Even planetary bodies can be observed in 'opposition'
At first sight, the animation seems unremarkable. The 26-kilometre-long body can be seen from different angles, so that the moon first becomes lighter and then darker again. For scientists, however, such images acquired at different phase angles (as the angle between the Sun, the object, and the observer is known, see Image) are of particular interest. Because of the different shadows that are cast and the varying amounts of reflected sunlight at different angles, it is possible to draw valuable conclusions about the surface material properties, particularly its roughness and porosity.
Of particular interest is observation at a phase angle of exactly zero degrees; that is, when the Sun is directly behind the observer (Part B of Image). At this point, the Sun shines vertically on to the surface and all shadows disappear. This leads to an increased surface brightness at the centre of the image. If, for example, the observer’s shadow is captured in the image, a kind of halo around is created around it in the centre of the image. As this only occurs if the Sun, as the light source, is exactly in opposition to the imaged object, this phenomenon is referred to as the opposition effect. This can even be observed from Earth to some extent during a full moon, as not only is a large area of the Moon illuminated, but the Sun, Earth and Moon are in a relatively straight line (that is, in opposition), thus creating no visible shadows on the Moon’s surface. This is why a full moon appears up to 10 times brighter than a half moon. Many people may already have noticed that when looking down at the ground from an aircraft, a light halo can be observed around the shadow of the aircraft. Other well-known examples include photographs taken on the surface of the Moon. When the Apollo astronauts taking the photographs had the Sun directly behind them, a halo would appear around the shadow of their head. All of these phenomena are due to the opposition effect. Conclusions about the nature of the surface material can be derived from the strength of this effect.