seasons depend on the sun not the moon.
Back in the day (and in some countries even today, alongside the Gregorian one) many (if not most) calendars were lunar, because there’s a very obvious and regular pattern to the moon. So it didn’t require much effort to make one. But it also drifts out of alignment with the seasons, since a lunar cycle is 29.5 days, so it’s out by about a week compared to a solar year over all 12 months. So after a generation you’re half a year off.
That’s much worse than even the most rudimentary solar calendar, which is only a quarter day per year off.
As others have said, the solar calendar aligns with the seasons and the solstices and equinoxs. But why is that important?
First, it makes it easier for farmers to know when to plant and harvest. Shepherds to know when to move their flocks. But perhaps just as importantly for ancient cultures, it aligns the festivals with their associated events.
The festival honoring the gods of agriculture in hopes of a good crops should occur in the spring when the fields are sowed. The festival thanking the gods for a good harvest should be just after the harvest. For smaller communities, this isn’t such an issue, but for an organized civilization, such festivals require planning, and people like structure.
Part of the impetus for the Julian calendar reform was that the festivals had become misaligned with the time of year they should occur.
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