Every second, Earth is moving around the Sun at nearly 30 km/s, the Solar System is orbiting the Milky Way, and the Milky Way itself is racing through space. We can't feel any of it directly. Yet motion leaves fingerprints in waves, allowing astronomers to measure the speeds of stars, galaxies, and even the expansion history of the universe.

The Doppler effect is one of the reasons we can study objects we may never visit.

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