It is needless to add that the sky must be clear and the night moonless for observations of the Zodiacal Light to be possible.
Among the explanations that have been given, the most probable one is that which likens the Zodiacal Light to a flattened nebulous ring surrounding the sun at some distance. It is to be remarked that the direction of the axis of the cone, or of the pyramid, prolonged below the horizon, always passes through the sun.
It was believed at first that this direction precisely coincided with the solar equator; but it seems more certain that it coincides with the plane of the earth’s orbit, or the ecliptic.
Now, what is the nature of this luminous mass? Must it be considered as a zone of vapors thrown off by the sun, when in the process of consolidation, when our central star passed from a nebulous state to that of a condensed fluid sphere? This was the opinion of Laplace.
Another hypothesis, also connected with the first, is that the Zodiacal Light is formed of myriads of solid particles, analogous to the aerolites, possessing a general movement, but traveling separately around the focus of our solar world. The light of the ring would be thus produced by the accumulation of this multitude of brilliant points, reflecting toward us the light borrowed by each of them from the sun.
This explanation accounts for the intensity of the Zodiacal Light at different epochs; it would suffice to admit that the condensation of the particles or the density of the ring is not the same throughout its extent, and that its movement of circulation round the sun presents successively different parts to the earth. In this case, it becomes a question whether this lenticular ring of matter is distinct from the zone of aerolites.
Lastly, some astronomers regard the Zodiacal Light as a vaporous ring which belongs to the earth, surrounding it at some distance. But this is an opinion which appears somewhat wild, and is utterly at variance with observation.
Are the stars that are visible to the naked eye spread orderless on the celestial vault? or is there not between those apparently most closely connected some real or physical connection which requires us to rank them in natural groups?
These questions have been already partly solved by what is known of the double and multiple star systems. Soon, exploring the regions of the sky visible by means of the telescope, we shall have to pass in review a multitude of stellar associations, in which suns are found so compact and so numerous, and the form of the groups so regular, that it is impossible to deny their reciprocal dependence.
But long before the discovery of these islands, these archipelagos as worlds, scattered with such astonishing profusion over the infinite, the naked eye had already distinguished a certain number of groups, the stars composing which were so near together that it was impossible to doubt their physical connection.