::::::This seems to be a good example of how simpleminded analyses of concepts that work well in our daily lives do not work very well in different contexts, specifically the idea of being "closest".  Planets are not static entities, and over time different planets will be closer to other planets than at other times due to the nature of objects orbiting a central body.  A more meaningful understanding of "closer" might be "more accessible" or "easy to get to", in which case Mercury is NEVER the closest planet to Earth as the required delta-V will always be higher than that needed to get to Mars or Venus.  Calling Mercury the "closest planet" to Earth is really quite misleading here, as it would imply that it trumps the two planets in the nearest orbits in terms of close proximity, and could, for example, leave the layperson wondering why it took so long to actually send a probe to orbit Mercury if it's so "close".  Similarly, other exploratory techniques, like bouncing RADAR reflections from terrestrial sites off the surface of another planet, were first done on Venus and Mars because they get much closer to Earth than Mercury does.  As was pointed out above, by this sort of thinking, the Sun is in fact the closest body to any of the planets, in a "time-averaged" sense; this whole concept of time-averaged proximity is kind of weird, since it's basically saying something like "Often, Earth is closest to Mercury, except when another planet is closer", which is frankly rather tautological.