It was really timely to see the horizontal rainbow picture today. Just last week I photographed a "sideways rainbow". It is similar to the photo posted on apod today! The bow was facing west here in Bandon-By-The_Sea, Oregon at about 6:30pm June 4. I had to lay flat on the sidewalk and point the camera straight up. The bow is pointed west located above the ocean, on a horizontal plane, ie. parallel to the ground and sky!
Larry Baker