Please rotate your screen

As you can see, the "closest" planet to Earth, or most planets, is not necessarily the one usually shown beside it in grade school textbooks. This is because planets don't orbit the sun at the same rate, so a lot of the time planets are actually on the opposite side of the sun from each other. Because of this, it turns out that on average Mercury is the closest planet to Earth, not Venus.

This visualization was inspired by CGPGrey's excellent video on which planet is closest to Earth. I should make a brief note here that the planets are not exactly co-planar, so this further affects distance.

Obviously, planet size is not to scale with distance, otherwise the planets would be invisible. The planet sizes are correct relative to each other. The sun is, of course, much smaller than in reality.

The effect is covered in much more detail in the source paper.