David Aguilar (617) 495-7462
Christine Pulliam (617) 495-7463

What's New: The Zodiacal Light

The zodiacal light is a dim, roughly triangular glow seen in the west after the faintest glow of evening twilight has disappeared or in the east before the faintest signs of morning twilight have appeared. It is caused by sunlight being reflected by small (a few microns to a millimeter across) dust particles that orbit the Sun in the ecliptic plane of the Solar System - or zodiac - hence its name. The best times to see it are the occasions when the zodiac as seen from Earth makes a steep angle relative to the horizon. Favorable times are during February and March evenings in the western sky, and September and October mornings in the eastern sky. Then it appears as a cone-shaped or triangular glow, wider at the base and narrowing as it follows the ecliptic.

The dust particles and the glow they produce are exceedingly faint and diffuse. The zodiacal light is visible only under the darkest of skies; any light pollution or poor sky transparency will obscure it. Even faint moonlight is enough to swamp the glow, so the presence and phase of the Moon must be taken into account.

Nevertheless, when all is said and done, this is a favorable time of year to look for the zodiacal light. Try looking after nightfall between March 18 and 31, when the Moon will be out of the way.

Some of us have been urban dwellers for much of our lives, and have rarely been under skies dark enough to truly see the Milky Way. Yet the zodiacal light is a glow even more feeble and delicate that that of our galaxy.

There are related phenomena that are even harder to see. Under ideal conditions, on very dark nights, it is possible to trace the subtle strand of the zodiacal light all around the sky - a phenomenon known as the zodiacal band. Again, we are seeing the dust plane of our Solar System.

Somewhat easier to see - because it is more concentrated - is a diffuse glow directly opposite the position of the Sun (which at the moment, presumably, is deep under our feet!). It can be perceived as a thickening or concentration of the zodiacal band, and is known as the gegenschein. This is caused by the phenomenon of forward-scattering of sunlight coming from a point 180° behind us.

Zodiacal Light from Slopes of Kilimanjaro, ©Kwon, O Chul / www.twanight.org/kwon (click image to enlarge*).

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