Terence Tao on the cosmic distance ladder
Why Hacker News recommends it
HN recommends it as an excellent explanation of how we know cosmic distances.
What it is
Terence Tao and Grant Sanderson of 3Blue1Brown walk through the cosmic distance ladder, showing how humans measured distances from the Earth to the Moon, Sun, planets and beyond. It includes animations and the history behind each step.
Recommended by 2 HN commenters across 2 different months, 2025 to 2026.
What Hacker News said
Strong rec GuB-42 · 2025-02 · on “Undergraduate shows that searches within hash tables can be much faster”
About geocentrism vs heliocentrism, 3blue1brown has recently released a video [1] that talks about about it. It is about the cosmic distance ladder, but geocentrism is mentioned, and it makes a lot of sense in context. To summarize: heliocentrism was known to the ancient Greeks, who realized the Sun was much bigger than the Earth, so it would seem logical to have the Earth go around the sun. But the counterargument was that if the Earth goes around the Sun, the stars should move relative to each other during the year, because of parallax, and they didn't have instruments that were precise enough to see it, so they assumed that they didn't. Copernicus major contribution wasn't heliocentrism,
Strong rec adolph · 2026-02 · on “Medieval Monks Wrote over Ancient Star Catalog – Particle Accel Reveals Original”
This is so true. There is an awesome Terry Tao / 3blue1brown collaboration that explicates the epistemological basis: Terence Tao on the cosmic distance ladder Pt 1: [this video] Pt 2: [LINK] Commentary and Corrections: [LINK]
Cites dfabulich · 2025-07 · on “Slow”
Here's another good example of a series of slow experiments: the cosmic distance ladder. [this video] [LINK] You can compute the distance to the moon if you know the radius of the earth by looking at how long lunar eclipses take, data gathered over years of observations. Eratosthenes computed the radius of the earth by clever trigonometry in ancient times, and Aristarchus computed that a 3.5-hour lunar eclipse indicates that the moon is ~61 earth radii away. Once you have the distance to the moon, you can compute the size of the moon by measuring how long it takes the moon to rise. It takes about two minutes, and so the radius of the moon is about 0.0002 of the distance to the moon. By cosmic coi
Cites epistasis · 2026-08 · on “Scientists release biggest 2D map of the universe”
They do this mostly with spectroscopy, breaking out the light by frequency. The longer the light has been travelling, the more frequency shifts into redder frequencies, because the universe is continually expanding. But those distances are all built on top of the cosmic distance ladder (Terence Tao explaining on 3blue1brown) [this video] Maps like DESI add the 3d distances. [LINK]