This week in What’s Up is a phenomenon called the zodiacal light. If you have access to dark skies, starting this coming week on March 20 and continuing the week after, you may be able to see it from the northern hemisphere. This is a relatively unique phenomenon, and you don’t need any special equipment to see it, just the unaided eye.

Zodiacal light is caused by the Sun illuminating dust in interplanetary space. This cloud of dust is in the plane of Earth’s orbit around the Sun, so it appears from horizon to horizon. In the spring, the zodiacal light is visible after sunset. In the fall, it’s visible before sunrise. It is best seen in the western sky of the sky in the spring, and the reverse in the fall.

The densest portion of the dust light forms a slight triangle-shaped light and has been called “false dawn”. In the modern era, it’s mostly washed out by city light pollution, which is why you need to go to dark skies to see it. In fact, it contributes to the sky brightness in very dark skies, making them brighter than they otherwise would be. Skyglow is also caused by different gases in the atmosphere scattering light. It doesn’t sound intuitive, but it’s how it works.

A related phenomenon is “gegenschein”, or “counter glow” in German, which is just a fun word to say. It’s also caused by sunlight reflecting off of dust particles but in the opposite direction as the zodiacal light.

Observations from Pioneer 10 showed that the interplanetary dust cloud may have come from the main asteroid belt and extends to two astronomical units from the Sun. The dust gets pulled into the Sun from radiation pressure, while new dust comes from asteroids and comets hitting each other and making many small pieces of debris. Astronomer and guitarist from Queen, Brian May, did his Ph.D. thesis on the zodiacal light, contributing to the science on the subject despite using 30-year-old data from before he started the band.

The Moon will be past full next week, so go out to a dark spot and try to spot this faint light. While you’re there, you can look at some deep-sky objects in your telescope. Milky Way season, when the best deep-sky objects will be visible, is starting soon. This time of year is when the dense core of the Milky Way is visible high in the sky during the evening in the northern hemisphere. A good place to start for deep sky objects is the Messier catalog, which we’ve talked about before.

More Information

Zodiacal light: All you need to know (EarthSky)

How to See and Shoot the Zodiacal Light (Sky & Telescope)