Seeing Over the Horizon

March 23, 2012

Seeing over the horizon 2

: Alain Origne 
Summary Author: Alain Origne

March 2012 Viewer's Choice The photo above showing a gorgeous magenta sunset, the city of Marseille, France (foreground), the Gulf of Lion (mid-ground) and the Pyrenees Mountains (background) was captured about 40 minutes after sunset on January 22, 2012. The Pic du Canigou is the portion of the Pyrenees visible on the distant horizon -- some 165 mi (265 km) from Marseille. On a planet without an atmosphere, the Pic du Canigou, with peaks reaching around 9,130 ft (2,785 m), couldn’t be seen from Marseille. On Earth, however, the natural gradient of air pressure and density decreases from sea level to the top of the atmosphere. This causes far-away objects such as the mountain peaks seen here to appear as if lifted above the horizon. This phenomenon of atmospheric refraction makes extraterrestrial objects such as the Sun and Moon appear above the horizon for a few minutes after they've set and is well-known to astronomers as astronomical refraction. What’s being observed is looming -- an exaggeration of normal refraction, produced by a steeper than usual decrease in density with height. This should not be confused with a mirage since there’s no image inversion. Note that the elevated position of the photographer plays a greater role in seeing the distant mountains than does atmospheric refraction.

Still, it's rare to see this distant range because of the lack of contrast between the mountains and the background sky. Mid-level clouds and pollution usually blocks the view. Fortunately, fair weather prevailed over France at the time the picture was taken, and the anticyclonic conditions resulted in an extraordinary clear sky over the Gulf of Lion. Such conditions are likely responsible for the unusually large refraction as well as the unusually good visibility. The low relative humidity of the clear air helped to shrink hygroscopic aerosols, thus improving visibility.

Photo Details: Camera: NIKON D7000 on tripod; Nikon ED 400mm lens; Focal Length: 500mm (35mm equivalent: 750mm); Aperture: f//5.6 (manual focus); Exposure Time: 0.010 s (1/100); ISO equiv: 1000; Software: Adobe Photoshop CS5 Macintosh.