The Moon keeps presenting the same face to Earth because its rotation period is the same as its orbital period around Earth. In other words, the Moon is tidally locked to our planet: it takes about 27.3 days to complete one rotation on its axis and the same 27.3 days to orbit Earth. As a result, the same hemisphere is always geared toward us. Key points to understand:
- Tidal locking: Over long timescales, Earth's gravity stretched and slowed the Moon's rotation until it synchronized with its orbit. This is a common fate for moons orbiting larger bodies.
- Apparent “slight wobble” (libration): We can see a bit more than half the Moon over time due to the ellipticity of the orbit and a slight tilt of the Moon's axis. This libration lets us glimpse slightly beyond the average edge, but not the entire far side.
- Far side vs near side: The “far side” is the hemisphere never visible from Earth, not the dark side. Both the near and far sides receive sunlight; the dark side is only relative to the Sun’s illumination at any moment.
If you’d like, I can add a simple analogy or a couple of diagrams to illustrate orbital resonance and libration without getting too technical.
