A broad, softly glowing pyramid of light that rises from the horizon is easy to miss, but this week offers one of the year's best chances to see it. The phenomenon is zodiacal light, sunlight scattered by dust spread through the inner solar system and concentrated around the plane in which the planets orbit. From Earth, that dusty structure is seen edge-on along the ecliptic, the path through the sky followed by the sun, moon and planets. The ecliptic is home to the 12, actually 13, constellations of the zodiac, from Pisces the fish to Leo the lion.
The new moon occurs on Sept. 10-11, leaving the predawn sky free of moonlight. What remains are the usual obstacles for any skywatcher: clouds, haze and artificial light. Zodiacal light is enormous and can be fainter than the Milky Way, with no hard edges. It appears as a broad triangular glow, widest and brightest near the horizon and narrowing as it climbs.
From the Northern Hemisphere around the September equinox, observers should look east before morning twilight, when the ecliptic rises steeply away from the horizon and the faint glow becomes easier to separate from the murk below. It will reach about 30 to 40 degrees above the horizon. An unobstructed eastern horizon, preferably over the ocean or a large body of water, and a location free of light pollution are the essential ingredients. Viewing should begin before astronomical twilight starts, not after the sky is already visibly brightening.
Artificial skyglow tends to form a rounded dome above one location on the horizon, while zodiacal light reaches diagonally or almost vertically upward along the ecliptic. Eyes need at least 20 to 30 minutes to adapt to the darkness. A camera almost always detects the light before the eyes become fully convinced, with a tripod and a wide-angle lens around 14-24mm among the easiest approaches. A starting point of about f/2.8, ISO 1600 and an exposure of roughly 10-20 seconds usually puts a photographer in useful territory, though sky brightness and lens speed change everything. Shooting RAW and focusing manually on a bright star are recommended.
After the new moon on Sept. 10, a slender waxing crescent moon returns to the evening sky. On Sept. 14 it appears shortly after sunset beside Venus, extremely low in the western twilight. By Sept. 16-17 the moon moves toward Antares, a reddish star at the heart of Scorpius. Sept. 18 brings a first quarter moon, suited to binoculars or a small telescope, and Venus at the greatest brilliancy of its current apparition as the Evening Star.
Orion, normally considered a winter constellation, is in the eastern sky before dawn in September, with its stars along the same general strip occupied by the zodiacal light. Its belt stars will be up, as will bright Sirius below. Jupiter and Mars will also be visible, right on the ecliptic where the false dawn may appear. The constellations climbing in the September dawn are the ones Earth will face during winter evenings a few months later.
