Why a Day on Mercury Is Longer Than a Year

One rotation on its axis takes almost 59 Earth days. One orbit around the Sun takes about 88 days. As the planet rotates, it moves noticeably along its orbit, so the Sun returns to the same place in the sky much later.

This creates a strange fact that's hard to wrap your head around. A solar day there lasts two years. A person standing on the surface would celebrate New Year's twice without seeing a sunset even once.

The planet is close to the Sun, and tidal forces slowed its rotation over billions of years. Usually, this slowing ends with a body always keeping the same side facing its star, as the Moon does toward Earth.

Here, things turned out differently. The orbit is highly elongated, making a different ratio stable: three rotations on its axis for every two orbits around the Sun. This combination is stable too, but it occurs less often.

The mistake persisted for almost a century, and for an understandable reason. Observing the surface through a telescope is difficult: the planet is always close to the Sun in the sky. Astronomers saw similar features at similar times and reached a reasonable but incorrect conclusion.

Because of its slow rotation and lack of an atmosphere, the temperature difference there is enormous: the sunlit side is hotter than an oven, while the night side is colder than anything found on Earth. At the same time, ice appears to survive in permanently shadowed craters near the poles.