moonbows

A moonbow, or lunar rainbow, is a rainbow produced by moonlight rather than sunlight. It forms when the light from the Moon is refracted, reflected, and dispersed by raindrops or mist in the atmosphere. The arc appears opposite the Moon in the night sky, and the rainbow’s cone angle is about 42 degrees, similar to a daytime rainbow. Because the Moon is much dimmer than the Sun, moonbows are typically faint.

In appearance, moonbows are often mostly white to the naked eye, since the dim light makes color difficult to see. Under favorable conditions and with long exposure, colors can become visible. They require a bright Moon, rain or spray in the foreground, and a dark sky with little light pollution. Droplets must be of suitable size to produce a clear spectrum.

Observing tips: stand with your back to the Moon and look toward rain or mist in the opposite part of the sky. A nearly full or gibbous Moon enhances visibility, but the night should be dark enough for the bow to stand out. A tripod and long exposure can reveal subtle colors in photographs, though the naked eye often perceives mainly white.

Moonbows can be seen near waterfalls, coastal spray, or rain showers where bright moonlight is cast opposite the observer. They are a relatively rare nocturnal counterpart to solar rainbows, and their visibility depends on Moon phase, weather, and ambient light conditions.