The Subtle Glow in the Night Sky Could Be the Gegenschein

No matter how much we believe we understand about our planet, there’s always something new to learn. For instance, imagine you're out enjoying a peaceful evening of stargazing under a dark sky and suddenly notice a brighter patch of light. Could it be a UFO? (No.) Perhaps the aurora borealis? (Unlikely, unless you're in a region where it's visible.)

Instead, what you might be witnessing is the phenomenon known as the gegenschein. Directly translating from German as 'faint light,' the gegenschein occurs under specific astronomical circumstances when the sun is positioned directly opposite the Earth from where you're stargazing. Here’s what causes it—and how you can try to spot it for a unique astronomical adventure on your next night under the stars.

The Intriguing Gegenschein Phenomenon

So, what exactly is the gegenschein? Astrophotographer and astronomy expert Mary McIntyre sheds some light on the phenomenon—pun intended.

"The gegenschein is a faint, oval patch of light that may appear in a very dark, clear sky, directly opposite the sun," McIntyre explains. "It’s closely linked to zodiacal light, with both phenomena caused by sunlight scattered by a disc of interplanetary dust. This dust extends likely beyond Jupiter’s orbit, with the dust particles being tiny—ranging from 0.001 to 0.3 millimeters in size." Yes, Earth is like Peanuts' Pigpen, surrounded by a cloud of dust.

McIntyre continues: "The gegenschein is dimmer than zodiacal light but brighter than the surrounding dust band. Its formation is tied to the opposition surge and the backscattering of light by the dust particles." Confused about the surge? It turns out, understanding the gegenschein involves some complex astronomical effects.

"The opposition surge is the brightening of a rough surface or an area filled with small particles when illuminated from directly behind the observer," McIntyre explains. "In astronomy, planets are best observed during opposition, or when they’re directly opposite the sun in the sky. A more familiar example is the full moon, which is fully illuminated by the sun from the opposite side of the sky each month."

The opposition surge, sometimes referred to as the opposition effect, makes the interplanetary dust particles on Earth's far side from the sun appear somewhat brighter – this phenomenon is called the gegenschein.

Conditions Must Be Perfect

Understanding the gegenschein is as intricate as explaining its origins. To observe it, specific astronomical conditions must be met. First, you'll need a dark, clear sky – and by clear, we mean no clouds or other atmospheric particles like pollution or humidity. This is why summer isn't ideal for spotting the gegenschein, although it's still possible in certain places.

When it comes to the best seasons for viewing the gegenschein, winter offers a distinct advantage: "You'll have a better chance of spotting the gegenschein during the winter months because the antisolar point will be at its highest position at local midnight," says McIntyre. "If you'd like to locate the antisolar point exactly, newer versions of the free planetarium software Stellarium can help you mark it against the stars in the night sky."

"You also need a moonless night, as moonlight can wash out the sky," McIntyre advises. To catch a glimpse of the gegenschein, try stargazing during the new moon, or even a day or two before or after it.

Timing is crucial for spotting the gegenschein: "It is most likely to be visible around local midnight when it reaches its highest point in the sky," McIntyre advises.

To sum it up, you'll want to search for the gegenschein during a winter new moon (or close to it), with a clear sky, around local midnight, from a location far removed from city lights and other sources of light pollution. Not too difficult, right?

Tips to See the Gegenschein

If you're able to meet all the conditions for timing, season, and clear skies, there are a few more tips to help you spot the gegenschein if it's visible.

First, make sure your eyes are fully adjusted to the dark. This typically takes about 20-30 minutes, according to McIntyre. Avoid checking your phone or other devices during this time, as it will reset the adjustment process for your eyes.

"You may also need to use averted vision to observe it," she explains. "Astronomers often use this technique to view faint objects. Simply look slightly away from the object, and the more sensitive rod cells in your eye will help you perceive it more brightly in your peripheral vision than when you focus directly on it."

The gegenschein is not the only phenomenon of its kind; there's also "heiligenschein," which occurs when water droplets in the form of dew are present. "If you're out early on a sunny morning with dew covering the ground and the sun still low, look around the head of your shadow – you might notice a white glow surrounding it," McIntyre says.

How does the heiligenschein phenomenon occur? "Each dew droplet acts as a tiny lens, scattering light in various directions. Some of this light, however, is reflected directly back along its original path, resulting in back-scattering and thus brightening the area around the antisolar point [in your shadow]," McIntyre clarifies.