Phases of the Planets
The planets, as viewed in the sky, exhibit characteristic aspects
and phases. "Aspects" refers to the location of the planet
with respect to our overhead sky reference (objects on the celestial sphere);
"phases" refers to the fact that the planets, through a telescope,
exhibit phases (differing amounts of lighted hemispheres as viewed from the
earth). The terminology associated with these aspects and phases is different,
depending on whether we refer to an inferior planet or a superior planet.
Aspects and Phases of the Inferior Planets
The inferior planets exhibit the aspects and phases illustrated in the following
diagram.
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. Thus, we see that the inferior planets 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.
Aspects and Phases of the Superior Planets
The aspects and phases of the superior planets differ from those of the inferior
planets because of geometry: their orbits are outside that of the Earth. These
aspects and phases are indicated in the following diagram.
When a superior planet is at quadrature, it is on our celestial meridian at
sunrise or sunset. Comparing with the preceding diagram for the inferior
planets, we notice two basic differences: (1) The superior planets do not
exhibit a full range of phases; they are always gibbous or full. (2) The
superior planets can be located at any distance East or West of the Sun in our
sky, unlike the inferior planets where there is a limiting angle away from the
Sun (greatest elongation).
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