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jackFor the purposes of this story, I'm going to assume that days, lunar months, and solar (vernal) years have a fixed duration of 24h, 29.53xx days and 364.2429 days respectively. (If you want the details, someone will explain in the comment section below.)
In the old days (TM) [1] people tended to have days and nights, and lunar festivals (eg. at full moon), and annual festivals (eg. at spring), but not to have complicated accounting practices. Thus it was easy to say "A year begins at solstice (or equinox, or etc etc), a month begins with a new moon (etc etc), and a day begins at sunset (or midnight, or dawn, etc)" and not worry whether you knew exactly what day the year started on, or if there were always the same numbers of months. And many, many ancient people had calendars based on many variations on this theme.
However, if you want some sort of regular pattern, you quickly run into problems, such as "365¼ isn't a whole number". What happened doesn't always follow the neat arithmetical solutions I'm about to describe, but generally did so from the same principles. (I imagine there was typically some astronomer who had a fair idea, and then a lot of tradition and politics which made things more complicated.)
The first step is to define the month in terms of a whole number of days. In fact, this happens fairly automatically if the month begins when someone sees a new moon -- naturally they'll only do so at night.
Next, define a year in terms of days or months, for instance, say a year begins with the first month that starts after the spring equinox. And this is all fine and dandy, with the exceptions of the hiccups that you can't be sure in advance when things are going to happen -- how can you have a new month if it's cloudy? How many days will it be until the equinox? If you start feeling organised, you might want everything to be a fixed number of days known in advance.
This works fairly well too. For instance, just define a year to be 365 days, and you're done. The start of the year is always about the equinox, and you know it in advance, even if you're unsure when the equinox will be.
However, about this point some smart-alec astronomers come along and say "actually, we've worked it out, and we DO know when the equinox will be. It'll be about 365 days and six hours after the previous one." And you say "365 is good enough, right?" and they say "Sure, as long as you don't mind spring getting 6h later every year."
And indeed, some people decided they DIDN'T care. For instance, the Gregorian calendar has full moons at any time of the month and the Islamic calendar has spring at any time of the year.
Another approach is to wait until spring gets too late and then panic and shove an extra month in there every few years to hold your May-equivalent up until it's actually spring. Ideally you'll give the job of deciding when that's necessary to a bunch of interested political appointees, and forget about it entirely when there's a war on, and as long as you don't mind having the date jump around randomly under everyone's feet, this works just fine. This is what the Romans did until the Julian calendar reform[2], when someone worked out that 365¼ days was the same as 365 days but 366 days every four years, and got Caesar to listen, and this keeps the Julian calendar aligned with the seasons for thousands of years to come.
(In fact, the Gregorian's calendar's slight adjustment to this will last until the slowing rotation of the earth makes planning that many thousand years in advance meaningless.)
Similarly, lunar calendars had to have 29½ day months, ie. alternating 29 and 30 day months to keep the months aligned with the moon (and an extra day every so often to make up the slight difference between 29.5 and 29.53xx).
If you're used to having both months=moons and years=seasons though, you need a compromise of both those adjustments. This is called a lunisolar calendar. It's good in that it keeps months based on moons and years based on seasons, so you can see your approximate position by looking about you. But if you're used to, eg. financial years, it's rather inconvenient because the number of little adjustments you have to make to both months and years line up make it look made by the same people who designed the camel[3].
Continued...
[1] I was originally going to say "In Ye Olde Days", but in fact, IIRC this is all back way before Britian was even regarded as a far off mystical island vaguely regarded as a potential trade-source for tin, at some point after the post-iceage recollonisations, but before anyone in Britain spoke a definitely recorded language.
[2] I AM NOT MAKING THIS UP
[3] I mean, committees, not arabs. Muslims ended up with a lunar calendar. Most N000 year old calendars, eg. hindu and chinese, are apparently in fact lunisolar. Since the particular example I'm thinking of is the Jewish calendar, it no doubt WAS designed by a committee[4].
[4] That's a joke about Jews. In fact, lots of details were in fact specified by God directly when He spoke to Moses, back when everything was still based on observation and before the Israelites had adopted new-fangled babylonian notions like naming the months.
