Zodiacal Light

ZODIACAL LIGHT, a faint illumination of the sky, sur rounding the sun and elongated in the direction of the ecliptic on each side of the sun. It is lenticular in form, brightest near the sun, and shades off by imperceptible gradations, generally be coming invisible at a distance of 90° from the sun. Until a recent time it was never observed except in or near the zodiac ; hence its designation. The most favourable time for evening observation in the northern hemisphere is during the months of February and March. In the tropics, where the ecliptic is nearly perpendicular to the horizon, it may be seen after the end of twilight on every clear evening, and before twilight on every clear morning, unless blotted out by moonlight. It then presents a nearly vertical wedge shaped form, the base of which extends 15° or 20° on each side of the point at which the ecliptic intersects the horizon. The point of the wedge is quite indefinite, the extremely diffuse light grad ually fading into invisibility at a height which may range from 50° to 70° or even more, according to the keenness of the ob server's vision.

It is clear that the light proceeds from a region surrounding the sun, lenticular in form, the axis of the lens being nearly per pendicular to the ecliptic, while the circumference extends at least to the orbit of the earth. The hypothesis which best explains all

the phenomena is that the light is that of the sun reflected from an extremely tenuous cloud of particles having the form and extent described, and becoming more and more tenuous as the earth's orbit is approached until, immediately outside the orbit, it fades into complete invisibility. It has been shown by Fath that the spectrum is identical with that of sunlight.

Intimately connected with the zodiacal light is the Gegenschein, or counter-glow, a faint illumination of the sky in the region op posite the sun, which may generally be seen by a trained eye when all the conditions are favourable. The Milky Way renders it in visible during the months of June, July, December and January. Its light is so faint and diffuse that it is impossible to assign di mensions to it, except to say it covers a region of several degrees in extent. Barnard, the most successful observer, assigns diam eters of 5° or even 1 o° or more. The explanation is uncertain.