October 31, 2013
Photographer: Hans Juergen Heyen
Summary Author: Hans Juergen Heyen; Andy Young
This photo shows a view of a portion of a container ship (black rectangles) off the German coast of the Wadden Sea, approximately 7-9 mi (12-15 km) away. In the background is the port of Wilhelmshaven, approximately 20 mi (32 km) distant. Wilhelmshaven's power plant with its tall chimneys as well as the port's cranes can easily be seen. Note the inferior mirages of the cranes and chimney. In particular, notice that the inverted images are slightly reduced in size from their erect counterparts. This is best seen in the oblique lines of the cranes. The effect increases with the height of the eye (or camera) above sea level -- estimated to be about 10 ft (3 m) here.
Partly because of the curvature of the Earth, the lower part of the vessel and the lower parts of the cranes are hidden. However, much of this occlusion is due to the height of the vanishing line where the image folds over from erect to inverted. It seems anything lower than about 32 ft (10 m) is beneath the viewer's apparent horizon. At the docks in the background only structures higher than about 82 ft (25 m) can be seen. Note that the height of this boundary increases with distance beyond the apparent horizon.
There appear to be breaks in the image near the apparent horizon; this may result from variations in the local height of the waves. Near shore, where the waves are extremely low, the apparent horizon is strongly depressed because we see deeply into the surface layer, close to the water surface. A bit farther away, the water is deeper and the waves are higher interrupting our view of the mirage. There, we see the water itself -- or rather, the sky reflected in the shoreward-facing facets of the waves. Finally, a bit higher in the picture, our line of sight is again oblique enough to the water surface to allow the inverted image of the mirage to be seen again.
The drawing at right helps to illustrate some of what's being observed here. The photo at right shows the container ship in a different position. Photos taken on September 4, 2013.
Photo details: Top - Camera Model: Canon EOS 600D; Lens: EF300mm f/4L IS USM +1.4x; Focal Length: 420mm; Aperture: f/9.0; Exposure Time: 0.0016 s (1/640); ISO equiv: 100. Insert - Same except: Exposure Time: 0.0013 s (1/800); ISO equiv: 125.