---
title: "Planetary Vision"
description: "Peter Galison, a historian of science and a member of the Event Horizon Telescope collaboration, uses the first image of a black hole to give \"planetary\" a precise technical meaning."
type: "reading-notes"
authors:
  - "Peter Galison"
published: "2025-11-21"
original_url: "https://planetaryvision.antikythera.org/"
doi: "10.1162/ANTI.5D09"
canonical_url: "https://antikythera.wiki/work/journal/planetaryvision"
md_url: "https://antikythera.wiki/md/work/journal/planetaryvision"
last_updated: "2026-08-23"
site: "Antikythera Wiki"
---

# Planetary Vision

> Independent, unofficial reading notes from Antikythera Wiki (https://antikythera.wiki). Written with AI from the published text, with citations; not by the work's author and not affiliated with Antikythera (https://antikythera.org). Read the original at the link below.

- **Authors:** Peter Galison
- **Published:** 2025-11-21
- **Kind:** Journal · Journal article
- **DOI:** https://doi.org/10.1162/ANTI.5D09
- **Original:** https://planetaryvision.antikythera.org/
- **Length:** 3.0k words
- **This page:** https://antikythera.wiki/work/journal/planetaryvision
- **Notes generated:** 2026-08-23

## Summary

[Peter Galison](https://antikythera.wiki/people/peter-galison), a historian of science and a member of the Event Horizon Telescope collaboration, uses the first image of a black hole to give "planetary" a precise technical meaning. The 2019 picture of M87* required an aperture the size of Earth, so the EHT synthesized one from radio telescopes on five continents and used the planet's rotation to fill in the gaps. Galison's claim is that this makes Earth part of the instrument rather than the object of inquiry, a shift from earlier planet-spanning science that measured the globe itself. The essay then looks forward to the Black Hole Explorer satellite, which would resolve the black hole's photon rings: concentric bands of light that have orbited the hole one, two, or three times and so preserve the universe's past as a kind of film. Designed by [Robert Gerard Pietrusko](https://antikythera.wiki/people/robert-gerard-pietrusko) and [Stewart Smith](https://antikythera.wiki/people/stewart-smith), the piece belongs to the journal's Planetary Sapience symposium and is its most literal case of a planet seeing.

## The argument

Galison starts from a resolution problem. In 1969 the best telescope, Palomar's 200-inch Hale, could resolve a stadium-sized feature on the Moon. M87* is larger than the solar system but fifty-five million light-years away, so imaging it required an aperture of 500 million inches, enough to pick out the straps on Neil Armstrong's boots. The EHT's answer was to link existing radio telescopes across the world into one virtual instrument.

He then distinguishes three senses in which this instrument was planetary. It was planetary in scale, a "skein over Earth itself". It was planetary in collaboration, with more than 200 scientists across thirty-odd countries. And it was planetary in interdependence, which he calls the subtle and important sense. The EHT did not stitch together valid partial images as a hundred photographers might tile a park. Its basic unit was the baseline, the correlation between two telescopes, and data from any single dish "signified nothing". The violin-string analogy carries the point: each baseline responds to one spatial frequency, and enough strings of enough lengths reconstruct the song. Correlation also filtered the five petabytes of observing data down to the fraction that mattered. Integration, not aggregation.

The historical section contrasts this with earlier planetary science. Eighteenth-century arc measurements near Quito tested Newton's prediction that Earth was oblate; magnetic-declination surveys mapped compass error for mariners; 1930s cosmic-ray measurements near the poles showed Earth to be a giant magnet. All were planetary in scale, but all measured Earth. The EHT used Earth: its size set the baselines, and its rotation multiplied them by turning each one relative to the black hole, the way a twirled pencil shortens and lengthens in view.

The final movement goes beyond the planet. An orbiting dish 12,000 miles up, the Black Hole Explorer, would triple the array's resolution and reveal the photon ring inside the hot gas. Because light can orbit the hole any number of times before escaping, the ring decomposes into concentric sub-rings, each carrying light from one orbit further back in time. For the largest black holes an orbit takes a week, so the rings form a film of the visible universe as seen from the hole, running backward at one frame per week. A lunar dish would show the second ring, one at the second Lagrange point the third. The EHT made Earth an instrument and produced a planetary vision; BHEX and its successors would produce a beyond-planetary one. The black hole, usually imagined as outside causal history, becomes a "memory storehouse" of everything that can be seen.

