Moon
The Moon is the natural satellite of Earth, an airless rocky body about 384,000 kilometers away that orbits Earth in about 27.3 days.
Definition
The Moon is the large rocky ball that orbits Earth, with no air, no surface water, and no light of its own; it shines by reflecting sunlight and is the only other world people have walked on. It is Earth's only natural satellite, about $3{,}474\ \text{km}$ across (roughly a quarter of Earth's diameter), orbiting at an average of $384{,}400\ \text{km}$ once every $27.3$ days with surface gravity about one sixth of Earth's; with essentially no atmosphere, its surface swings from about $120^\circ\text{C}$ in sunlight to $-130^\circ\text{C}$ at night and is covered with craters, dark lava plains called maria, and bright highlands. It has a small iron core, a mass of $7.35 \times 10^{22}\ \text{kg}$, and a mean density of $3.34\ \text{g/cm}^3$. The giant-impact hypothesis holds that it formed about $4.5$ billion years ago from debris ejected when a Mars-sized body struck the early Earth, which explains its low iron content and its isotopic match to Earth's mantle, and tidal locking has synchronized its rotation with its orbit.
Example
Driving to the Moon at highway speed without stopping would take about six months. Because the Moon takes $27.3$ days both to orbit Earth and to rotate once, the same hemisphere always faces us, while a full cycle of phases takes about $29.5$ days because Earth is also moving around the Sun. Apollo astronauts left retroreflectors on the surface between 1969 and 1971; laser pulses bounced off them return in about $2.5$ seconds, giving the distance to millimeter precision and revealing that the Moon recedes about $3.8\ \text{cm}$ per year.
Key Insight
The Moon always shows the same face because it spins exactly once per trip around Earth, so the far side is never visible from the ground. Its gravity is the main cause of ocean tides and it steadies Earth's axial tilt; without it, the seasons would swing far more wildly over millions of years. With no plate tectonics, water, or weather, the Moon's craters record $4$ billion years of impacts that Earth has long since erased, making it a preserved archive of solar system history.