If you saw a peculiar sight in the evening sky over northwestern Pennsylvania — and neighboring states — Tuesday evening, you’re not crazy, but it wasn’t a UFO either.

Many area residents reported spying an unusual, spiral-shaped light moving slowly across the sky around 10:45 p.m.

Warm, clear weather and several recently occurring astronomical phenomena have had folks with their eyes to the sky. The sturgeon moon peaked at 3:55 a.m. Saturday and coincided this year with the Perseid meteor shower. That celestial spectacle reached its height between 2 and 4 a.m. Wednesday.

The August sturgeon moon as photographed by Bradford resident Devin Grice.

So what, exactly, did folks see Tuesday evening?

According to multiple news reports, the unusual visual was caused by a rocket launch — which one, exactly, wasn’t entirely clear. Some outlets stated early on that it was the result of a SpaceX Falcon 9 rocket launch from Vandenberg Space Force Base in California. Others held that it was the Ariane 6 rocket launch from French Guiana, which occurred earlier that night. That theory has gained traction based on trajectory, timing and expert analysis. The Ariane 6 rocket was launched by the European Space Agency to carry a weather and climate satellite into polar orbit, per earthsky.org.

However, all sources agree that the strange spiral seen over much of the northeastern United States Tuesday night was created by the fuel that rockets vent during deorbit, also according to earthsky.org.

When excess propellant is released, it freezes in the thin air into ice crystals that reflect sunlight and create a glowing spiral visible to those of us on the ground. The spiral forms when the rocket spins slightly during the venting procedure. Sunlight can illuminate the frozen fuel particles if the rocket is high enough in the atmosphere due to the twilight effect, even if it’s dark on the ground.

It happens above the earth’s shadow, in the stratosphere and mesosphere and can create glowing shapes like the spiral, or a plume or halo.