Abstract
A FAIRLY large group of sunspots has recently been visible whilst crossing the sun's disc on June 23–July 6 in long. 305° and lat. 24° S. Its area on June 28 was 1000 millionths of the sun's hemisphere, of which 850 millionths was the area of the leader spot. When near central meridian passage, which occurred on June 29.9, the group was visible to the naked eye. Though itself not a return of an individual spot of the previous rotation, the recent spot represented a continuation of localised disturbance which has been apparent for the last two months in this part of the solar surface. In an ordinary telescope, the group has been an interesting object with conspicuous ‘bridges’ across the leader spot in particular. The spectroscope offered, as usual, a further range of observation. On June 26 at 8h 45m G.M.T. a bright eruption took place that was observable visually in the hydrogen C line of the solar spectrum and should have been possible to photographic records in the H and K lines of ionised calcium. The spectrohelioscope showed with perfection the contour of the areas of the brilliant hydrogen (Ha) emission and the dark filaments of gas bordering them at a later stage. The largest radial velocities measured at Greenwich for the absorption filaments were 20 km. a sec. outwards and 60 km. a sec. inwards to the sun. On June 29 at about 8h 45m, several moderately large radial velocities were recorded for about 1 hour. The minimum of the preceding 11-year cycle having occurred at 1933-8, the present spot with respect to size is not unusual, but it is the largest one seen for more than a year, and its appearance is one of several indications that the new cycle is now well-established.
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A Recent Sunspot. Nature 136, 18 (1935). https://doi.org/10.1038/136018d0
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DOI: https://doi.org/10.1038/136018d0