But this is not so. Mercury turns slowly on its axis, taking 58.6 Earth days to make one revolution. Its year—the time it takes to complete one orbit around the Sun—is 87.97 Earth days.

This leads to a strange situation. Mercury’s rotation rate of nearly fifty-nine Earth days is precisely two-thirds of the planet’s year. A person standing on the surface of the planet would see the huge Sun move from east to west across the dark airless sky, but it would slow down noticeably, then reverse its course and head back east for a while before resuming its westerly motion. At some locations on Mercury, the Sun rises briefly, then dips down below the horizon before finally rising again for the rest of the Mercurian day. After sunset the Sun peeks back up above the horizon before setting for the length of the night.

Counting the Mercurian day from the time the Sun appears directly overhead (local noon) to the next time it reaches that point, it measures one hundred seventy-six Earth days. From the standpoint of noon-to-noon, then, the Mercurian day is twice as long as its year!

The Sun looms large in Mercury’s sky. It appears twice as big as we see it from Earth when Mercury is at the farthest point from the Sun in its lopsided orbit and three times larger at the closest point. And it is hot. Daytime temperatures soar to more than 400° Celsius, four times higher than the boiling point of water, hot enough to melt zinc. At night the temperature drops to –135° C because there is no atmosphere to retain the day’s heat; it radiates away into space.

With a diameter of only 4,879 kilometers, Mercury is the smallest planet in the solar system except for distant—most Pluto. Jupiter and Saturn have moons that are larger than Mercury. The planet is slightly more than one-third larger than Earth’s own Moon.