Zhu Yao's year, month and day, regardless of the sun day, lunar day and Gongri, all have their parallelism. From the first year, month and day, after several rotations, its parallelism will return to an integer with no profit or loss. The value of this time is called period (that is, the time required for a celestial body to return to its relative position is the constant week of the celestial body).
Extended data:
In chemistry, it often refers to a series of elements in each line of the periodic table.
The periodic table of elements is divided into seven periods. 1 period (containing only two elements) is called extremely short period; The second and third periods (each containing 8 elements) are called short periods; The 4th and 5th periods (each period contains 18 elements) are called long periods;
Phases 6, 7 (including 32 elements) and 7 (undiscovered) are called superlong. 1, phases 2 and 3 are also called short periods, and phases 4, 5, 6 and 7 are called long periods.