Thursday, March 27, 2008

Secrets of the Zodiacal Light

Sometimes the great adventure of observational astronomy feels like an initiation into a secret society. You learn arcane preparatory rites like dark adapting, you learn to read with a red flashlight, and you learn about celestial objects and phenomena that were within your visual grasp all along, hiding in plain sight.

One example of the latter is the zodiacal light. If you’re a beginning stargazer, this may be an unfamiliar term. But even if you know what it is and what it’s supposed to look like, it can still elude you. The zodiacal light is in the observing category I call “Things That Are Barely There.”

The zodiacal light (pronounced zoh DYE uh cull) is a large, roughly triangular or cone-shaped glow in the night sky, extending upwards from either the eastern or western horizon. The base of the triangle or cone--the widest part of the glow--rests on the horizon. The zodiacal light is typically seen above the western horizon after sunset and above the eastern horizon before sunrise.

There are several reasons why the zodiacal light is somewhat elusive:

a) Its glow is easily confused with the glow of twilight. Twilight is the time before sunrise and after sunset when the Sun, although below the horizon, lights up Earth’s upper atmosphere. Some of that light is reflected toward Earth’s surface, creating a condition of faint illumination.

Avoid confusion with twilight by only trying for the zodiacal light before morning twilight or after evening twilight. To find out when twilight begins and ends at your location on a given date, use Form A at this U.S. Naval Observatory link. Under “Type of Table,” select “astronomical twilight.” Enter your location information, and then click the Compute Table button.

b) Its visibility is dependent upon good transparency. Transparency means atmospheric clarity. Clouds, haze, dust, or humidity can cause poor transparency. The glow of the zodiacal light is subtle, so it's best to have a crisp-looking sky, with good contrast between the bright stars and the deep blackness that surrounds them.

Avoid trying for the zodiacal light when transparency is poor, for example, if a big windstorm just blew through your area. A milky-looking sky makes spotting the cone of light difficult to impossible. In addition, it's critical to be in a dark-sky site, away from urban and suburban light pollution that degrades sky contrast. Finally, you need to avoid times when the Moon is up.

c) It is best seen at specific times of the year. In the Northern Hemisphere, the visibility of the zodiacal light varies greatly throughout the year, but it's most prominent when the ecliptic is nearly perpendicular to the western or eastern horizon. Some good times to view it are February and March after sunset, and September and October before sunrise.

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, Moon, and planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system.

The zodiacal light is so named because it aligns with the ecliptic, which coincides with the zodiac. The zodiac is the band formed by the twelve constellations that straddle the ecliptic, as seen from Earth. To understand why the zodiacal light coincides with the ecliptic, we must unlock the secret of its mysterious glow.

Interplanetary space is not empty. Our solar system contains a vast number of minute particles, some left over from the time of planet formation and some ejected from passing comets or asteroid collisions. Light from our Sun illuminates the myriad particles lying in the solar system plane, so we see the reflected sunlight as a luminous pillar of light along the ecliptic. If you're fortunate enough to view the zodiacal light when transparency is good and the sky is inky black, you may notice the light pillar has a yellowish cast. Our Sun is, after all, a yellow star, so it shines with a golden light.

Here are some links to good photos of the zodiacal light:

This is a great week to search for the zodiacal light after sunset, so let’s head out to the darkest spot we can find.

1) The zodiacal light should be visible about an hour and a half after sunset. The Moon doesn't rise until after midnight on the evening of Thursday, March 27. After that, it rises later each night. So it won't interfere with your quest.

2) Face west, the direction where the Sun set.

3) Look for a very large, faint cone of light rising from the western horizon high into the sky. It will slant slightly toward the southeast, following the curve of the ecliptic. If it's a clear night and you don’t see it, try this tip. Look toward the southwest and locate the constellations Orion and Canis Major. Examine this area of the sky and notice how black the space between the stars is. Now turn back to the west and look at the sky there. Do you see a difference between the blackness of the Orion/Canis Major region and the blackness of the region above the western horizon? The western sky should appear more milky. If you see this difference, try to define the boundaries of the milkiness. You should begin to see the pillar of the zodiacal light emerge.

If at first you don’t succeed, gaze gaze again. With a phenomenon so dependent on sky conditions, you may want to make spotting it a long-term goal. I’ve historically had a hard time seeing the zodiacal light; I really have to work at it. However, just last month I saw my most spectacular zodiacal light ever. Transparency and contrast were superb, and the cone of light was so obvious it was like a smack in the face.

Thank you, may I have another?