Note 16 — Chapter III

The best description of the Zodiacal Light occurs in a letter furnished by Sir John Herschel to the Times newspaper in March, 1843:—“The zodiacal light, as its name imports, invariably appears in the zodiac, or, to speak more precisely, in the plane of the sun’s equator, which is 7° inclined to the zodiac, and which plane, seen from the sun, intersects the ecliptic in longitude 78° and 258°, or so much in advance of the equinoctial points: in consequence it is seen to the best advantage at, or a little after, the equinoxes; after sunset, at the spring, and before sunrise, at the autumnal equinox; not only because the direction of its apparent axis lies at those times more nearly perpendicular to the horizon, but also because at those epochs we are approaching the situation when it is seen most completely in section.

“At the vernal equinox the appearance of the zodiacal light is that of a pretty broad pyramidal, or rather lenticular, body of light, which begins to be visible as soon as the twilight decays. It is very bright at its lower or broader part near the horizon, and, if there be broken clouds about, often appears like the glow of a distant conflagration, or of the rising moon, only less red, giving rise, in short, to amorphous masses of light such as have been noticed by one of your correspondents as possibly appertaining to the comet. At higher altitudes, its light fades gradually, and is seldom traceable much beyond the Pleiades, which it usually, however, attains and involves, and (what is most to my present purpose) its axis at the vernal equinox is always inclined (to the northward of the equator) at an angle of between 60° and 70° to the horizon, and it is most luminous at its base, resting on the horizon, where also it is broadest, occupying, in fact, an angular breadth of somewhere about 10° or 12° in ordinary clear weather.”