The Event Horizon Telescope Published the First Image of a Black Hole on 10 April 2019 and Our Galaxy's on 12 May 2022, Using Eight Radio Observatories Synchronised by Atomic Clocks

On 10 April 2019 the Event Horizon Telescope collaboration published the first image of a black hole: a lopsided ring of radio light around a dark centre, at the heart of the galaxy Messier 87. ESO’s release puts that black hole 55 million light-years away, with a mass 6.5 billion times the Sun’s. Three years later, on 12 May 2022, the same team showed the black hole at the centre of our own galaxy, Sagittarius A*, about 27,000 light-years away and roughly four million Suns in mass.

Neither picture is a photograph in the everyday sense. Both were assembled from radio signals recorded at observatories from Chile to Antarctica, and the recordings travelled by courier.

What the first picture shows

The ring in the 2019 image has a diameter of 42 microarcseconds, give or take 3, according to the collaboration’s first paper in The Astrophysical Journal Letters. A microarcsecond is a millionth of an arcsecond, which is itself 1/3,600 of a degree. The ring surrounds a darker patch, and the brightness ratio between ring and centre is about 10 to 1.

Heino Falcke explained the logic in ESO’s release: “we expect a black hole to create a dark region similar to a shadow.” The paper describes a dark shadow caused by gravity bending light and capturing it at the event horizon. The ring is brighter on one side, which the paper attributes to relativistic beaming from plasma rotating near the speed of light.

The mass came from comparing the ring against computer simulations of gas falling into a spinning black hole. The result was 6.5 billion Suns, give or take 0.7 billion. The image itself used data from 11 April 2017, one of four nights of observing. Sheperd Doeleman said in ESO’s release: “We have taken the first picture of a black hole.”

How eight dishes behave as one telescope

A dish as wide as the Earth is impossible, so the team did the next best thing. The method is called very long baseline interferometry. Telescopes far apart record the same patch of sky at the same moment, and a computer later combines the recordings pair by pair. The widest separation between any two dishes sets how fine a detail the combination can resolve.

In April 2017 the array had eight stations at six locations: ALMA and APEX in Chile, the Large Millimeter Telescope in Mexico, the IRAM 30-metre telescope in Spain, the Submillimeter Telescope in Arizona, the James Clerk Maxwell Telescope and the Submillimeter Array in Hawaii, and the South Pole Telescope. Baselines ran from 160 metres to 10,700 kilometres, and the paper says this builds a virtual telescope spanning nearly the full diameter of the Earth. NSF’s release gives the angular resolution as 20 microarcseconds, at a wavelength of 1.3 millimetres.

The hard part is timing. Each station carries a hydrogen maser, an atomic clock, and ESO’s release says these let the telescopes synchronise their recordings. GPS aligned the recordings to within tens of nanoseconds, the paper says, and the masers keep the signal coherent over the roughly 10 seconds that atmospheric conditions allowed. The 2019 team observed M87 in scans of three to seven minutes, alternating with a bright quasar called 3C 279 for calibration.

A petabyte per telescope, flown by courier

Each station recorded two frequency bands at a combined 32 gigabits per second, according to the first paper. MIT News reported that each telescope took in about one petabyte over the campaign, and that after each run researchers at each station shipped the hard drives by air to MIT Haystack Observatory in Massachusetts and the Max Planck Institute for Radio Astronomy in Bonn. “Air transport was much faster than transmitting the data electronically,” the MIT piece says.

At the two sites a specialised supercomputer called a correlator compared every possible pair of telescopes. A TechTarget report put the data behind the final image at about 3.5 petabytes, a different total from MIT’s per-telescope figure, and noted that the South Pole site is closed to flights between February and November. The calibration paper describes three independent software pipelines for cleaning the signal. Publication came two years after the observing nights.

The Milky Way’s turn

Sagittarius A* is far closer than M87* and harder to image. The ESO release for 12 May 2022 says it appears about as large in the sky as a doughnut on the Moon. NSF’s explainer says fast-moving gas surrounds it, so the scene keeps changing while the telescopes watch. Chi-kwan Chan called it “a bit like trying to take a clear picture of a puppy quickly chasing its tail.”

The team used observations from several nights in 2017, compiled a library of simulated black holes to compare against, and averaged many candidate images into the final one. The ring measures 51.8 microarcseconds across, give or take 2.3, per the collaboration’s paper, and the mass is about four million Suns. ESO says the work took five years and involved more than 300 researchers at 80 institutes. Geoffrey Bower said the team was “stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity.”

The distance is not given identically everywhere. ESO’s release says about 27,000 light-years, and NSF’s explainer says about 25,640. The mass of the object was first inferred from stars orbiting it, work covered in our piece on the Nobel Prizes of 2016 to 2026, where Reinhard Genzel and Andrea Ghez appear for that tracking.

Where India appears in the credits

ESO’s 2019 release says the East Asian Observatory partner in the project represents many regions of Asia, and its list includes India. The release does not say what India’s role was.

We also searched the full affiliation lists of the first M87 paper and the first Sagittarius A* paper for Indian institutions. We found none. The Tata Institute of Fundamental Research, the Indian Institute of Astrophysics, ARIES in Nainital and the Raman Research Institute do not appear in either. We could not open the collaboration’s own membership page, and we did not check the later papers, so a newer Indian affiliation is possible.

One link is historical rather than institutional. D. T. Emerson of the US National Radio Astronomy Observatory records that J. C. Bose worked in Calcutta in the 1890s at wavelengths as short as 5 millimetres. The telescope’s 1.3 millimetres is shorter, and no source we opened claims the EHT drew on his work. India’s own black-hole instrument under construction is a gravitational-wave detector, covered in our article on LIGO-India at Hingoli. What such detectors have recorded so far is in We Can Now Hear Black Holes Collide.

What the same black holes have shown since

On 24 March 2021 the team released M87* in polarised light, which maps magnetic fields. ESO’s release says only models with strongly magnetised gas fit the data. The release names eleven facilities, among them the Greenland Telescope, NOEMA in France and Kitt Peak in Arizona.

On 18 January 2024 the collaboration published 2018 data. The ring was the same size as in 2017, which general relativity predicts, while the brightest patch had moved about 30 degrees, to roughly the 5 o’clock position, the Max Planck institute’s release says. On 27 March 2024 it released Sagittarius A* in polarised light, showing organised, twisted magnetic fields, a pattern ESO says resembles M87*‘s.

On 16 September 2025 came a result the team had not expected. Across observations from 2017, 2018 and 2021, the magnetic pattern around M87* flipped direction. The authors say the cause could be the black hole’s own magnetic structure, matter along the line of sight twisting the polarisation, or both. The same paper reported the first faint jet emission in EHT data, and the Max Planck institute’s release says more than 400 researchers belong to the collaboration.

On 28 January 2026, NRAO reported a compact source about 0.09 light-years from M87*‘s black hole, at the expected position of the base of its jet, found in 2021 data.