OPOD - Opposition Streak

Opposition Effect Streak, Brazil

Linear opposition glow observed by Fabiano Diniz (atoptics & astronomy).

" During the final approach of a flight from Curitiba to Porto Alegre (both cities are state capitals in southern Brazil) I saw an opposition effect that caught my attention. It appeared as a bright line rather than a circular spot. As I was sitting in the last row, the plane shadow appears a little bit ahead of the line center." �Fabiano Diniz, shown with permission.

Shadow hiding gives apparent glows opposite the sun (at the antisolar point, ASP). Soil and rock minerals also backscattering light. When the ground is bedewed a heiligenschein adds its own brightness.

The glow is usually roughly circular. This bright streak implies an asymmetry in the shadow casting objects.

Imagine an extreme, a large array of parallel horizontal rods set a short distance above the ground. The rods form an equally spaced array of dark shadows.

Now look downwards on the rods in a direction along the sun�s rays (i.e. towards the ASP). Rods very near the ASP have their shadows neatly tucked in and hidden beneath them. Further away the shadow of each rod appears shifted towards the antisolar point and thus increasingly visible.

Areas away from the ASP have more shadows and so on average appear darker. But, with our extreme array, the effect only occurs along one angular direction. We see a bright linear streak instead of a circular bright area.

Crops so carefully arrayed would be unusual indeed. However, some degree of alignment due to wind or planting is sufficient to produce a bright opposition streak. See another example here.

These subtle effects are best seen when you are moving. They can be seen when walking through fields as well as from airplanes.