The Michelson-Morley experiment (1887) yielded a null result for the expected aether wind of v ≈ 30 km/s (Earth's orbital velocity). Standard explanations invoke Lorentz contraction and time dilation (special relativity). Here we show that the null result is a direct consequence of the historical and dimensional denition of c itself. The speed of light c is not a universal vacuum property but the maximum areal velocity (L²/T) of a point on Earth's equator relative to the Earth's surface as the reference frame. This follows from the original denitions of metre (1/40 000 000 of Earth's meridian) and second (1/86 400 of mean solar day). The Michelson-Morley null result is therefore not surprising but inevitable: both interferometer arms are embedded in the same rotating reference volume (Earth). No aether wind can exist relative to the dening frame itself.