Earth & Cosmos / Space & Astronomy

Very-Long-Baseline Interferometry

In April 1967 Canadian astronomers link two radio telescopes 3,074 km apart, each with its own atomic clock and tape recorder: a telescope as wide as a continent.

The signals recorded at the Dominion Radio Astrophysical Observatory in British Columbia and the Algonquin Radio Observatory in Ontario were played together afterwards. The hard part was building recorders and clocks stable enough to keep the two signals coherent for many minutes; the team saved money with surplus video recorders bought from the Canadian Broadcasting Corporation. VLBI later measured plate motions to millimetres, imaged active galactic nuclei, tracked spacecraft, and with telescopes on several continents produced the first black hole image (2019).