Jennifer’s Tips for Chasing Moonbows
I still photograph the Yosemite moonbows quite a bit, but often don't decide
whether to go until the last minute with work, weather, and the late drive home.
Saturday, June 2 is the next predicted date, but I have not decided whether I'll
go that night or not. Water levels may be dropping off some by then, but with
the full moon being so early in June this year, hopefully it will still work. I
anticipate June likely will be the last opportunity for 2012, though, as I
cannot imagine the water levels will make a July attempt worth the trouble
unless the park gets a significant amount of late June rain.
I should have added that I fear the days of having the moonbow nearly to myself
are behind me. Last weekend, there were easily over 100 people in the viewing
area for the lower falls, and probably at least that many spread out along the
bike path near the chapel for the upper falls. At the lower falls, we ended up
sitting on the ground in front of people standing, which was okay since the
ground wasn't wet and since Robert and I can still manage to get down to and up
from the ground at our ages. However, that is not a good solution for many club
members, and having dry ground at the lower falls at moonbow time is quite
unusual. If there is a group of any size trying to stay together, they either
need to show up extra early, or try the upper falls as the probable better
option.
Just what is a moonbow, anyway?
A moonbow is a lunar rainbow created by the refraction of bright moonlight off
spherical water droplets at an angle of nearly 42 degrees.
Where can I find a moonbow?
Yosemite National Park’s Yosemite Falls is the best-known local location for
moonbows. Other popular locations for moonbows are Cumberland Falls in Kentucky
and Victoria Falls in southern Africa. For those who cannot travel, apparently
it is possible to create a moonbow with spray from a garden hose, although I
have not attempted that.
When does the Yosemite Falls moonbow occur?
The moonbow can occur any time the conditions are favorable (darkness, clear sky
near the moon, bright moonlight, ample water/mist, and proper geometric angle).
A team of astronomers and physicists at Texas State University in San Marcos,
Texas created a program to predict the occurrence of the Yosemite Falls Moonbow,
assuming favorable conditions. For their predictions, see
http://uweb.txstate.edu/~do01.
It appears that the predictions are slightly conservative. Others have
photographed the moonbow a few days before/after the dates predicted.
*** Unfortunately, the low water levels this year have been to the moonbow’s
detriment. I had very little refraction in my photos taken on the night of May
31 (although on the positive side of things, the camera and I both stayed dry,
even from the viewing area footbridge where it normally is like standing in a
shower this time of year.) There are predictions given for June 28 through July
1, 2007, but conditions are not looking favorable for those dates unless the top
of the falls receives a lot of rain near that time.
I went to the Lower Yosemite Falls viewing area at an ideal time, but I didn’t
see anything that looked like a moonbow. What happened?
Possibly a couple different things went wrong. First, were you looking in the
right place? The moonbow forms in different locations in relation to the falls
at different times and on different dates. When there is a high arch across the
falls, it is easier to see than when it has dropped down into the rocks at the
base of the falls or when it forms to the right of the falls instead of arching
across the water from the viewer’s left to right.
That is another reason to check the Texas State predictions. They not only give
the times, but they advise whether the moonbow should form to the left, right,
or in the falls at a particular time. Also, the moonbow is yet another prime
example of the camera and the eye seeing differently. The long exposure on an
SLR camera will capture the colors, but the human eye has less color sensitivity
in dim light, so most people see the moonbow as a white, gray or silver arch
rather than full color.
How can I get a good moonbow photo?
Unless you can hand-hold a camera steady for 30 seconds or longer (good luck!),
use a tripod. If you have a remote or cable release for the shutter, use it. Any
camera movement likely will blur the image, and redoing 2+ minute exposures is
time-consuming. You also need a camera that will allow you to make exposures of
30 seconds or more. Set both the camera and the lens to manual. (Take a deep
breath, the manual settings are not that scary!) Turn the lens focus to the
infinity setting (a sideways 8 on most marked lenses, usually reached by
turning the focus ring most or all the way to your left). Set the camera’s
manual setting to bulb. Find and enable the camera’s mirror lockup
setting. If shooting digital, you may want to enable long exposure noise
reduction as well..
A small flashlight can be useful to check the camera and lens settings (and
light the pathway on the walk to the falls), but compose the image in the
viewfinder first. You won’t see much of anything in the viewfinder right away if
you’ve been looking at the light from the flashlight!
Chose the ISO setting (lower is less noisy, but requires longer exposures) and
bracket exposures. I generally check focus and composition at ISO 800 or 1600
where I can make a 20-30 second exposure, then reduce the ISO when I am ready to
take the actual photo (if it isn’t too wet to make exposures of over 30 seconds
in length).
At ISO 100 and f/4, I start with an exposure of about 2 minutes and bracket
up/down from that. Go early or at the beginning of the predicted times. The
moonbow at the lower falls drops down and gets harder to photograph as the hour
gets later. If it is too wet, try moving out to Cook’s Meadow and photographing
Upper Yosemite Falls instead, as moonbows also can form there. Good luck!
Jennifer Doerrie