Mercury keeps the same face permanently pointed toward the Sun.

Not a fact anymore

Mercury keeps the same face permanently pointed toward the Sun.

What we know now

Mercury rotates three times for every two orbits around the Sun. It is in a 3:2 spin-orbit resonance, so every part of the planet experiences daylight over time.

Why it changed

Radar observations in 1965 measured a rotation period of about 59 Earth days, invalidating the accepted 88-day synchronous-rotation model.

Status: Corrected
Category: Astronomy
Accepted approximately: Late 19th century to 1965
Changed approximately: 1965–1966

The older result arose because Mercury is difficult to observe from Earth. At the times when it was best positioned for telescopic observation, its actual 3:2 resonance caused nearly the same region to be visible, creating a misleading pattern.

This entry uses Corrected because a specific measured property, Mercury’s rotation period, was revised after a better observational method became available.

Evidence

Sources and what they establish

Historical context

The Planet Mercury NASA Technical Reports Server
States that Mercury was believed to keep the same hemisphere facing the Sun and that radar analysis invalidated this view.

Primary research

A Radar Determination of the Rotation of the Planet Mercury Nature / NASA ADS
The 1965 radar observations that measured Mercury's rotation at approximately 59 days.

Current evidence

A Solar System Survey of Forced Librations in Longitude NASA Technical Reports Server
Describes Mercury's 3:2 spin-orbit resonance: three rotations for every two revolutions around the Sun.