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A Naked-Eye Sunspot

Nature volume 142, page 713 (15 October 1938) | Download Citation



A GIANT sunspot, easily visible to the naked eye, is crossing the sun's disk in latitude 17° north from October 5 until 18, the time of central meridian passage being October 11.9. The area of this spot on October 6, corrected for foreshortening, was nearly 2,500 millionths of the sun's hemisphere, and on October 8 its area measured 2,800 millionths, the measurements being made at Greenwich Observatory. The spot on the latter date extended in solar longitude for about 125,000 miles. Sunspots as large as this one are usually associated with terrestrial magnetic disturbances commencing about one day after the time of central meridian passage. On October 7, a small magnetic storm was recorded at the Greenwich magnetic observatory at Abinger, the disturbance reaching a maximum between 16h and 20h U.T. The ranges of the magnetic elements were: in declination 51' ; in horizontal force 200? ; and in vertical force 260?. It seems uncertain, however, whether this magnetic disturbance can be linked to the big sun-spot ; the most probable time of one related to the disturbed area of the sun containing this sunspot would be about October 12-13.

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