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Frequently Asked Questions

  • How come there are data with values between 0 and 11?
  • Why counting sunspots by eye nowadays, in the space era?
  • Ins't a visual sunspot number completely subjective compared to modern quantitative indices?
  • Why is the International Sunspot Number always lower than simultaneous Wolf numbers published by other sources ? (The 0.6 factor)
  • What is the special role of the pilot station ? (Specola Solare Ticinese, Locarno)
  • What does smoothed monthly mean sunspot number actually mean?
  • How is the yearly mean sunspot number calculated?
  • Which time tag should be attached to each daily International Sunspot Number?
  • Who can contribute as an observer to the SILSO sunspot network?
  • What are the motivations and personal benefits to be a SILSO observer ?
  • Will the original Sunspot Number data serie (V1.0) be continued in the future?
Sunspot Index and Long-term Solar Observations

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Data policy

SILSO images and data can be freely downloaded as public data. However, any public use, web based or paper publication of those data must include an explicit credit to the source: (SILSO data/image, Royal Observatory of Belgium, Brussels)

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