The Unwavering Truth about the Zodiacal Light?

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?”

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