Opposition effect
by Eva Seidenfaden

Opposition effect on a dirty street.
Photograph by © Eva Seidenfaden

Opposition effect seen from a plane
Opposition effect seen from a plane.
Photograph by © Eva Seidenfaden

Opposition effect and streak on a field in Norfolk
Opposition effect and streak on a field in Norfolk.
Photograph by © Les Cowley

The opposition effect is related to the heiligenschein and thus looks similar. But normally it is very much fainter, so that you have to look precisely to see it. Like in case of the heiligenschein, the observer must have the sun behind him. A low sun elevation (long shadow!) is favourable. So in winter it will be easier to observe the opposition effect.

Opposition effect on cypress hedge
Opposition effect on a completely dry cypress hedge (looking to the south).
Photograph by Eva Seidenfaden.

The opposition effect or “dry heiligenschein” (Minnaert) is in principle based upon the fact that things placed in the antisolar point hide their own shadow (shadow hiding). Thus it can also be observed on dry grass or from a plane on a forest. Here there is also a streak effect possible when the light is reflected from vertical cylinder-shaped things like tree trunks or straw stubbles. The opposition effect can be observed especially well on sand, for example on the beach, on dirty streets or sandy park alleys. Some woods have a resinous or waxy cover on their leaves, which can extremely increase the opposition effect without any water being involved.

The right photograph below shows the opposition effect and an “opposition streak”. The streak is the extension of the “dry heiligenschein”. The opposition streak forms because the short stalk stumps vertical to the observer above the antisolar point (shadow of the observer) also hide their own shadow. The diagram below on the right is made to illustrate the principle of the opposition effect: Each of the bright disks hides a part of its own shadow, so that there appears a brightening in the centre.

Opposition effect diagram
Opposition effect diagram; Copyright Eva Seidenfaden