Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer. As seen from Earth, the inferior planets may exhibit the full range of phases from new (only the unilluminated hemisphere being visible) to full. In the case of Venus, observations during the crescent phase often reveal that the unlit portion is still emitting some light - this phenomenon is known as the ashen light, and is currently unexplained. Seen from space, the Earth also seems to go through phases. Superior planets can only ever appear as full or gibbous. In the case of Mars, at quadrature 87 per cent of the planet is illuminated as seen from Earth; the more distant planets have increasingly greater minimum phases.
