Terence Tao on the cosmic distance ladder

Summary by Krater

This video explains the historical progression of methods used to calculate cosmic distances, starting from measuring the Earth's radius and the distance to the Moon using lunar eclipses and ancient geometry, up to Johannes Kepler's discovery of planetary orbital shapes.

From the video

Answers: How do astronomers calculate cosmic distances and measure the size of Earth, Moon, and Sun?

• cosmic distance ladder
• Earth radius measurement
• Aristarchus method for Moon distance
• Erastosthenes method for Earth size
• Kepler's laws of planetary motion
• parallactic motion in astronomy

What it concludes

• Eratosthenes calculated the Earth's circumference using shadows at noon in Syene and Alexandria with roughly 10 percent accuracy.
• Aristarchus used lunar eclipse durations and relative geometry to estimate the distance to the moon as approximately 60 earth radii.
• Copernicus placed the sun at the center of planetary motion and calculated orbital periods for the known planets.
• Kepler used Tycho Brahe's observational data to prove that planetary orbits are elliptical rather than circular.