The Shape of Gravity: Why On Earth Are Planets Spherical? - Alain Goriely

The Shape of Gravity: Why On Earth Are Planets Spherical? - Alain Goriely

by Alain Goriely

Formal & Physical Sciences Astronomy & Cosmology PGAstronomy, space and timePGSSolar system: the Sun and planets
🎙 Alain Goriely 👥 450K 📅 April 17, 2026 ⏱ 48 min 👁 24K 📄 science communication 🧭 2026-08-03
Available in: English (current) Français

Keywords

spherical planetsgeoidNewtonCassinigeodesyellipsoidsbifurcationGladys WestGPSexoplanets

Summary

In this Gresham College lecture, mathematician Alain Goriely explores the question of why planets are spherical, tracing the historical and scientific journey from ancient Greek ideas to modern geodesy. He begins by noting that the Earth is not a perfect sphere but an oblate spheroid, and discusses the 17th-century debate between Cassini (prolate) and Newton (oblate). Newton’s theoretical arguments, based on pendulum clock observations and fluid equilibrium, suggested oblateness, while Cassini’s triangulation measurements in France seemed to indicate prolate. To settle the debate, the French Academy sent expeditions to Lapland and Peru in the 1730s; their measurements confirmed oblateness. Goriely highlights the contributions of key figures like Huygens, Clairaut, Maupertuis, and Émilie du Châtelet, and later Gladys West, whose work on satellite geodesy underpins GPS. The lecture then moves to fluid planets, discussing Maclaurin and Jacobi ellipsoids and Poincaré’s pear-shaped figures, which arise from bifurcation theory. Goriely touches on the relevance of these shapes to moons like Europa, Earth tides, and the search for life, and concludes with elastic planets and gravitational collapse, noting that exoplanets may have diverse shapes. The talk emphasizes the deep interplay between mathematics, physics, and observation in understanding planetary shapes.

195 words

Critical Evaluation

The lecture provides a comprehensive and engaging overview of the historical and mathematical aspects of planetary shapes. Alain Goriely, a distinguished applied mathematician, delivers a well-structured narrative that balances accessibility with technical depth. The historical account is accurate and well-documented, covering the ancient Greek contributions, the Newton-Cassini debate, and the subsequent expeditions that resolved it. Goriely effectively explains the scientific principles involved, such as pendulum clock behavior, triangulation, and the concept of hydrostatic equilibrium, without oversimplifying. The mathematical derivations, while not fully detailed in the talk, are presented in a way that conveys their significance. The inclusion of modern developments, such as Gladys West’s role in GPS and the discussion of fluid and elastic planetary models, demonstrates the ongoing relevance of the topic. The lecture is well-referenced, with mentions of primary sources and the Gresham College website for further materials. The Q&A session is available separately, indicating a thorough treatment. The only minor critique is that some technical concepts, like bifurcation theory, are introduced briefly and may require additional explanation for a general audience, but this does not detract from the overall quality. The title accurately reflects the content, and the lecture successfully challenges the assumption of perfect sphericity while providing a rigorous scientific perspective.

205 words

Title / Content Match

The title accurately reflects the content, which explores why planets are spherical and the historical and mathematical developments that refined this understanding.

Quality & Reliability

9/10

Lecture by a renowned mathematician (Alain Goriely, Oxford) with rigorous historical and mathematical content, based on established scientific knowledge and documented historical events. The presentation is well-structured and includes references to primary sources (Newton, Huygens, etc.) and modern geodesy. Minor simplifications for a general audience do not undermine accuracy.

Chapters

Cited Sources

  • Gresham College — Official website of the institution hosting the lecture.
  • Support Gresham College — Page for supporting the college's educational mission.
  • Lecture page: The Shape of Gravity — Dedicated page for this lecture, likely containing further resources and transcript.
  • Q&A session — Link to the question and answer session following the lecture.

Concurring Sources

  • Gresham College lecture page — Official page for the lecture, likely containing additional resources and references.

Contribution & Novelties

The lecture offers a fresh perspective on a classic topic by weaving together historical narrative, mathematical theory, and modern applications. It highlights the often-overlooked contributions of figures like Émilie du Châtelet and Gladys West, and connects the historical debate to contemporary research on exoplanets and fluid dynamics. The presentation of Poincaré’s pear-shaped figures and bifurcation theory provides insight into advanced mathematical concepts in an accessible manner.

Pour aller plus loin :

  • Hydrostatic equilibrium — Fundamental concept for planetary shapes.
  • Geoid — The precise shape of Earth as modeled by geodesy.
  • Maclaurin spheroid — Equilibrium shape of a rotating fluid body.
  • Jacobi ellipsoid — Another equilibrium shape for rotating fluids.
  • Poincaré pear-shaped figure — Bifurcation in planetary shapes.
  • Gladys West — Mathematician whose work contributed to GPS.

126 words

Radar Profile

The radar profile shows high scores across all dimensions, indicating a well-rounded and reliable lecture. The quantity and quality of information are excellent, with a strong technical level suitable for an interested audience. The overall reliability is high, reflecting the expertise of the speaker and the institutional backing.

Reliability 9/10