Phases of a planet

In astronomy, the phases of a planet or natural satellite refer to the various appearances of the illuminated part of the body as seen by a distant observer.

The simplest example is of course the phases of the Moon but the phases of the planet Venus, discovered by Galileo in 1610, are also well known. This was an argument in favour of the Ptolemy's heliocentric. In the case of Mercury, the Moon and Venus, crescent phases can be seen from the Earth because these are inner planets, but this is not the case for the outer planets. Thus Mars is always seen full or gibbous.