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Zodiacal light

Updated on November 16, 2014

“ … it was so bright that I couldn't believe I'd never seen it before. I was in Chile … I commented to my host … that the light pollution from nearby La Serena was unbelievably intense. The light in the west was washing out even the Sagittarius Milky Way, which was quite high in the sky at the time.” These words describe the first time Tony Flanders saw zodiacal light (March 25, 2008), which he mistook for pollution.

Most people are not familiar with the term zodiacal light, and often mistake this white cone of light for pollution or city lights. This is a phenomenon which is always ‘there’ but is often washed out by the brightness of the sun. Therefore one will recognize it most readily at specific times of day, at particular times of the year, and in geographical areas nearer the equator. Sky watchers in the northern United States or Canada are familiar with the term but are sad to say they have never seen it.

The zodiacal light appears as a white, “foggy” cone or long triangle, wider at the horizon. In the southern hemisphere, the cone leans toward the right; in the north it leans to the left. The appearance is the result of countless tiny particles, debris from comets, asteroids and the ‘big bang’, which orbit the sun. As with moonshine, the light is a reflection of the sun. According to H. C. van de Hulst, these dust particles are about 0.04 inches in diameter and are separated by an average of five miles.

Re-emitting the sun’s rays as infrared radiation, the reradiation causes the particles to spiral slowly into the sun, then being replaced by new comet dust and collisions between asteroids.

To understand the shape of the zodiacal light, one needs to understand a little simple astronomy. The planets orbit along a plane called the ecliptic, so the shape of the solar system is effectively a disc. Densest near the sun and fanning out from the sun to beyond Mercury, Mars, Earth and Venus are dust particles, on the same plane. With the earth’s tilt and orbit, the optimum time of year to see this phenomenon is during the spring and autumn equinoxes, when the ecliptic is perpendicular to the horizon. The ecliptic is the pathway the sun and moon travel in relation to Earth. This pathway was named the Zodiac, or Pathway of Animals, by ancient man. Hence the term zodiacal.

Considered one of the rarest phenomena in the night sky, the zodiacal light captured the fascination of Brian May, founding member of the rock group Queen. He started his doctoral thesis in 1970, but then took time away from it to become a musical celebrity. Thirty-five years later he returned to complete the thesis in 2007, receiving his PhD in astrophysics from the Imperial College in London.

A person could view the light in the southern part of the northern hemisphere, especially when well away from city lights and when the moon is new or just a sliver. The light is actually milkier than the Milky Way. The Milky Way appears as a band across the summer sky, while the zodiacal light is a cone.

To see this “false dawn”, as it is sometimes called, look toward the east horizon around the time of the autumnal equinox just before sunrise, to avoid the glare of the sun washing out the view. It is viewable from late January through early April.

Around the spring equinox, the light is called a “false dusk” and is visible on the west horizon after sunset. These are actually incorrect nicknames, since the zodiacal light is always white and never has any other color in it. Look for it at least an hour and a half before sunrise or after sunset. The false dusk can be seen from September through November

In the northern parts of the southern hemisphere the timing is reversed. If it is still difficult to envision, there are some great photographs at http://earthsky.org/astronomy-essentials/everything-you-need-to-know-zodiacal-light-or-false-dawn. Like the meteor showers of August, the zodiacal light is something anyone should try to spot if the opportunity arises.

© 2014 Bonnie-Jean Rohner

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