The Zodiacal Light and Gegenschein

According to Gruson and Brugsch, the Zodiacal Light was known in ancient times, and was even worshipped by the Egyptians. Strabo does not mention it; but Diodorus Siculus seems to refer to it (B.C. 373), and he probably obtained his information from some Greek writers before his time, possibly from Zenophon, who lived in the sixth century B.C.[255] Coming to the Christian era, it was noticed by Nicephorus, about 410 B.C. In the Koran, it is called the “false Aurora”; and it is supposed to be referred to in the “RubÁiyÁt” of Omar Khayyam, the Persian astronomical poet, in the second stanza of that poem (Edward Fitzgerald’s translation)—

“Dreaming when Dawn’s Left Hand was in the Sky,[256]
I heard a voice within the Tavern cry,
Awake, my Little ones, and fill the Cup,
Before Life’s Liquor in its Cup be dry.”

It was observed by Cassini in 1668,[257] and by Hooke in 1705. A short description of its appearance will be found in Childrey’s Britannia Baconica (1661), p. 183.

The finest displays of this curious light seem to occur between the middle of January and the middle of February. In February, 1856, Secchi found it brighter than he had ever seen it before. It was yellowish towards the axis of the cone, and it seemed to be brighter than the Milky Way in Cygnus. He described it as “un grande spectacle.” In the middle of February, 1866, Mr. Lassell, during his last residence in Malta, saw a remarkable display of the Zodiacal Light. He found it at least twice as bright as the brightest part of the Milky Way, and much brighter than he had previously seen it. He found that the character of its light differed considerably from that of the Milky Way. It was of a much redder hue than the Galaxy. In 1874 very remarkable displays were seen in the neighbourhood of London in January and February of that year; and in 1875 on January 24, 25, and 30. On January 24 it was noticed that the “light” was distinctly reddish and much excelled in brightness any portion of the Milky Way.

Humboldt, who observed it from Andes (at a height of 13,000 to 15,000 feet), from Venezuela and from Cumana, tells us that he has seen the Zodiacal Light equal in brightness to the Milky Way in Sagittarius.As probably many people have never seen the “light,” a caution may be given to those who care to look for it. It is defined by the Rev. George Jones, Chaplain to the “United States’ Japan Expedition” (1853-55), as “a brightness that appears in the western sky after sunset, and in the east before sunrise; following nearly or quite the line of the ecliptic in the heavens, and stretching upwards to various elevations according to the season of the year.” From the description some might suppose that the light is visible immediately after sunset. But this is not so; it never appears until twilight is over and “the night has fully set in.”

The “light” is usually seen after sunset or before sunrise. But attempts have recently been made by Prof. Simon Newcomb to observe it north of the sun. To avoid the effects of twilight the sun must be only slightly more than 18° below the horizon (that is, a little before or after the longest day). This condition limits the place of observation to latitudes not much south of 46°; and to reduce atmospheric absorption the observing station should be as high as possible above the level of the sea. Prof. Newcomb, observing from the Brienzer Rothorn in Switzerland (latitude 46° 47' N., longitude 8° 3' E.), succeeded in tracing the “light” to a distance of 35° north of the sun. It would seem, therefore, that the Zodiacal Light envelops the sun on all sides, but has a greater extension in the direction of the ecliptic.[258] From observations at the Lick Observatory, Mr. E. A. Fath found an extension of 46° north of the sun.[259]

From observations of the “light” made by Prof. Barnard at the Yerkes Observatory during the summer of 1906, he finds that it extends to at least 65° north of the sun, a considerably higher value than that found by Prof. Newcomb.[260] The difference may perhaps be explained by actual variation of the meteoric matter producing the light. Prof. J. H. Poynting thinks that possibly the Zodiacal Light is due to the “dust of long dead comets.”[261]

From careful observations of the “light,” Mr. Gavin J. Burns finds that its luminosity is “some 40 or 50 per cent. brighter than the background of the sky. Prof. Newcomb has made a precisely similar remark about the luminosity of the Milky Way, viz. that it is surprisingly small.” This agrees with my own observations during many years. It is only on the finest and clearest nights that the Milky Way forms a conspicuous object in the night sky. And this only in the country. The lights of a city almost entirely obliterate it. Mr. Burns finds that the Zodiacal Light appears “to be of a yellowish tint; or if we call it white, then the Milky Way is comparatively of a bluish tint.” During my residence in the Punjab the Zodiacal Light seemed to me constantly visible in the evening sky in the spring months. In the west of Ireland I have seen it nearly as bright as the brightest portions of the Milky Way visible in this country (February 20, 1890). The “meteoric theory” of the “light” seems to be the one now generally accepted by astronomers, and in this opinion I fully concur.

From observations made in Jamaica in the years 1899 and 1901, Mr. Maxwell Hall arrived at the conclusion that “the Zodiacal Light is caused by reflection of sunlight from masses of meteoric matter still contained in the invariable plane, which may be considered the original plane of the solar system.”[262] According to Humboldt, Cassini believed that the Zodiacal Light “consisted of innumerably small planetary bodies revolving round the sun.”[263]

The Gegenschein, or Counter-glow.—This is a faint patch of light seen opposite the sun’s place in the sky, that is on the meridian at midnight. It is usually elliptical in shape, with its longer axis lying nearly in the plane of the ecliptic. It seems to have been first detected by Brorsen (the discoverer of the short-period comet of 1846) about the middle of the nineteenth century. But it is not easy to see, for the famous Heis of MÜnster, who had very keen eyesight, did not succeed in seeing it for several years after Brorsen’s announcement.[264] It was afterwards independently discovered by Backhouse, and Barnard.

Prof. Barnard’s earlier observations seemed to show that the Gegenschein does not lie exactly opposite to the sun, but very nearly so. He found its longitude is within one degree of 180°, and its latitude about 1°·3 north of the ecliptic.[265] But from subsequent observations he came to the conclusion that the differences in longitude and apparent latitude are due to atmospheric absorption, and that the object really lies in the ecliptic and exactly opposite to the sun.[266]

Barnard finds that the Gegenschein is not so faint as is generally supposed. He says “it is best seen by averted vision, the face being turned 60° or 70° to the right or left, and the eyes alone turned towards it.” It is invisible in June and December, while in September it is round, with a diameter of 20°, and very distinct. No satisfactory theory has yet been advanced to account for this curious phenomenon. Prof. Arthur Searle of Harvard attributes it to a number of asteroids too small to be seen individually. When in “opposition” to the sun these would be fully illuminated and nearest to the earth. Its distance from the earth probably exceeds that of the moon. Dr. Johnson Stoney thinks that the Gegenschein may possibly be due to a “tail” of hydrogen and helium gases repelled from the earth by solar action; this “tail” being visible to us by reflected sunlight.

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 ? 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.”