If a planet disappears behind the sun because the sun is exactly between the planets, it is said to be in occultation . Superior planets can have only superior conjunctions with the sun. At superior conjunction the outer planet appears in its completely illuminated full phase, whether it is on the same side or the far side of the sun. The planet is said to be at opposition to the sun when, at superior conjunction, both it and the Earth are on the same side of the sun. Opposition is a good time to observe an outer planet with Earth-based instruments, because it is at its nearest point to the Earth and it is in its full phase.
Inferior planets can never be at opposition to the sun, from Earth's point of view.
Occultations, transits, conjunctions, and oppositions offer special opportunities for scientific observations by spacecraft. Studies of the solar corona and tests of general relativity can be done at superior conjunctions. Superior conjunctions also present challenges communicating with a spacecraft nearly behind the sun, which is overwhelmingly noisy at the same radio frequencies as those used for communications. At opposition, such radio noise is at a minimum, presenting ideal conditions for gravitational wave searches. These special opportunities and challenges are further discussed in later chapters.