Chasing Ghosts: Unveiling the Hidden Colors of the Lunar Rainbow, or Moonbow

A Rainbow in the Dark

Imagine standing under a full moon, near the thundering mist of a waterfall, and seeing a pale, ghostly arch stretch across the night sky. It looks like a colorless rainbow, an ethereal silver band against the darkness. This is a moonbow, also known as a lunar rainbow, and it's one of nature's most elusive and magical displays. While it may appear faint and white to our eyes, it holds a secret that only a camera can truly reveal.

The Science Behind the Specter

A moonbow is formed in much the same way as its daytime counterpart. It's an optical phenomenon caused by the refraction and reflection of light in water droplets—typically from mist, rain, or a waterfall. The key difference, of course, is the light source. Instead of direct sunlight, a moonbow is created by moonlight, which is simply reflected sunlight. Because the moon is so much dimmer than the sun, a moonbow is thousands of times fainter than a typical rainbow. The geometry must be perfect: the moon must be low in the sky (at an angle of less than 42 degrees) and the sky must be very dark. The viewer needs to be standing with the moon at their back to see the arc form in the water droplets ahead.

Why Do We See a 'White' Rainbow?

The most common question from those lucky enough to spot a moonbow is, "Where are the colors?" The answer lies not in the sky, but in our own biology. The human eye has two types of photoreceptor cells: rods and cones. Cones are responsible for color vision but require bright light to function effectively. Rods, on the other hand, excel in low-light conditions but are unable to perceive color. Since moonlight is too faint to activate our cone cells, we rely on our rods to see the moonbow. The result is the perception of a white or grayish arc. The ancient Greek philosopher Aristotle noted this in his work, Meteorology, around 350 BC.

"These appearances are rare, but have been seen. They are not, however, as richly colored or as distinctly marked as the solar rainbow... and are seen only in a whitish light."

Revealing the True Colors

This is where technology triumphs over human limitation. A camera with the ability to take a long-exposure photograph can do what our eyes cannot. By leaving the shutter open for several seconds, or even minutes, the camera's sensor can gather enough faint light from the moonbow to resolve its hidden colors. The resulting photographs are often breathtaking, revealing a vibrant, multi-colored arc that stands in stark contrast to the dark landscape. It's a powerful reminder that there is often more to reality than what we can perceive with our senses alone.

Where to Hunt for a Moonbow

While a moonbow can technically occur anywhere with the right conditions, a few locations around the world are famous for their reliable and spectacular displays, primarily due to their powerful waterfalls that generate constant mist. These include:

• Cumberland Falls State Resort Park, Kentucky, USA: Known as the "Niagara of the South," this is one of the most accessible and predictable places in the Western Hemisphere to see a moonbow.
• Yosemite National Park, California, USA: During the spring snowmelt, the park's waterfalls, particularly Yosemite Falls, produce enough mist to create stunning moonbows on clear, moonlit nights.
• Victoria Falls, Zambia/Zimbabwe: The immense spray from this massive waterfall, known locally as Mosi-oa-Tunya ("The Smoke That Thunders"), creates brilliant moonbows for several days around the full moon.

If you ever find yourself in one of these places during a full moon, turn your back to the light, face the mist, and you might just witness a ghost of a rainbow appear in the dark.

Sources

• Wikipedia - Moonbow
• National Park Service - Yosemite's Lunar Rainbow
• Kentucky State Parks - Cumberland Falls Moonbow