Into the Distance

With only a quick glance at this picture, we know that the mountain ridges are all at radically different distances from us. But how do we know this? The more distant the ridge, the brighter and bluer it appears. We are so used to seeing this -- in photos, paintings, or real landscapes -- that usually we don't even consciously notice it. This brightening, technically known as airlight, is due to sunlight being scattered by the atmosphere -- the same thing that makes the sky blue. The effect is more pronounced when looking toward the sun, and when there is dust or haze in the air. Under certain conditions, airlight can even cause distant mountain ranges to completely disappear.

This photograph appeared on NASA/USRA's Earth Science Picture of the Day site on May 17, 2005.

Date: March 19, 2005
Time: 9:19 a.m. MST
Location: Organ Pipe Cactus National Monument, Arizona
Camera: Olympus OM-1 35mm SLR
Film: Fuji Provia 100F slide
Focal length: 200 mm
Aperture: f/5.6
Exposure time: 1/500 second
Scanner: Nikon Coolscan LS-2000