Letter | Published:

The Remarkable Sunsets


AT the present stage of the discussion upon the “green sun” and rosy sunsets it seems to me it would be well to recall attention to a few facts, for there seems to be a tendency on the part of some correspondents to allow imagination to carry them beyond the region of fact into that of fancy. First, then, I would point out that my observations show conclusively that at the time of the green sun there was an altogether abnormal amount of moisture in the upper regions of the atmosphere, while the ordinary hygrometric observations showed the air near the ground to be comparatively dry. I have studied the rain-band spectrum almost daily for the last six or seven years, and I have never before known such a long continuance of the heavy rain-band in a comparatively clear sky—a sky in which there was only a light haze. At sunset and sunrise the intensity of the bands was such as I have before seen only from an altitude of some six or seven thousand feet, and even then rarely. In this connection it may be well to point out that the spectrum as observed by Mr. Donnelly (NATURE, vol. xxix. p. 132), though, as remarked by Mr. Lockyer, resembling that observed here in some respects, yet differed from it in some important points. The “low sun-bands” appeared weak rather than strong, partly perhaps by contrast with the great intensity of the rainband, and the rainband itself was easily divided into lines, of which eight are recorded in my note-book as being seen with a one-prism spectroscope. The band between b and F, observed by Mr. Lockyer, was also seen here, and was found to be one ascribed to aqueous vapour, W. L. 504. A spectrum almost in all respects similar to that observed here can be seen by any one who will examine the absorption produced by a small cloud passing over the sun as seen with the spectroscope, having a lens in front of the slit. The contrast with the bright spectrum of the sun shows ihe general absorption in the red very clearly, and if the sun be near the horizon the other bands will be, in most cases, fairly well seen.

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