The different observatories that make up the EHT are all can make different radiofrequency observations of different objects in space. In this instance, they were all aligned to look at the radiation emitted by each black hole's event horizon, working in concert to provide the sort of extreme optical resolution necessary to image something so small and so far away. Daniel Marrone, an astronomer at the University of Arizona and a member of the EHT team, told the audience at Wednesday's press conference that while the black hole is 6.5 billion times the mass of the sun, the event horizon is basically just one-and-a-half light-days across. For reference, M87 itself, already an impressive body to image at 55 million light-years away, is 120 light-years in diameter. Doeleman calls the feat the "equivalent of being able to read the date on a quarter in LA when we're standing here in Washington, D.C."