Aspects and Phases of the Inferior Planets

The inferior planets exhibit the aspects and phases illustrated in the adjacent 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.

Two Kinds of Conjunction

There are two kinds of conjunction, corresponding to the Earth, Sun, and planet lying almost on the same line: inferior conjunction, where the dark side of the planet is turned toward the Earth, and superior conjunction, where the lighted side of the planet faces Earth. Here is an animation illustrating aspects and phases of the inferior planet Venus.

Mercury Near Superior Conjunction

Inferior planets are difficult to see at conjunction because they are very near the Sun. The adjacent image shows an unusual view from the orbiting SOHO solar observatory. It uses a device called a coronagraph to partially block the light of the Sun with a disk so that the region near the Sun can be studied. In this image we see the planet Mercury very near superior conjunction. The horizontal streaks on either side of Mercury are artifacts generated by the detection system.