VLBI
VLBI is a technique in radio astronomy where signals from distant radio telescopes are combined taking into account the delay in wave arrival. Resolving power (ability to distinguish fine details) depends on the distance between antennas, not their size. With baselines of thousands of kilometers, an angular resolution of microarcseconds is achieved — one can see a lighter on the Moon. To combat atmospheric distortions, the method of closure phases is used, combining signals from three or more antennas.
History
The idea of combining widely separated antennas emerged in the 1960s. The first successful observations were made in 1967 by groups of scientists in the USA and Canada.
How it works
Imagine two people looking at the same star and precisely noting when its light reaches each of them. If the distance between them is known, from the time difference one can reconstruct details inaccessible to a single observer. VLBI radio telescopes work the same way, only they record radio waves and synchronize by atomic clocks.
💡 The largest virtual telescope — the Event Horizon Telescope — combined stations from Antarctica to Europe to show the silhouette of a black hole in 2019.