VLBI: Measuring the Earth with Radio Waves
🌐 Measuring the Earth from space
Question
How can radio waves from space measure the Earth to the millimetre?
Very Long Baseline Interferometry: radio telescopes thousands of kilometres apart observe the same quasar at the same instant, each recording the arrival time of the signal. The tiny differences between those times depend on the exact 3D positions of the telescopes on a curved Earth. By comparing the recordings, VLBI maps the movement of tectonic plates within millimetres, measures the Earth's rotation rate (UT1), and fixes the positions of the telescopes themselves.
The math
Timing precision sets distance precision: distance = c × Δt. A 1 ns timing error → 0.3 m; a 1 ps (picosecond) error → 0.3 mm. The baselines are up to ~10,000 km - the diameter of the Earth - which is why the millimetre precision is possible.
Why it matters
This is arguably the most elegant 'the Earth is round' experiment humans do every day: the arrival-time geometry only works if the telescopes sit on a curved surface at precisely known 3D positions. On a flat Earth the geometry of the signal arrival times cannot be solved - yet it is solved, daily, to the millimetre.