Great IdeasNature and science
The heavens and the planets
The sun, moon, planets and stars, and how people worked out their motions.
The question
How are the sun, moon, planets and stars arranged, and what makes them move?
- 1 Plato 360 BC
- 2 Aristotle 350 BC
- 3 Claudius Ptolemy AD 150
- 4 Dante Alighieri 1316
- 5 Nicolaus Copernicus 1543
- 6 Johannes Kepler 1620
- 7 Isaac Newton 1687
- 8 Albert Einstein 1916
- 9 Arthur Eddington 1933
- Earth at the center
- