For the next week or so sky-watchers in the Northern Hemisphere will have the chance to see an elusive celestial pyramid known as the zodiacal light.
The triangular tower of light is easiest to spot around the spring and fall equinoxes. Look for it over the eastern horizon about an hour before sunrise in the fall, and over the western horizon just after sunset in the spring.
At best, the zodiacal light is no brighter than the dim plane of our home galaxy, the Milky Way, so atmospheric haze, a bright moon, or light pollution can hide the faint cone from view. (See a picture of the Milky Way arcing over Iran.)
In general, the zodiacal light is easiest to see in dark, rural areas, particularly on moonless nights.
September 8 saw a new moon—when the unilluminated side of the moon faces Earth—so viewing conditions should be favorable for the next few days as the waxing moon slowly returns to full brightness.
Since the light appears close to dawn in the fall, the phenomenon was often mistaken in antiquity for the light of the rising sun, and it became known as the false dawn.
"Knowing the difference between this false dawn and the real dawn is important for the timing of prayers in the Islamic world," said Geza Gyuk, an astronomer with the Adler Planetarium in Chicago.
But the unusual shape of the light should help sky-watchers tell the difference.
"It will look like a mostly vertical column or pyramid, in contrast to the horizontal true dawn," Gyuk said. "Look for a soft, triangular, pearly glow that precedes the sun and sunrise."
Pyramid of Light Born of Comets?
Unlike the stars and gases of the Milky Way, which stretch away from Earth for light-years, the source of the zodiacal light lies between the inner planets of our solar system.
There, billions of dust grains orbit the sun in a flattened disk spread out along the ecliptic—the plane of the solar system, which also contains the paths of the 12 constellations of the zodiac.
The dusty disk, also called the zodiacal cloud, radiates from near the sun out beyond the orbit of Mars, toward Jupiter. The dust reflects and scatters sunlight in such a way that it creates a visible glow for observers on Earth.
"Because the dust in the solar system is concentrated along the ecliptic plane, the zodiacal light is likewise concentrated," Gyuk said.
It's possible to see the zodiacal light year-round, but for mid-northern latitudes, the ecliptic sticks up almost vertical to the horizon during spring and fall, making the light easier to distinguish from the horizontal glow of sunrise or sunset.
Until recently, astronomers had thought most of the space debris in the zodiacal cloud came from asteroid collisions. But a study released in April suggests that the dust is instead shed by comets that swing close to Jupiter's orbit.
The gas giant planet's strong gravitational pull scatters dust off these comets, according to the study. This dust then falls into orbit around the sun, continuously replenishing the zodiacal cloud.