Astronomy

Page 104:

To describe the appearance (of the flash, spectrum) in 1870 one cannot do better than to quote from the words of the discoverer:

"The observation is possible only... at a total eclipse of the sun, at the moment when the advancing moon has just covered the sun's disc- the solar atmosphere [chromosphere] of course projects somewhat at the point where the last ray of sunlight [from the brilliant photosphere] has disappeared. If the spectroscope (visual in this case) be then adjusted with its slit tangent to the sun's image at the point of contact, a most beautiful phenomenon is seen. As the moon advances, making narrower and narrower the remaining sickle of the solar disc, the dark (Fraunhofer) lines of the spectrum for the most part remain sensibly unchanged, though becoming somewhat more intense. A few, however, begin to fade out, and some even turn palely bright a minute or two before totality begins. But the moment the sun is hidden, through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by hundreds and thousands, almost startlingly, as suddenly as stars from a bursting rockethead, and as evanescent, for the whole thing is over in two or three seconds. The layer seems to be only something under a thousand miles in thickness, and the moon's motion covers it very quickly." (G. A. Young)

Fig. 1. The flash spectrum at the conclusion of totality (third contact) on March 7, 1970, by William C. Atkinson and Frank B. Dow, Jr. The arcs are emission lines of the solar chromosphere, conspicuous ones being the D₃ line of neutral helium in the yellow (between red and green) at 5876 angstroms and the H and K lines of ionized calcium at 3968 and 3934 in the violet (here rendered blue). The Balmer lines of hydrogen extend from H_à in the red (overexposed) at 6563... In the green at 5303 angstroms, the ringlike image is the corona itself in the light of iron atoms ionized 13 times. Chromospheric lines from within the ring are from the element magnesium. Enlarged...

See also S+T issue cover photo (left panel) and article in "Gleanings for ATMs", A Slitless Spectrograph for the Flash Spectrum (pp.318-323)

This photograph appears in several astronomy books (J. Pasachoff), in a NASA publication, Geo magazine, and was- until its renovation ca. 1999- displayed in the "Hall of the Sun" at the Planetarium of the New York City Museum of Natural History. Photograph taken at Nantucket island airport with a homemade slitless spectrograph using a concave diffraction grating of 15,000 lines/inch and 50cm focal length.

Weight-driven clock equitorial mounted 3,000mm camera and slitless spectrograph

Spectrograph- cover off











Collimator- carbon arc source, condensing lens, and adjustable slit

Spectrograph- cover off revealing film plane and test spectrum
Parabolic film plane; focus test