The Unwavering Truth about the Zodiacal Light
Aug 10, 2009

The zodiacal light is a case in point. This little-known phenomenon is a roughly triangular cone of light, produced by sunlight reflecting off a cloud of minuscule dust particles, of cometary origin, that is scattered across the ecliptic plane. At mid-latitude, the light is often mistaken for a ‘false dawn’, as it is best seen in the east preceding the dawn or in the west following the evening twilight in respectively spring and autumn. As the glow is so faint – 10,000 times weaker than that of a strong aurora – it is not very surprising that, in Europe, the zodiacal light was only discovered in 1683 by the Italian astronomer, Giovanni Cassini (1625-1712), while the classical philosophers do not seem to have known it at all.

That there is more to the zodiacal light than reflected sunlight follows from a spate of eyewitness accounts of pulsations and other unexpected fluctuations observed in it. Cassini himself had noted intermittent variations in the brightness of the light and concluded after just ten observations that the axis of the zodiacal light rose and sank not with the ecliptic, but with the equator of the sun. The intrepid explorer and pioneer of geomagnetism, Alexander, baron von Humboldt (1769-1859), witnessed similar perturbations during his travels in South America:

“I have occasionally been astonished, in the tropical climates of South America, to observe the variable intensity of the zodiacal light. As I passed the nights, during many months, in the open air, on the shores of rivers and on llanos, I enjoyed ample opportunities of carefully examining this phenomenon. When the zodiacal light had been most intense, I have observed that it would be perceptibly weakened for a few minutes, until it again suddenly shone forth in full brilliancy. In some few instances I have thought that I could perceive – not exactly a reddish coloration, nor the lower portion darkened in an arc-like form, nor even a scintillation, as Mairan affirms he has observed – but a kind of flickering and wavering of the light. Must we suppose that changes are actually in progress in the nebulous ring?”

Until today, dithering scientists have not addressed the question in unison, but a number of inquisitive scholars did at least acknowledge the challenge. Kristian Birkeland, Norway's most famous auroral researcher, was gripped by the problem. Birkeland was confident he had witnessed oscillations in the zodiacal light and was eager to measure these, prompting his impulsive departure to Sudan to study the phenomenon. Birkeland’s suspicion was that the pulsation in “the intensity and shape of the light … surely testifies to an electric origin …” Indeed, it would have to be “akin to the pulsation which is sometimes seen in auroral lights and the oscillations in terrestrial magnetism.”

Birkeland buttressed this impression with the testimony of George Jones (1800-1870), a chaplain of the United States Navy who had had a marked interest in the zodiacal light. On one occasion, Jones reported “a swelling out, laterally and upwards, of the zodiacal light, with an increase of brightness in the light itself; then in a few minutes, a shrinking back of the boundaries, and a dimming of the light; the latter to such a degree as to appear, at times, as if it was quite dying away; and so back and forth for about three quarters of an hour …”

Birkeland accepted that a substantial constituent of the zodiacal light was due to reflected sunlight, yet also reasoned that some of the light is produced in the same way as the aurorae – by an excitation of particles. Unfortunately, although his terrella experiments seemed to support this hypothesis, he was never able to measure the oscillations.

Naked-eye observations of pulsations, fast moving waves, a temporary invisibility and unexpected ‘parallaxes’ in the zodiacal light continued to be made, sometimes on timescales of a few days. In 1990, the zodiacal light was seen to brighten following a fairly vigorous auroral display. This led at least one modern researcher, the late Neil Bone (1959-2009), to admit that “the diffuse glow of the zodiacal light…, produced mainly by reflection of sunlight from myriads of small particles in the inner solar system does have some association with auroral effects.”

Clearly, the mystery of the zodiacal light has not yet been fathomed and there is much to be learned. In modern terms, does Birkeland’s proposed explanation in terms of ‘corpuscular rays’ or streams of electrons emitted from the equatorial plane of the sun translate to a direct influence of the solar wind on the zodiacal dust? In that case, space scientists ought not to vacillate, as, before long, so little dust will be left that the zodiacal light itself, unless replenished, will become a thing of the past. Though it may still be long before the dust settles on the scientific debate, now is the time to act and obtain the measurements Birkeland had so much desired to make.

Contributed by Rens Van der Sluijs

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Further Reading:The Mythology of the World Axis; Exploring the Role of Plasma in World Mythology

www.lulu.com/content/1085275

The World Axis as an Atmospheric Phenomenon

www.lulu.com/content/1305081