CoursesCourse 20
Thinking about the Earth
How our picture of the planet changed. The earth was once a flat floor, then a sphere that nobody could see, then a still center, a moving planet, a flattened ball, a world of unknown age, and finally a shell of drifting plates. This course follows each change from the first guess to the measurement that settled it, and asks what each one cost the people who had to give up the old picture.
Start lesson 1Lessons
- 1 The Earth becomes a sphereNobody saw the whole earth until the twentieth century, yet the Greeks knew its shape two thousand years earlier. How they reasoned their way there, how it was measured, and what “round heaven, square earth” really meant.
- 2 A moving Earth, a flattened EarthIn 1851 a pendulum in Paris showed the floor turning. Why the earth was thought to stand still, what finally showed that it moves, and the long argument over whether it is an olive or an onion.
- 3 How old is the Earth?An archbishop dated creation to a night in 4004 BC. How rocks, fossils, a cooling clock, and a meteorite from Arizona pushed the answer back to 4.5 billion years, and what the old “religion against science” story leaves out.
- 4 Drifting continentsA meteorologist saw that the coastlines fit, and most geologists said no. Fifty years later sonar and magnets showed he was right about the picture and wrong about the engine. Plate tectonics, earthquakes, and what a picture of the earth means.
You’ll be able to
- Explain how the Greeks reasoned that the earth is a sphere, how Eratosthenes measured it, and what the Chinese phrase “round heaven, square earth” does and does not say.
- Describe why the earth was thought to stand still, what finally showed that it moves, and how pendulums and surveys settled the argument over its shape.
- Trace the argument over the earth’s age from Ussher’s chronology to radioactive dating, and say why Kelvin’s cooling clock ran fast and what the figure of 4.54 billion years rests on.
- Describe Wegener’s drifting continents, the sea-floor evidence that decided the matter, and the difference between earthquakes at plate boundaries and inside continents.
About this course
This course covers 地球论 (Thinking about the Earth), the history of the idea of the earth. It follows Course 3, The cosmos before Newton, which told the story of the Greek cosmos and of Copernicus, and Course 4, Newton’s clockwork universe, which told how Newton’s laws explained the sky. Here the subject is the planet itself: its shape, its motion, its age, and the slow motion of its crust. Course 1, Why science began in Greece, asked why the Greeks asked such questions in the first place; Course 8, The journey of life, takes up the story of life on the same planet.
The argument and the examples lead. Around them we add what is ours: every date and number checked against sources (several popular figures, such as the date of Columbus’s landfall, the place of Magellan’s death, the instruments that mapped the sea floor, and the age of the oldest rocks, turned out to be wrong in the usual telling), hands-on activities, background for readers in China and the United States, and an honest “Another view” in each lesson, including on how much of a “revolution” story is hindsight.