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The start of solar cycle 25 was declared by SIDC on September 15, 2020 as being in December 2019. [9] This makes cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years. Details of cycles 1 to 25
Solar cycle 23 lasted 11.6 years, beginning in May 1996 and ending in January 2008. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 120.8 (March 2000), and the minimum was 1.7. [29] A total of 805 days had no sunspots during this cycle. [30] [31] [32]
The solar cycle is a 28-year cycle of the Julian calendar, and 400-year cycle of the Gregorian calendar with respect to the week. It occurs because leap years occur every 4 years, typically observed by adding a day to the month of February, making it February 29th. There are 7 possible days to start a leap year, making a 28-year sequence. [1]
Converse progression - one day before birth equals one year of life, (and so on backwards in time). Ascendant arc progression - planets are moved the same distance as the secondary progressed ascendant; Symbolic arc progression - planets are moved an arbitrary number of degrees a year e.g. 3 degrees for each year, 5 degrees and so on.
The Islamic calendar is a purely lunar calendar and has a year, whose start drifts through the seasons and so is not a solar calendar. The Maya Tzolkin calendar, which follows a 260-day cycle, has no year, therefore it is not a solar calendar. Also, any calendar synchronized only to the synodic period of Venus would not be solar.
As at Oct 13, 2024, solar cycle 25 is averaging 40% more spots per day than solar cycle 24 at the same point in the cycle (Oct 13, 2013). Year 1 of SC25 (Dec 2019 to Nov 2020) averaged 101% more spots per day than year 1 of SC24. Year 2 of SC25 (Dec 2020 to Nov 2021) averaged 7% more spots per day than year 2 of SC24.
Since 1978, solar irradiance has been directly measured by satellites with very good accuracy. [11]: 6 These measurements indicate that the Sun's total solar irradiance fluctuates by +-0.1% over the ~11 years of the solar cycle, but that its average value has been stable since the measurements started in 1978.
Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began. [2] [3] The solar cycle lasted 11.4 years, beginning in October 1964 and ending in March 1976. The maximum smoothed sunspot number observed during the solar cycle was 156.6 (November 1968), and the starting minimum was 14.3. [4 ...