The claim that the distance from Earth to other planets varies dynamically based on their orbital positions is a matter of everyday astronomical observation and basic orbital mechanics, requiring no citation.
The claim is a foundational, trivially true statement of celestial mechanics and everyday observation. Planets move in orbits of differing radii and speeds, meaning the distance between any two planets changes continuously over time. Following Step 0, this is classified as COMMON_KNOWLEDGE, so no papers are attached and no citation is needed.