How space works
Tidal locking, tidal heating, and changing orbits
Gravity pulls more strongly on the near side of a world than on its far side. Over time, that difference can change its spin, warm its interior, and reshape its orbit.
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A tide is a difference across a body
The near and far sides of a moon do not feel exactly the same gravitational pull from its planet. The resulting differential force can stretch the moon. Rock responds as well as water, although the deformation and energy loss depend on the material and its internal structure.
As a body rotates, its deformation may not point precisely toward the object raising the tide. The resulting torque transfers angular momentum and dissipates energy. Over long times, spin and orbit can evolve together. Earth's ocean tides are influenced by coastlines, water depth, and resonance, making actual tidal patterns more complicated than two perfect bulges moving around an ideal globe.
The Moon still rotates
The Moon takes roughly one rotation in the same time it takes to orbit Earth relative to distant stars. This synchronous rotation keeps almost the same hemisphere facing Earth. From above the lunar orbital plane, the Moon turns once during every circuit. Without that turn, observers on Earth would eventually see all sides.
Small changes in viewing geometry allow us to see somewhat more than half the lunar surface over time, an effect called libration. The far side is not permanently dark: it receives sunlight during its day just as the near side does. Far side and dark side therefore describe different ideas and should not be used interchangeably.
Tidal heating needs changing deformation
A moon in an eccentric orbit experiences changing gravitational stresses. Repeated flexing can release heat inside it. Neighboring moons can help maintain eccentricity through gravitational interactions, keeping that energy source active. Io's intense volcanism is a striking result, while tidal energy also contributes to the story of icy ocean worlds.
Being tidally locked does not mean the body has stopped evolving. It can still deform, orbit, precess, exchange angular momentum, and lose internal heat. The rate depends strongly on orbital distance, eccentricity, and how efficiently material dissipates energy. Similar-looking moons can have very different geological activity.
Close exoplanets require a careful inference
Tides can influence the rotation of planets on tight orbits, especially around low-mass stars. Synchronous rotation is a common expectation in some cases, but it is usually inferred from tidal models rather than observed directly. Mercury demonstrates that a spin–orbit relationship need not be one-to-one: it turns three times during two solar orbits.
A synchronously rotating planet could have persistent day and night hemispheres. Whether temperatures become extreme depends on atmospheric and oceanic heat transport as well as incoming starlight. A short orbital period does not reveal the weather. Profiles should keep the measured year separate from assumptions about the day, clouds, or surface climate.