Ghostly cosmic glow provides an early Halloween treat for stargazers

As we approach Halloween, the cosmos has its own celestial treat in store for keen-eyed sky-watchers.

Just before dawn across the entire Northern Hemisphere for the next two weeks, observers can view a faint cone of light extending halfway up the eastern sky for about an hour before local sunrise.

That ghostly glow is called zodiacal light, and can be most easily observed from deep in the countryside, away from the light pollution of cities. You may have already seen it but mistaken it for high-altitude clouds or just the lights of a distant city on the eastern horizon.

However, in this is case, their origins are truly cosmic.

Ancient Romans thought this spooky haze was far-off campfires just past their horizon, while the Greeks thought it was the byproduct of distant volcanic explosions. By the 12th century the thinking was that it was simply the first hint of the coming dawn. In fact, Persian astronomer, mathematician and poet Omar Khayyam made reference to it as “false dawn” in his poem The Rubaiyat.

Today we know that the zodiacal light is actually caused by a fortuitous geometrical alignment between the Earth and sun and countless grains of microscopic interplanetary dust spread out within the ecliptic or flat plane of the inner solar system out to the orbit of Mars and toward Jupiter.

For just a few weeks each year, the sun’s light rays hit all this dust, making their combined light appear along the ecliptic or pathway each planet appears to follow. This faint light show is best seen during early spring and autumn when the ecliptic line is nearly perpendicular to the horizon in the North Hemisphere.

The astronomical phenomenon is called ‘zodiacal’ light because it is usually seen projected against zodiacal constellations like Virgo and Leo, which were conjured up by ancient astronomers and poets to fit the ecliptic.

Visible from the tropics to mid-latitudes, this celestial glow will be particularly easy to catch on some of the upcoming moonless early-morning hours, looking toward the eastern sky before dawn for a diffused light that resembles a tilted cone, wedge or slanted pyramid. At the base of the cone the light may extend some 20 to 30 degrees along the horizon (a fist on your outstretched arm covers about 10 degrees of sky).

Under really clear skies, zodiacal light can be as bright as the Milky Way and stretch more than halfway up in the sky – a truly memorable sight!

It’s amazing to think that this subtle sky glow is the result of billions of tiny dust grains left over from the formation of the planets some 5 billion years ago.