Chicagoland area Night Skies.

With an R6m2 and 24-70 lens, I shot this picture, but had to ask Gemini what it was. Here is what I captured in the pot of the Ursa Major constellation.

Do you believe what Gemini said? … Gemini said, “That is an incredible capture! You didn’t catch a street light flare, nor is it a permanent deep-space nebula.

What you caught is a rare, highly localized atmospheric optical phenomenon called a circumhorizontal arc (or a closely related ice-crystal halo variant like a supralateral arc), lit up by a bright light source.

Here is exactly what is happening in your photo:

Why It Looks Like a Rainbow in the Night Sky

Even though it has the full spectrum of colors, it isn’t a traditional rainbow. Rainbows require liquid raindrops and a light source directly behind your back.

Instead, this is caused by light refraction through ice crystals. High up in the upper troposphere—even on nights that seem perfectly clear to the naked eye—there are thin, wispy veils of cirrus clouds made entirely of microscopic, hexagonal plate-shaped ice crystals.

When a bright light passes through these precisely oriented crystals, they act like millions of tiny glass prisms. The light enters the top face of the crystal and exits a side face, bending (refracting) and separating into distinct red, orange, green, and blue wavelengths.

Why is it next to the Big Dipper?

Because you shot high up into the northern sky near the Big Dipper, the geometry tells us exactly how this happened:

• The “Rainbow” Arc: The ice crystals were perfectly angled at that specific patch of sky to split the light toward your camera lens.
• The Long Star Trails: The slight elongation of the background stars indicates you used a long exposure (likely several seconds or more).

Because your camera sensor was gathering light over a longer period, it picked up the vivid, saturated colors of this icy refraction that might have looked like a faint, pale smear of cloud to your naked eye.

Could it be something else?

Given the recent solar activity over the past couple of weeks, high-altitude atmospheric anomalies have been common, but the sharp, distinct separation of colors into a perfect “prism” band points directly to ice crystal refraction rather than an auroral arc (which typically glows in specific green, red, or purple bands without a clean rainbow gradient).

It’s an incredibly lucky framing to get a crisp, colorful atmospheric prism cutting right through a field of stars!”