The iPhone 5 has a pano mode. You set that mode, tap the start button and pan the camera. Finally tap the button again to stop. The camera stitches things together to create the pano image.
I made about a dozen images in this way until I got one right. Most were ruined when I moved the camera up or down while panning. That caused either a break in the rainbow or a squashed look. This one was my best attempt.
I processed the image some in PhotoForge2 on my iPad. Processing included cropping and a slight saturation boost.
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I'm going to visit my neighbors later today to see if they discovered a pot of gold somewhere in their house last night.