## Section by section

### Opening and the three senses of planetary

The first four headings build the triad. "Planetary in scale" lists the dishes, from the western United States, Mexico, Chile, Hawaii, Spain, and the South Pole to later additions in Greenland, France, and Korea. "Planetary in collaboration" covers the human structure, from operating dishes on 15,000-foot peaks to correlating data on supercomputers. "Planetary interdependence" introduces the baseline and the violin string and explains why correlation extracts signal: unwanted light falling on one dish but not the other is eliminated in comparison.

### Measuring Earth

The history of geodesy, magnetism, and cosmic rays establishes what was ordinary about planet-scale science (expeditions with theodolites, pendulums, and platinum rulers; the 43-kilometer equatorial bulge eventually fixed by satellites) so that the EHT's novelty shows by contrast: measurement with Earth rather than of it.

### Earth as instrument

The violin and pencil analogies are restated, and the section refers to a three-panel animation in the online piece showing baselines sweeping into view and the image filling in. Without Earth's size and rotation together there would have been no image of M87* in 2019 or of Sagittarius A* in 2022.

### Measuring a black hole

A short section on scale. M87* has six and a half billion solar masses; its light left fifty-five million years ago, shortly after the Chicxulub impact. Galison suggests zooming out from Earth's orbit through the Milky Way's hundred billion stars to the local cluster and beyond.

### Philosophy of the black hole: the photon ring

The longest section. It records the seminar-room question after April 2019 (what more could the orange donut yield?) and the BHEX answer. The photon-ring explanation is careful to say that decoding the sub-rings into images is "not remotely practical". The philosophical payoff is the two one-line paragraphs, "It is the ultimate movie" and "It is the memory of everything", and the closing speculation that the rings may be the last record of what we do.

### Black Hole Explorer

A specification block closes the piece: a 3.4-meter antenna, simultaneous 100 and 300 GHz observation, a 100 gigabit-per-second laser downlink, 20,000-kilometer altitude, six microarcsecond resolution by space-ground interferometry. The final lines, "We watch the black hole. The black hole watches us," invert the essay's opening sentence.

## Key concepts

- **Planetary vision** — seeing in which Earth's size and rotation are components of the instrument rather than its object; the essay's three-part definition of "planetary" as scale, collaboration, and interdependence. See [Planet-Sized Instrument](https://antikythera.wiki/terms/planet-sized-instrument).
- **[Integration vs. Aggregation](https://antikythera.wiki/terms/integration-vs-aggregation)** — the distinction between assembling data that is meaningless in isolation (the EHT) and stitching together independently valid pieces (the park photographers).
- **Baseline** — the correlation between two telescopes, the EHT's basic unit; each baseline samples one spatial frequency of the image.
- **Photon ring** — the thin ring of light orbiting just outside a black hole's horizon, decomposable into sub-rings of light that circled once, twice, or more times and therefore record successively earlier states of the visible universe.
- **Beyond-planetary vision** — the resolution achieved by joining orbital, lunar, or Lagrange-point dishes to the terrestrial array; Galison's name for the next epoch after planetary vision.

## Connections

- The piece is part of the [Planetary Sapience Symposium](https://antikythera.wiki/work/journal/planetarysapience) and is its most concrete instance of a planet becoming self-aware through computation. [Planetary Sapience](https://antikythera.wiki/work/media/noema-planetary-sapience) asserts that planetary computation is a "distributed sensory organ"; Galison supplies the worked example.
- Bratton cites the Event Horizon Telescope in [Antikythera](https://antikythera.wiki/work/journal/research) and [A New Philosophy Of Planetary Computation](https://antikythera.wiki/work/media/noema-a-new-philosophy-of-planetary-computation) as part of computation's genealogy from the Antikythera mechanism onward; the essay confirms the citation in detail. Agreement.
- [For a General Theory of Simulations](https://antikythera.wiki/work/journal/theoryofsim) uses the EHT image, and names Galison, as its case of simulation inferring what must be true before observation confirms it. Galison's own account emphasizes correlation rather than simulation; the two are compatible but stress different stages of the same project.
- [The History of Now](https://antikythera.wiki/work/journal/historyofnow) and [Infinity Mirror](https://antikythera.wiki/work/journal/infinitymirror) are the symposium's other treatments of deep time and planetary self-observation; the photon-ring "memory of everything" is a literal counterpart to their figurative ones.


## Terms used

- [Integration vs. Aggregation](https://antikythera.wiki/terms/integration-vs-aggregation)
- [Planet-Sized Instrument](https://antikythera.wiki/terms/planet-sized-instrument)

## Related works

- [Infinity Mirror](https://antikythera.wiki/work/journal/infinitymirror)
- [The History of Now](https://antikythera.wiki/work/journal/historyofnow)
- [Antikythera](https://antikythera.wiki/work/journal/research)
- [After Alignment](https://antikythera.wiki/work/journal/afteralignment)