From Eric Kvaalen
In his list of revolutionary human ideas, Colin Barras includes lunar calendars, implying these allowed people to plan for seasonal events (25 October, p 32).
But the lunar calendar is not much good for this. It has an error of plus-or-minus 15 days compared with the seasons, and you have to calibrate it by adding a 13th month every two or three years.
A better way of keeping track of the seasons is simply to observe where the sun rises – and ignore the moon.
Les Essarts-le-Roi, France
