Abstract
Two new large groups of sunspots have recently appeared. Particulars from the Royal Observatory, Greenwich, are as follows, the areas being measured on April 21 and expressed in millionths of the sun's visible hemisphere.
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Large Sunspots. Nature 143, 717 (1939). https://doi.org/10.1038/143717f0
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DOI: https://doi.org/10.1038/143717f0