Opposition Effect Above Ketchikan, Alaska

April 21, 2011

Ketchikan_Opposition_SG (2) 
Photographer: David K. Lynch
Summary Author: David K. Lynch

As shown above, the opposition effect is an apparent brightening of the landscape near the antisolar point -- the location of an observer’s shadow. The antisolar point (ASP) is that point on the celestial sphere exactly opposite the Sun. The opposition effect occurs on virtually all landscapes except water and is due to the absence of shadows from this direction. The opposition effect was so named because the Moon brightens dramatically when it approaches full. The full Moon occurs when the Moon is opposite the Sun as seen from Earth and is considerably brighter than the gibbous (near full) phase.

When an observer looks precisely opposite the Sun, his or her line of sight is parallel to and coincident with the Sun’s rays. Shadows can only be seen when the line of sight is not parallel to the Sun’s rays, for example when the observer looks away from the ASP. Since the average brightness of the landscape is a combination of sunlit and shaded regions, it seems brightest where there are no shadows -- at the ASP. The opposition effect usually appears slightly “warmer” or yellower than the landscape surrounding the ASP. Shadows tend to be bluish because without direct sunlight the only light that reaches them is blue skylight. Reflected blue skylight is part of the landscape, and when it's absent (as the ASP), the scene appears slightly less blue and more yellow.

In this aerial photograph of a forest in Alaska near Ketchikan, the shadow of the airplane clearly marks the location of the ASP. On average, the trees are brightest and look yellower surrounding the shadow of the plane because their shadows cannot be seen. Away from the ASP, the tree shadows are more evident and contribute their own dark component to the average landscape brightness. Photo taken on May 10, 2010.