Superior planet can have only one X with the Sun during its synodic period, while an inferior planet can have two.
What is X
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The answer is "CONJUNCTION POINT'
Superior planets have only one conjunction point, when they are on the opposite side of the sun to the earth. Inferior planets have two conjunction points, one where the planet is on the opposite side of the sun to the earth (superior conjunction), and one where the planet is directly between the earth and the sun (inferior conjunction). Conjunction is not a good time to observe a planet.
A synodic period is the time it takes for an astro-body to return to
the same angular position from the Sun as seen from the Earth.