Sunspot number

Rarity: Uncommon

A topic a little better known than average, read and translated beyond its own country.

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Notion

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Science and technology

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What Wikipedia says

The Wolf number (also known as the relative sunspot number or Zürich number) is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the Sun. Historically, it was only possible to detect sunspots on the far side of the Sun indirectly using helioseismology.

Excerpt from the Wikipedia article “Sunspot number”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Leland McInnes at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons.