By John Lindell – Updated Mar 24, 2022
Mercury, the planet closest to the Sun, is notoriously difficult to observe from Earth because it appears only near dawn or dusk. Despite this, it holds key clues about planetary dynamics. Scientists once believed that Mercury’s day—its rotation on its axis—matched its year, but modern observations have corrected that view.
Early theories posited that Mercury was tidally locked to the Sun, meaning one hemisphere would always face our star. This would imply a 88‑day rotation period equal to its orbital period. In 1965, radar ranging from Earth disproved this assumption, revealing that Mercury rotates in 58.65 Earth days—exactly two‑thirds of an orbital year. Astronomers describe this ratio as a 3:2 spin‑orbit resonance: for every two orbits, Mercury completes three rotations.
A solar day—time between successive solar noons—is considerably longer on Mercury. Because of its rapid orbital motion and slow spin, a solar day lasts 175.85 Earth days, roughly two Mercurian years. This long day results from the planet’s elliptical orbit, where its speed varies significantly between perihelion (closest to the Sun) and aphelion (farthest).
Observers on Mercury would witness the Sun rise in the east, move westward, pause, then briefly reverse before continuing east to west. This apparent reversal occurs because Mercury’s orbital speed exceeds its rotational speed near perihelion, temporarily “overrunning” the Sun in the sky.
Mercury’s lack of a substantial atmosphere means the day side can reach 840 °F (450 °C), while the night side plummets to –300 °F (–180 °C). The 58.65‑day rotation allows each surface point to experience both extremes over a Mercurian year.
For more details, visit NASA Solar System Exploration.
