Work seems to have been organized on the basis of a nine-day week. Three nine-day weeks, or 27 days, is the approximate time between new moons. Every third year was made up of 13 moons, with the others having 12. This formed a cycle of 37 moons, and 20 of these cycles made a period of 60 years. A true year, as opposed to a calendar year, may be defined as the time the Earth takes to return to the same point on its orbit around the sun.
Astronomers therefore recognize different kinds of year.
The simplest reference point is one on the orbit in which the Earth aligns with the sun and a particular star. Another reference is a point on the orbit where the Earth's axis is perpendicular, or at a right angle, to a line from the sun. This occurs twice a year, in the spring and fall. A year measured between successive crossings of one of these points is called the tropical year. The seasons keep in step with the tropical year because both are based on the position of the Earth's axis.