Radio astronomers can get around this problem by linking together lots of smaller radio dishes into a single array, where they effectively act as one giant telescope. Each dish in the array collects light from a target object, like the glowing disk around a black hole, and converts the radio waves it receives into an electronic signal.They then used a computer called a correlator to combine all the electronic signals from the various dishes into what’s called an interference pattern. Finally, astronomers tapped a special kind of math ( Fourier transforms for the curious) to decode that pattern, showing what the target would look like in the sky if our eyes could see in radio wavelengths.With an array of radio telescopes, it’s the distance between individual dishes, rather than the diameter of a single dish, that determines the array’s resolving power. The farther apart two dishes are, the better the resolving power of that array. That’s why radio observatories like the Very Large Array in New Mexico and the Atacama Large Millimeter Array in Chile can have dishes miles apart. But even these arrays aren’t big enough to resolve the tiny radio speck in the sky that is the supermassive black hole in the Milky Way, or the one in M87.So, to really crank up the resolution, astronomers combine data from telescopes in entirely different locations, using precise atomic clocks and GPS systems to carefully time the observations and keep everything in sync.“You can put your antennas anywhere on the Earth that you like. You can put one in California, and put one in West Virginia,” says Jim Braatz, an astronomer at the National Radio Astronomy Observatory who is not part of the EHT collaboration. “With those two antennas, you can kind of simulate or mimic a telescope with the diameter of the whole country.”

The EHT has taken that to a global scale to make a telescope as big as our entire planet. Radio telescopes in Arizona, Hawaii, Mexico, Chile, Spain, and even Antarctica all observed their black hole targets in tandem. With dishes spread as far as possible, the EHT aims for nothing less than the maximum resolution a radio array can get without leaving Earth.

“In general, the more pairs of antennas that you have,” Braatz says, “the better the image you'll get at the end of the day.” That’s why this first black hole image is so impressive — and it’s just the first of many results we’ll soon see from the EHT.The research was published today in The Astrophysical Journal Letters.Editor's note: An earlier version of this story incorrectly stated that M87 was 1,000 times farther than Sgr A*.