The mistake many tourists make, he said, is to attempt a shot of the moon framed by an arch when they're much too close to the arch.
"They take that shot," Webster said, "and they're disappointed because the moon was so spectacular when they saw it, and they get it home and it's just a little white dot."
Webster's secret is to hike a long way away from the arch on a trek that's carefully planned in advance. He spends hours at his home computer with mapping software calculating distance, elevation, azimuth, the angle of the rising moon and the timetable for moonrise. On his hike, he carries data on his cellphone and on a laptop in his backpack.
It may not be obvious why a photographer's distance from the arch will make the moon appear bigger. Webster explains that it's because the moon is vastly farther away than the arch. No matter where a photographer stands, the moon seems relatively small. It appears to be about the size of a fingernail held at arm's length. Whether the photographer stands close to the arch or miles away, the moon will always appear to be about that same size — a fingernail at arm's length.
As Webster hikes away from the North Window, the arch itself seems to get smaller and smaller as it recedes into the distance. Since the moon's apparent size remains unchanged, a rising moon will fill a larger and larger portion of the arch. If a photographer zooms in with a high-quality telephoto lens, he can get an impressive image of a looming moon filling the arch.
"If everything goes right, we can get a nice shot of it," Webster said. "It's delightful to try and do."
Getting to the right spot, though, is a big challenge.
"We are right on it," he said as he arrived at the spot indicated by his calculations about a mile away from the arch. "We are looking pretty good now."
Webster knows from experience that he doesn't always find exactly the right spot for his tripod.
"I have gone home completely empty-handed sometimes," he said.
Disappointingly, as the moment of moonrise arrived, the moon did not appear in the arch. Webster waited tensely, hoping his timing calculation was slightly off. Moments later, his hopes were dashed.
"Oh, there it is," he exclaimed. "Gosh! We missed it! We missed it!"
To Webster's dismay, the moon could be seen rising above the cliff a bit to the left of North Window arch. Webster realized he and his tripod were about 20 yards north of where they needed to be. It was apparently due to a slight error in correlating map data with features on the ground.
Webster took it in stride.
"More often than not," he said, "I've gone home with nothing."
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