Lashed to the Sun

In the days before space-based telescopes and probes, earthbound astronomers did the best they could to gauge the rotation of Mercury. The nineteenth-century astronomer Giovanni Schiaparelli observed the movement of what few, indistinct surface features he could discern and concluded that, unlike any other planet’s, Mercury’s rotation was synchronous with its orbit around the sun.
Synchronous orbit means that Mercury always keeps one face toward the sun, and the other away from it, much as the moon always presents the same face to the earth. Technology marches on. In 1965, by means of radar imaging, unavailable to Schiaparelli in the nineteenth century, astronomers discovered that Mercury’s rotation period was not 88 days, but only 59 days. This discovery implied that Mercury’s rotation was not precisely synchronous with its orbit, but that it rotated three times around its axis every two orbits of the sun.