Have you ever seen a moonbow?

Moonbow and Big Dipper, Lower Yosemite Fall, Yosemite

I’m fortunate to have a ringside seat for many of Mother Nature’s most exquisite phenomena, but few excite me more than the shimmering arc of Yosemite’s moonbow. A “moonbow”? I thought you’d never ask….

As you may have figured, a moonbow is a rainbow caused by moonlight. (Don’t be fooled by the fact that your spellcheck doesn’t recognize “moonbow”–it’s a very real thing indeed, and the more technically correct “lunar rainbow” designation just doesn’t seem to convey the magic.) Because a moonbow is a rainbow, all the natural laws governing a rainbow apply. But all this physics isn’t as important as simply understanding that your shadow always points toward the center of the rainbow/moonbow; the rainbow/moonbow will only appear when the sun/moon is 42 or fewer degrees above the horizon (assuming a flat horizon)–the higher the moon/sun, the lower the rainbow. When the moon or sun is above 42 degrees, the rainbow disappears below the horizon.

Each spring, High Sierra snowmelt surges into Yosemite Creek, racing downhill and plunging into Yosemite Valley below. A Yosemite icon, Yosemite Falls drops 2,500 feet in three magnificent, mist-churning steps. On spring full moon nights, light from the rising moon catches the mist, which bends it into a shimmering arc. John Muir called this phenomenon a “mist bow,” but it’s more commonly known today as a moonbow.

While a bright moonbow is visible to the naked eye as a (breathtaking) silver band, revealing the bow’s color requires the camera’s ability to accumulate light. The above image, from a couple of years ago, was captured near the bridge at the base of Lower Yosemite Fall. Not only was it crowded (the moonbow is no longer much of a secret), wind and mist made the necessary 20- to 30-second exposures an exercise in persistence. To include the Big Dipper in this frame (I love the way it appears to be the source of the fall), I composed vertical and wide (19mm). This was a 30 second exposure at f4 and ISO 400.

Moonbow and Big Dipper, Yosemite Falls, Yosemite

Understanding the basic physics of a rainbow makes it possible to photograph a moonbow from other, less crowded locations in Yosemite Valley. In the image on the left, the moon had climbed so high that the moonbow had almost dropped from view. And it was so small at this point that I couldn’t see it at all with my unaided eyes. But I knew it would be there, so I exposed the scene enough to make it nearly daylight bright, again orienting the composition vertical and wide to include the Big Dipper.

<<  FYI, as of this writing, I still have a couple of openings in my two 2012 Yosemite Moonbow photo workshops, April 2-5 and May 2-5.  >>

13 Comments on “Have you ever seen a moonbow?

  1. That’s actually a double “moonbow!” That is so cool! It will definitely be added to my ever growing list of things I’d like to see.

  2. You are truly amazing…Your photography is exquisite…I love the “moonbow”, never saw one before…

  3. Re the lack of star trails: I notice that the farther away a star is from the North Star in this photo, the more “blurred” it is–actually seeming to show a very short trail. The ones that are near the North Star haven’t moved enough in 30 seconds to appear blurred.

    • Very astute observation, Ellen. Since the North Star (Polaris) is (more or less) directly above the North Pole (Earth’s northern axis of rotation), it appears stationary, and the other stars appear to revolve about it. From our vantage point here on Earth, each star makes a complete circle (360 degrees) in 24 hours, and covers exactly the same number of degrees in any unit of time (seconds, minutes, hours, days, whatever). In other words, the farther the star is from the North Star, the larger its circle and more distance it appears to cover. There are 1440 minutes in 24 hours (60 x 24), which means every celestial thing we view from Earth travels 4 degrees each minute (1440 / 360). So during this half-minute exposure, each star covered 2 degrees (half of 4 degrees), but the motion of the stars closer to the North Star appears less because they travel a smaller circle. I hope that makes sense.

      • Thanks for the techno-term explanation! It makes 100% sense; I just didn’t feel like getting into it that deeply in my comment. It’s sort of like when playing an old LP–the outer tracks sound MUCH better than the inner tracks, as there is much more linear distance covered per rotation on the spindle and therefore much more audio information can be included in each rotation. The inner couple tracks are painful to listen to, as there is not nearly enough linear coverage per second of sound to cram in all the necessary audio information–making it sound “gritty”. Keep up the awesome work, Gary; I love each post and each photo! You have a way of capturing the beauty that God has placed in this world that speaks to the soul! It also makes me frustrated to have to sit behind a computer and desk all day and not be out poking around with my camera in the highways, byways, ditches, weeds instead! 🙂 But better armchair travel and photography than none at all… 🙂

  4. Really beautiful and transcendent. Gary, I’m definitely one of your new fans!

  5. Pingback: Photographic reality: Accumulate light « Eloquent Nature by Gary Hart

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