The first image of a black hole: how they did it
A telescope the size of the planet, and data too big to send over the internet.
2 min read · Published 27 August 2026
The answer
The Event Horizon Telescope linked radio observatories across the globe into one instrument using very long baseline interferometry, giving an effective aperture the size of Earth. Each site recorded data with atomic-clock timestamps onto physical drives, which were flown to central facilities and correlated. The M87 image came out in 2019, the Sagittarius A* image in 2022.
Why an Earth-sized telescope was necessary
Resolution depends on aperture relative to wavelength, and the target, the shadow of a black hole, is comparable to an orange on the Moon. No single dish could do it. Very long baseline interferometry combines telescopes observing at once and gives the resolution of a dish as wide as the distance between them. Spread across continents, that is roughly Earth's diameter.
The steps
- Simultaneous observation, needing clear weather at every site at once.
- Atomic clocks, so signals could be aligned to fractions of a nanosecond afterwards.
- Correlation, reconstructing what a single Earth-sized dish would have measured.
- Image reconstruction from a sparse sample of the aperture.
- Independent verification by four teams kept apart until each had a result.
The blind analysis is the part I would teach. Four teams, no peeking, the same ring.
Why the blind analysis counted
Reconstructing an image from sparse data involves choices, and the expected answer, a bright ring, was known from theory. That is a textbook setup for producing what you expect. The teams used different algorithms and were forbidden to talk until each had converged. All four produced a ring of similar size.
That is exactly the discipline astrology lacks. The researchers knew what they hoped to see and built a procedure to defeat the bias. The result is credible because of the precaution, the principle in is astrology a science.
What the image shows
Not the black hole, which emits nothing. The ring is superheated material in the accretion flow; the dark centre is the shadow, about two and a half times larger than the event horizon because gravity bends light around the hole. Its size matched general relativity's prediction. M87's black hole is around 6.5 billion solar masses; Sagittarius A* is roughly four million.
How was the first black hole image taken?
By linking radio telescopes across continents into an Earth-sized aperture, with data flown on drives to correlation facilities.
What does the black hole image show?
The bright ring is superheated material in the accretion flow. The dark centre is the shadow, about two and a half times the event horizon.
Why were four separate teams used?
The expected result was known in advance, so four teams using different algorithms were kept apart to stop them producing it unconsciously.
Natalie Hasselbank