When we observe Venus an inner planet, we see a phase.
Question: Why don’t we see a phase when we observe the outer planets?
Let’s look at the Viewing Geometries that describe what’s going on…
Here is a picture of the orbital configuration for both Inner and Outer Planets with respect to Earth. For the Inner Planet positions and phases:
Gibbous phases are phases between quarter and full phases. Greatest Elongation refers to the largest separation of the planet from the Sun in our sky, either to the East, or to the West. The inner planet Venus exhibit a complete set of phases (just like the Moon) as viewed from the earth, and can never be further from the Sun than the angles defined by greatest elongation.
(Images courtesy http://csep10.phys.utk.edu/astr161/lect/celestial/aspects.html)
Outer planets viewing geometry are different because their orbits are outside that of the Earth. They are brightest and the disk is ‘fullest’ at Opposition. However when we see them at other times, because of their distance, the phase is not perceptible. At Quadrature, when we see them on our horizon at Sunrise and Sunset, (Sun, Earth, Planet make a right triangle) .
Solar System Configurations: Best time for Viewing
The best time to view the Inner planets is Greatest Elongation. At Inferior Conjunction, they disappear in the glare of the Sun. At Superior Conjunction, they are hiding behind the Sun as shown below . The inner planet will show phases as rays of sunlight illuminate the planet positioned at the angle of Greatest Elongation from Earth.
Geometry of Venus (inner planet at Greatest Elongation) shows phase. Geometry of Jupiter at Quadrature shows full illumination on the disk.
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