4. The year preceding that of minimum spots.
5. The year of minimum spots.
6. The year succeeding that of minimum spots.
7. An intermediate period of increasing number of spots,—average length about l1⁄2 years.
8. The year preceding that of maximum spots.
The irregular periods numbered 3 and 7 are necessary because of the variation of the length of the sun-spot cycle. Number 3 may vary in length from 1 to 7 years, and number 7 from 0 to 3.