A planet 10 light-years away is a single unresolved pixel. You can't photograph its surface. But if a civilization on that planet emits radio signals, the planet's rotation creates a Doppler signature that depends on where on the surface the transmission originates. The side rotating toward you is blueshifted; the side rotating away is redshifted. A city at the equator shifts more than one at the poles. A city at longitude 90° shifts most when it's on the limb.
Socas-Navarro (arXiv:2603.01032) shows you can invert this: given a time series of Doppler-shifted radio emissions from an unresolved planet, reconstruct a longitude-latitude map of the emitters. Rotational Doppler cartography — mapping an alien civilization's geography from a point source.
The method was tested by treating Earth as the target. Using known positions of major cities and Earth's rotation rate, the technique recovers the distribution of transmitters: dense clusters at the longitudes of Europe, East Asia, and the Americas, with gaps over the Pacific and Atlantic.
The resolution depends on the planet's rotation rate and the observation duration. A fast-rotating planet gives more Doppler separation per unit time. A longer observation covers more rotation phases. The fundamental limit is the planet's inclination — a pole-on planet shows minimal Doppler variation because its rotation is in the plane of the sky.
The conceptual inversion is clean: instead of building a bigger telescope to spatially resolve a distant surface, use the planet's own rotation to spread spatial information across the frequency domain. The planet does the aperture synthesis. You just need to listen long enough and decompose the Doppler modulation.
A civilization that transmits is a civilization that broadcasts its own map, encoded in the rotation it can't prevent.