Astro Bob: Zodiacal light stands tall at dawn

By Bob King

October 05, 2024 at 3:48 PM

You might have heard the term zodiac before. It refers to the broad belt around the sky that contains the 12 constellations of the zodiac. Familiar examples include Gemini the twins, Leo the lion and Scorpius the scorpion. The dozen figures are special because the sun travels through them over the course of a year.

In October, our star shines from the constellation Virgo, while in January you'll find it in Sagittarius. The sun's path through the zodiac is called the ecliptic. The planets follow the same path because they all lie either on or very close to the plane that includes the sun and Earth. As far as the sun and planets are concerned, the solar system is as flat as a pancake.

Zodiacal dust — The dust responsible for the zodiacal light lies within the plane of the solar system from near the sun to beyond the orbit of Mars.

Within that plane there lies a vast cloud of fine dust particles shed by comets, expelled into space by asteroid collisions and even ejected by Mars (though we're still clueless how) that extends all the way from the sun to beyond the orbit of Mars. Known as the zodiacal (zo-DIE-uh-cul) light, it's one of the largest entities in the solar system and one of the most tenuous.

C/2023 A3 dawn Oct. 1 — I've been out at dawn a lot the past two weeks in pursuit of Comet Tsuchinshan-ATLAS, photographed here on Oct. 1. The early hour has provided lots of opportunities to see the winter stars and planets along with the zodiacal light.

The zodiacal light lies along the zodiac, hence the name. From mid-northern latitudes it's visible with the unaided eye around the end of dusk in March and April and again before the start of dawn in late September through early November. At these times the ecliptic lies at a steep slant to the horizon, tipping the light upward into good view.

Zodiacal light countryside — Although you can see the zodiacal light from the eastern limits of medium-sized cities it's best viewed from a dark, rural sky. There you can experience the full glory of its breadth. I took this photo 5 miles northeast of Duluth on Oct. 1.

To the eye the zodiacal light looks like an enormous, wedge-shaped pillar of diffuse light slanting upward from the eastern horizon and extending halfway up the sky. It glows brightest and broadest at its base and gradually narrows and fades the higher up you look. It's BIG! The full wedge stands at least five fists tall and about 1 1/2 to 2 fists wide.

I like to get out about 2 hours before sunrise (around 5:15-5:30 a.m. CDT in the Duluth area) and watch until the light begins to fade with dawn's arrival. The zodiacal light resembles the fuzzy look of the Milky Way but has a much smoother texture compared to the chunky appearance of the galactic band. More like breath on a mirror.

Corona breath — Water vapor in our breath condenses into a cloud of water droplets on a cold day. It glows brightly when backlit by the sun, as shown in this photo. Similarly, the sun backlights interplanetary dust to create the glowing pillar of zodiacal light.

Like the planets, comets and asteroids, we view the zodiacal light by reflected sunlight. Since we're looking in the sun's general direction at dawn, the dust is backlighted and glows the same way a person's breath does on a cold day when they stand between you and the sun.

The closer you look in the sun's direction (toward the eastern horizon) the brighter the zodiacal light, the reason why the bottom of the "finger" glows brighter than the top.

Anyone can see and enjoy this remarkable sight. All you need are your eyes and a location with a dark eastern sky on moonless mornings. Viewing will be best now through Oct. 15 and again from Oct. 30 through Nov. 14.