It was observed under favourable circumstances in January and February, 1903, by the French astronomer, M. F. Quénisset. He found that it was better seen when the atmosphere was less clear, contrary to his experience of the Zodiacal Light. Prof. Barnard’s experience confirms this. M. Quénisset notes that—as in the case of the Zodiacal Light—the southern border of the Gegenschein is sharper than the northern. He found that its brightness is less than that of the Milky Way between Betelgeuse and γ Geminorum; and thinks that it is merely a strengthening of the Zodiacal Light.[267]
A meteoritic theory of the Gegenschein has been advanced by Prof. F. R. Moulton, which explains it by light reflected from a swarm of meteorites revolving round the sun at a distance of 930,240 miles outside the earth’s orbit.
Both the Zodiacal Light and Gegenschein were observed by Herr Leo Brenner on the evening of March 4, 1896. He found the Zodiacal Light on this evening to be “perhaps eight times brighter than the Milky Way in Perseus.” The “Gegenschein distinctly visible as a round, bright, cloud-like nebula below Leo (Virgo), and about twice the brightness of the Milky Way in Monoceros between Canis Major and Canis Minor.”[268]
Humboldt thought that the fluctuations in the brilliancy of the Zodiacal Light were probably due to a real variation in the intensity of the phenomenon rather than to the elevated position of the observer.[269] He says that he was “astonished in the tropical climates of South America, to observe the variable intensity of the light.”
CHAPTER XIV
The Stars
Pliny says that Hipparchus “ventured to count the stars, a work arduous even for the Deity.” But this was quite a mistaken idea. Those visible to the naked eye are comparatively few in number, and the enumeration of those visible in an opera-glass—which of course far exceed those which can be seen by unaided vision—is a matter of no great difficulty. Those visible in a small telescope of 2¾ inches aperture have all been observed and catalogued; and even those shown on photographs taken with large telescopes can be easily counted. The present writer has made an attempt in this direction, and taking an average of a large number of counts in various parts of the sky, as shown on stellar photographs, he finds a total of about 64 millions for the whole sky in both hemispheres.[270] Probably the total number will not exceed 100 millions. But this is a comparatively small number, even when compared with the human population of our little globe.
With reference to the charts made by photography in the International scheme commenced some years ago, it has now been estimated that the charts will probably contain a total of about 9,854,000 stars down to about the 14th magnitude (13·7). The “catalogue plates” (taken with a shorter exposure) will, it is expected, include about 2,676,500 stars down to 11½ magnitude. These numbers may, however, be somewhat increased when the work has been completed.[271] If this estimate proves to be correct, the number of stars visible down to the 14th magnitude will be considerably less than former estimates have made it.