Months follow the moon
Edo · before 1873
Edo’s calendar counted months by the moon, but also kept track of the solar year. A month had twenty-nine or thirty days. Twelve such months were shorter than a full cycle of seasons.
To keep the two reasonably aligned, calendar makers sometimes inserted an extra month. Some years therefore had thirteen months.
This was a lunisolar calendar: moon and sun working together. Its numbered months stayed near particular seasons, without matching the months on a modern calendar.
A calendar principle, not the month pattern of a particular historical year.
- Month length
- A lunar month has twenty-nine or thirty days.
- Adjustment
- An extra month occurs in some years, not every year.
- Season
- The year also follows the solar cycle; this is not a purely lunar calendar.