Mercury turns exactly three times on its axis for every two orbits of the Sun, so a single day there lasts two of its years, and from the right spot on the surface you would watch the Sun rise, stop, reverse, and rise a second time.

Mercury turns exactly three times on its axis for every two orbits of the Sun, so a single day there lasts two of its years, and from the right spot on the surface you would watch the Sun rise, stop, reverse, and rise a second time.

Imagine standing on Mercury before dawn. There is almost no atmosphere to soften the darkness. The ground is a hard field of craters and broken rock. Above it, the sky remains black even as the eastern horizon begins to glow. Then the Sun appears. From Mercury it looks more than three times as large in the sky as it does from Earth, and the light can be seven times brighter. Yet it does not climb with the familiar steadiness of an earthly sunris…