If you’ve ever been out and about after dusk in March, chances are you’ve seen the zodiacal light, but didn’t recognize it for what it was; most likely you thought it was just evening twilight, or perhaps the Milky Way poking up from the horizon. The fact is, it’s neither: it’s dust orbiting the sun along with the planets and asteroids and comets, reflecting sunlight back into our eyes.
The zodiacal light appears as a triangular-shaped glow rising up from the western horizon and coming to an apex high in the night sky in the vicinity of the Pleiades star cluster. The presence of a bright planet such as Venus–such as we have this year–is both a blessing and a curse. Venus will show you the exact direction in which to see the glow, but its brightness also obscures its delicate structure. The light itself is pearly white, the color and spectrum of reflected sunlight.
Our solar system is flat like a pancake, with all of the planets, the majority of the asteroids, and most of its gas and dust swirling around the sun in a single plane. That plane aligns with “the ecliptic,” the path Earth takes around the sun–or, as seen from our vantage point, the plane in which the sun seems to circle us. We call it the ecliptic because it is only when the moon crosses it can we have solar or lunar eclipses.
The planets, the dust, and even our own moon can always be found within a band extending about six degrees to either side of the ecliptic. This is a region of the sky which our ancestors envisioned was populated by a circle of animals called the Zodiac: a celestial zoo where the twelve constellations representing the familiar astrological signs reside. The zodiacal light gets its name, of course, from the strip of sky to which it is confined.
The diffuse glow is seen primarily by forward scattering, when sunlight is deflected only slightly from its original path. It’s an effect similar to the silver linings of clouds, or the twilight glow itself, except that this process happens in interplanetary space rather than in our own atmosphere. Its brightness has been calculated to be equivalent to sunlight reflecting from a single one-millimeter speck of dust for every five miles of space. It’s therefore quite faint and difficult to see except under very favorable circumstances. The light is most easily visible when seen closest to the sun where the scattering is strongest, and when the ecliptic is angled well away from the murk of our own horizon.
Such favorable configurations happen only twice a year around the time of the equinoxes: Just before morning dawn in the fall, or after evening twilight in the spring. For folks in Lyons, the chance of seeing the morning glow (called “false dawn”) is remote because it competes with the light pollution from the urban corridor to our east. Hence the best opportunity for us comes on clear moonless evenings, looking westward after dusk, within a month or so of the Spring Equinox.
To avoid confusing the glow with evening twilight, you need to wait until the sun is at least 16° below the true astronomical horizon (not just the tops of the mountains). That means waiting at least 90 minutes after the news channels announce the time of sunset. Equivalently, don’t attempt to see it before around 8:30 p.m. once daylight savings time has gone into effect. Head outside and allow your eyes to become well dark-adapted for about 15 minutes. After that, you might be surprised at how well you can locate the plane of the zodiac glowing softly in the night sky, a triangle of light angled upwards from the horizon and tilted slightly to the south.
One final observing caveat: Moonlight readily washes out the subtle glow from the reflected light of the interplanetary dust. The evenings from March 10 through March 26 mark the window when the moon won’t interfere with your observing quest this year. I wish you good seeing and clear skies!