Solar cycle 21

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The Sun with some sunspots visible.

Solar cycle 21 was the 21st solar cycle since 1755, when recording of solar sunspot activity began.[1][2] The solar cycle lasted 10.3 years, beginning in June 1976 and ending in September 1986. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 164.5, and the minimum was 12.3.[3] There were a total of 273 days with no sunspots during this cycle.[4][5][6] The largest solar flare of this cycle (X15) occurred on July 11, 1978.[7]

See also[edit]


  1. ^ Kane, R.P. (2002). "Some Implications Using the Group Sunspot Number Reconstruction". Solar Physics 205(2), 383-401.
  2. ^ "The Sun: Did You Say the Sun Has Spots?". Space Today Online. Retrieved 12 August 2010. 
  3. ^ SIDC Monthly Smoothed Sunspot Number. "[1]"
  4. ^ Spotless Days. "[2]"
  5. ^ What's Wrong with the Sun? (Nothing) more information: Spotless Days. "[3]"
  6. ^ Solaemon's Spotless Days Page. "[4]"
  7. ^