C. Natural Systems
C.4 Earth and Cosmos
Course description
- Geology. Study Earth's structure, core to crust, and the rock cycle: igneous, sedimentary, and metamorphic formation, read against deep time and radiometric dating. Alfred Wegener proposed continental drift in 1912 and was largely dismissed for decades; seafloor-spreading evidence in the 1960s vindicated him and became plate tectonics, and the case shows how long a correct idea can be rejected when nobody, its author included, can supply a mechanism. Tarbuck and Lutgens's Earth Science or comparable online modules cover the material; examining rock samples or fossils in a natural history museum connects evidence to theory directly.
- Climate and atmosphere. Learn the physics of the greenhouse effect and the carbon cycle, and how paleoclimatologists read past climate from ice cores, including the correlation between CO₂ and temperature over the past 800,000 years. David Archer's Global Warming: Understanding the Forecast is a good guide; the Keeling Curve is the single dataset worth returning to repeatedly. Rockström and colleagues framed nine planetary boundaries for a stable Earth system in 2009; treat that paper as the origin of the idea. The 2023 reassessment found six of the nine crossed, and the 2025 Planetary Health Check put the count at seven, adding ocean acidification.
- Astronomy and cosmology. Kepler's laws of planetary motion preceded and fed into Newton's law of gravitation. Islamic astronomers at Maragha, particularly Nasir al-Din al-Tusi in the thirteenth century, devised a geometric construction, now called the Tusi couple, that reappears in Copernicus's De revolutionibus (1543), nearly three centuries later; how it reached him is still debated. Chinese court astronomers independently recorded the supernova of 1054 CE whose remnant is now the Crab Nebula. Carl Sagan's Cosmos remains the best generalist entry point to the modern picture of stars and galaxies.
Checkpoints
Each checkpoint is tagged with the stage of the cycle it tests.| Stage | Checkpoint | Attempts |
|---|---|---|
skepticism | Explain why Wegener's continental drift was rejected in the 1920s and what seafloor evidence changed minds in the 1960s. | Post the first attempt |
formalism | Compute Earth's radiative equilibrium temperature with no atmosphere, about −18°C, and explain the gap to the observed average. | Post the first attempt |
formalism | Repeat Eratosthenes' measurement of Earth's circumference using shadow lengths from two cities, collaborating online or using published solar-noon data if you can't travel. | Post the first attempt |
abstraction | Explain the seasonal wiggle superimposed on the Keeling curve's upward trend. | Post the first attempt |
abstraction | Explain what the Tusi couple does geometrically and why its reappearance in Copernicus matters to historians of science. | Post the first attempt |
Discussion
Proposed edits
Found an error, a missing idea, or a better source? Propose a change. A moderator reviews each proposal.Sources
- Lüthi, D. et al. "High-resolution carbon dioxide concentration record 650,000–800,000 years before present." Nature 453, 379–382 (2008).
- Rockström, J. et al. "A safe operating space for humanity." Nature 461, 472–475 (2009).
- Richardson, K. et al. "Earth beyond six of nine planetary boundaries." Science Advances 9(37), eadh2458 (2023).
- Potsdam Institute for Climate Impact Research. Planetary Health Check 2025 (September 2025).
- Saliba, George. Islamic Science and the Making of the European Renaissance (MIT Press, 2007).