The solar year, for example, like the lunar month, is not a rational multiple of a day. If it were precisely 365.25 days, as we were told in school, then astronomical events related to the apparent position of the sun would repeat precisely over that period, and the year and the day would be back in step at exactly the same point. “So nothing ever repeats exactly. Calendars therefore have to make compromises, and it is the history of those compromises in different cultures that has led to a plethora of calendar systems.”20
