Why are there 365 days in a year?
“Full moons happen about every 29.53059 days.” Jim Moskowitz “The length of the tropical year is 365.24219 days.” Tom Cull “Originally, the Roman calendar had 12 months of 29 or 30 days” Dan Berger “The number of days in the months has originated through a long and tortuous history of the calendar.” John Link
In other words: if the period of rotation of the Earth about its axis is defined as a day; and if the period of revolution of the Moon around the Earth (from full moon to full moon) is 29.53059 days; and the period of revolution of Earth around the Sun is 365.24219 days; then there are 12.37 lunar months in a year. Lunar calendars correct for the third of a month by adding a leap month every few years, while solar calendars simply divide the year into 12 divisions (“calendar months”) and ignore lunar months altogether. Essentially, there is no physical reason why the ratios of the rotation of the Earth and the revolutions of the Earth and Moon should be rational numbers (the rotation of the Moon is a different story), even though humans have tried for years to impose order on the system.