Closest Planet to Earth?

What is the closet planet to Earth? At first glance this may seem an easy question. Venus, at closest approach is only about a third of an astronomical unit away. Mars is only about half of an astronomical unit away, but neither is the closest planet to Earth most of the time. That honor goes to Mercury, yes Mercury! But how is that possible? At closest approach Mercury is farther from Earth, at about two-thirds of an astronomical unit, than both Venus and Mercury!

The above chart, found in a tweet by Brian Brettschneider, plots the distance of Mercury, Venus, and Mars from Earth. Given that the four planets are not always on the same side of the Sun, their distance to Earth varies. Because Mercury has a quicker orbit around the Sun, it catches up to Earth faster than Venus catches up to Earth and even faster than Earth catches up to Mars in its orbit. Therefore, Mercury actually spends more time closer to Earth than Venus and Mars do! Just under half of the time, Mercury is closer, followed by Venus about a third of the time and Mars at about sixth of the time.

This makes sense, but it isn't immediately obvious when at closest approach, Mercury comes in third to Mars and Venus. Interesting!