284 



SCIENCE-GOSSIP. 



CHAPTERS FOR YOUNG ASTRONOMERS. 

 By Frank C. Dennett. 



THE SUN. 



(Continued from page 251.) 



There is a faint light known as the Gegenschein, 

 apparently connected with the Zodiacal light, which 

 has been repeatedly observed by Barnard and others. 

 It becomes visible in the Zodiac, about opposite to the 

 sun's place, and so, during the winter months, should 

 be looked for on dark, starry nights in the portion of 

 the ecliptic upon the meridian at midnight. 



Returning more directly to the sun itself, there is 

 much which, to the ordinary individual, becomes 

 visible only at the time of a total solar eclipse. 

 Then, when the sun's brilliance is hidden behind the 

 dark body of the moon, the corona arrests the eye, 

 encircling the sun like the glory around the head of 

 a pictured saint. Close study shows the corona to 

 be partly made up of rays. Comparing eclipse with 

 eclipse, variations are found to make themselves 

 apparent in this appendage. The times when sun- 

 spots are in greatest abundance have the corona 

 pretty equally distributed all round the disc, any rays 

 then occurring mostly about 45° from the poles of the 



Total Eclipse of the Sun, August 9TH, 18 



sun, and not extending to great lengths. On the 

 other hand, when sunspots are near the minimum, 

 the corona is rifted over the poles, whilst near the 

 equator there are great extensions. 



The finest photograph of the coronal rays yet 

 taken was that by Mrs. Maunder on a Sandell plate 

 during the eclipse of January 21st, 1898. The 

 longest ray extended a length about equal to six 

 diameters of the sun. Another photograph of the 

 corona, taken by the Russian expedition to Nova 

 Zembla during the eclipse of August 9th, 1896, is 

 reproduced for purposes of comparison. 



The corona, studied by help of the spectroscope, 

 is found to give a faint continuous spectrum — pre- 

 sumably reflected sunlight — and one green line, at 

 first thought to be one of the lines due to iron, but 

 which is now known to come from an element called 

 coronium. Hydrogen and helium are also found to 

 be present in the corona. Anything further with 

 regard to this wonderful appendage is clouded in 

 mystery. 



Another feature visible at the total eclipses of the 

 sun is the red prominence. These "flames" had 

 been observed previously to the Indian eclipse of 

 August 1868, but it was not until that occasion that 

 the nature of them was revealed. The spectroscopes 

 of Captain J. Herschel, M. Janssen, Colonel Ten- 

 nant, and M. Rayet showed these prominences to be 

 composed of incandescent gas, principally hydrogen. 

 Directly this was an ascertained fact, hopes were 

 raised that the spectroscope would show them at any 

 time when the sky was clear. This had already been 

 pointed out by Sir W. Huggins. Sir J. N. Lockyer 

 and M. Janssen succeeded in observing the spectrum 

 of the prominences on the day following the eclipse, 

 and before the news reached England Lockyer had 

 succeeded in making a similar observation. Very 

 soon it was found that by opening the slit of a 

 spectroscope of considerable dispersion, not only the 

 spectrum, but the entire prominence could be ob- 

 served. These objects are of enormous size, some- 

 times reaching 70,000 miles or more above the ap- 

 parent level of the sun. Notwithstanding their great 

 magnitude, very extensive changes take place, some- 

 times in the interval of less than a quarter of an hour. 

 Perhaps the grandest exhibition which it has fallen 

 to the lot of an observer to witness took place on 

 September 7, 1871. Professor Young, then of Dart- 

 mouth College, 

 Hanover, N.H., 

 saw a cloud of 

 hydrogen some 

 100,000 miles in 

 length lying with 

 its lower surface 

 about 15,000 

 miles above the 

 sun's limb, 

 whilst the upper 

 surface had an 

 altitude of 

 54,000 miles. 

 Thus it was at 

 12.30, but by 

 12.55 the whole 

 had been, as it 

 were, blown to 

 shreds, which 

 had already 

 reached a height 

 of nearly 100,000 

 miles. So rapid 

 was the motion 

 that by 1. 15 the height attained was over 200,000 

 miles ; so that the filaments were ascending at the 

 rate of about 166 miles per second. These beautiful 

 rose-tinted prominences seem to occur in almost any 

 latitude, and not to be confined to the zones in 

 which the spots appear, as would seem most natural 

 to expect. 



From time to time observers in various places have 

 fancied that they have observed planetary or cometary 

 bodies in transit over the sun, which some have sup- 

 posed to be a planet revolving within the orbit of 

 Mercury, and which has been even named Vulcan, 

 but which is now usually believed to have no real 

 existence. 



Note.— On p. 251, column 2, 2nd line from 

 bottom, for " I approximate," read " it approxi- 

 mates ; " last line, for " is, in my opinion, vertical," 

 read " is lenticular." 



'To he continued.") 





