Planetary phase
Encyclopedia
Planetary phase is the term used to describe the appearance of the illuminated section of a planet
. Like lunar phase
s, 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.
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
. Like lunar phase
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
s, the planetary phase depends on the relative position of the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, the planet and the observer.
As seen from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, 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
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, observations during the crescent
Crescent
In art and symbolism, a crescent is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points .In astronomy, a crescent...
phase often reveal that the unlit portion is still emitting some light - this phenomenon is known as the ashen light
Ashen light
Ashen light is a subtle glow that is seen from the night side of the planet Venus. This ashen light is said to be very similar to Earthshine on our moon, but not as distinguished in brightness. It was first sighted by the astronomer Giovanni Battista Riccioli on January 9, 1643, and has been...
, 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
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, at quadrature
Quadrature (astronomy)
In astronomy, quadrature is that aspect of a heavenly body in which it makes a right angle with the direction of the Sun. It is applied especially to the apparent position of a superior planet, or of the Moon at first and last quarters....
87 per cent of the planet is illuminated as seen from Earth; the more distant planets have increasingly greater minimum phases.