In the old days (TM) [1] people tended to have days and nights, and lunar festivals (eg. at full moon), and annual festivals (eg. at spring), but not to have complicated accounting practices. Thus it was easy to say "A year begins at solstice (or equinox, or etc etc), a month begins with a new moon (etc etc), and a day begins at sunset (or midnight, or dawn, etc)" and not worry whether you knew exactly what day the year started on, or if there were always the same numbers of months. And many, many ancient people had calendars based on many variations on this theme.
However, if you want some sort of regular pattern, you quickly run into problems, such as "365¼ isn't a whole number". What happened doesn't always follow the neat arithmetical solutions I'm about to describe, but generally did so from the same principles. (I imagine there was typically some astronomer who had a fair idea, and then a lot of tradition and politics which made things more complicated.)
The first step is to define the month in terms of a whole number of days. In fact, this happens fairly automatically if the month begins when someone sees a new moon -- naturally they'll only do so at night.
Next, define a year in terms of days or months, for instance, say a year begins with the first month that starts after the spring equinox. And this is all fine and dandy, with the exceptions of the hiccups that you can't be sure in advance when things are going to happen -- how can you have a new month if it's cloudy? How many days will it be until the equinox? If you start feeling organised, you might want everything to be a fixed number of days known in advance.
This works fairly well too. For instance, just define a year to be 365 days, and you're done. The start of the year is always about the equinox, and you know it in advance, even if you're unsure when the equinox will be.
However, about this point some smart-alec astronomers come along and say "actually, we've worked it out, and we DO know when the equinox will be. It'll be about 365 days and six hours after the previous one." And you say "365 is good enough, right?" and they say "Sure, as long as you don't mind spring getting 6h later every year."
And indeed, some people decided they DIDN'T care. For instance, the Gregorian calendar has full moons at any time of the month and the Islamic calendar has spring at any time of the year.
Another approach is to wait until spring gets too late and then panic and shove an extra month in there every few years to hold your May-equivalent up until it's actually spring. Ideally you'll give the job of deciding when that's necessary to a bunch of interested political appointees, and forget about it entirely when there's a war on, and as long as you don't mind having the date jump around randomly under everyone's feet, this works just fine. This is what the Romans did until the Julian calendar reform[2], when someone worked out that 365¼ days was the same as 365 days but 366 days every four years, and got Caesar to listen, and this keeps the Julian calendar aligned with the seasons for thousands of years to come.
(In fact, the Gregorian's calendar's slight adjustment to this will last until the slowing rotation of the earth makes planning that many thousand years in advance meaningless.)
Similarly, lunar calendars had to have 29½ day months, ie. alternating 29 and 30 day months to keep the months aligned with the moon (and an extra day every so often to make up the slight difference between 29.5 and 29.53xx).
If you're used to having both months=moons and years=seasons though, you need a compromise of both those adjustments. This is called a lunisolar calendar. It's good in that it keeps months based on moons and years based on seasons, so you can see your approximate position by looking about you. But if you're used to, eg. financial years, it's rather inconvenient because the number of little adjustments you have to make to both months and years line up make it look made by the same people who designed the camel[3].
Continued...
[1] I was originally going to say "In Ye Olde Days", but in fact, IIRC this is all back way before Britian was even regarded as a far off mystical island vaguely regarded as a potential trade-source for tin, at some point after the post-iceage recollonisations, but before anyone in Britain spoke a definitely recorded language.
[2] I AM NOT MAKING THIS UP
[3] I mean, committees, not arabs. Muslims ended up with a lunar calendar. Most N000 year old calendars, eg. hindu and chinese, are apparently in fact lunisolar. Since the particular example I'm thinking of is the Jewish calendar, it no doubt WAS designed by a committee[4].
[4] That's a joke about Jews. In fact, lots of details were in fact specified by God directly when He spoke to Moses, back when everything was still based on observation and before the Israelites had adopted new-fangled babylonian notions like naming the months.
no subject
Date: 2009-01-16 01:40 pm (UTC)