The Schwabe solar cycle is the eleven-year cycle of solar activity of the sun.

At periods of highest activity, known as solar maximum or solar max, sunspots appear. Periods of lowest activity are known as solar minimum. The last solar maximum was in 2001. The solar cycle is not strictly 11 years; it has been as short as 9 years and as long as 14 years in recent years.

As the image shows, the solar constant has varied by about 0.1% or 1 w/m² from its mean value of approximately 1366 w/m² over the last two solar cycles.

The Babcock Model explains the 11 and 22 year cycles as being due to a shedding of entangled magnetic fields.

For an details of solar cycles, see the solar variation article.

The solar cycle is also a name given to the 28-year cycle of Julian calendar with respect to the week. This cycle also occurs in the Gregorian calendar, but is interrupted by years such as 1700, 1800, 1900 and 2100 whose numbers are divisible by four but are not leap years.

See also: Dominical letter, Friday 13