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I have done more research on the Japanese calendar, and it's as I suspected: claiming to know the accuracy of a historical calendar down to the day is impossibl

I have done more research on the Japanese calendar, and it's as I suspected: claiming to know the accuracy of a historical calendar down to the day is impossible, because the traditional calendar was primarily lunar based, but they had to add an extra month to the year occasionally because the year would start to drift away from the seasons, but this wasn't done in a consistent, systematic manner. I found a tool that attempts to back-convert Japanese dates into Gregorian dates. The tool is called NengoCalc. There's a page (in German) here http://www.yukikurete.de/nengo_chronology.htm http://www.yukikurete.de/nengo_chronology.htm that talks about the challenges of the historical Japanese calendar. Here's a quote from the google-translate version of that page: "In Japan, the Gregorian calendar was introduced in the early Meiji period. The following applies: ５明治 12 月 3 日 corresponds to 1.1.1873. Before that, the bound lunar calendar from China was used. With this the new moon falls on the first of the month. There are months with 29/30 days. From time to time intercalendar months were added, which were named after the previous month. In the case of Japan, it is particularly difficult because this did not happen regularly and therefore there was no continuous time base and consequently no continuous year counting. Instead, you have the system of annual currencies, era names, 年号. These can be changed at any time, e.g. September 1615: 慶長 20 年 -> 元和１年. The first currency of the year was 大化 (645). Because of the lunar calendar, the annual currency year can have a time difference of 4-8 weeks to the Gregorian calendar, so you need appropriate tables for conversion. So it can be that at the end of an annual currency year in the Gregorian calendar you are already in the following year. At this point it should also be pointed out that in Japanese materials this fact is often not taken into account and simply incorrect information is given. Since 1868 there has only been one currency per reign. If the annual currency changes, the year is named after the new currency. In addition, this is also used as a posthumous name for the emperor." The tool NengoCalc, uses three different tables that have been developed for converting the dates and shows the discrepancies between those tables, and tries to use the "most correct" one, but verifying the dates is impossible. All of this is to say that making claims about "this is happening on the earliest day ever" is a good headline, but scientifically unverifiable because there's no way to tie the dates in Japanese historical records to an astronomical calendar.

Retric 6y ago
That’s only going to provide an off by exactly one lunar month adjustment to the dates rather than say a 4 day difference. As these dates are from the same period each year it’s very solid evidence when months where added to the calendar as being 2 months out of sync would be extremely noticeable. Anyway, feel free to look for a valid possible earlier day.
- prometheus76 6y ago
  I don't think what you are saying is accurate. If there is a different number of years between when they add leap months, that will alter how much the calendar has slipped by a decimal number of days.
  - Retric 6y ago
    You’re suggesting we know the offset aka full moon + N days, but not the month. Every calendar has specific days with full moons because that’s a physical event that takes place regularly. It doesn’t matter when you’re adding a lunar month, those full moons will always fall on the same days, thus full moon + 7 days for example can only ever be 7 days from one of those full moons on any calendar.
    - prometheus76 6y ago
      The lunar day is 29.56... days long. Some months, they had 29 days. Some months, they had 30 days. But there's still that pesky .06 days that accumulates over time, pushing the calendar out of sync with the astronomical events. In our calendar, we add a leap day every four years to catch that "leakage". They would add a month occasionally, but they did it at random intervals, so sometimes the calendar would have drifted 5 days off the gregorian before they corrected it, and other times it was apparently up to 8 weeks drifted from gregorian time. So the amount of drift was not in lunar month chunks. It was in accumulated drifts of .06, that got corrected at random intervals.
      - Retric 6y ago

Retric 6y ago

That’s only going to provide an off by exactly one lunar month adjustment to the dates rather than say a 4 day difference. As these dates are from the same period each year it’s very solid evidence when months where added to the calendar as being 2 months out of sync would be extremely noticeable. Anyway, feel free to look for a valid possible earlier day.

prometheus76 6y ago
I don't think what you are saying is accurate. If there is a different number of years between when they add leap months, that will alter how much the calendar has slipped by a decimal number of days.
- Retric 6y ago
  You’re suggesting we know the offset aka full moon + N days, but not the month. Every calendar has specific days with full moons because that’s a physical event that takes place regularly. It doesn’t matter when you’re adding a lunar month, those full moons will always fall on the same days, thus full moon + 7 days for example can only ever be 7 days from one of those full moons on any calendar.
  - prometheus76 6y ago
    The lunar day is 29.56... days long. Some months, they had 29 days. Some months, they had 30 days. But there's still that pesky .06 days that accumulates over time, pushing the calendar out of sync with the astronomical events. In our calendar, we add a leap day every four years to catch that "leakage". They would add a month occasionally, but they did it at random intervals, so sometimes the calendar would have drifted 5 days off the gregorian before they corrected it, and other times it was apparently up to 8 weeks drifted from gregorian time. So the amount of drift was not in lunar month chunks. It was in accumulated drifts of .06, that got corrected at random intervals.
    - Retric 6y ago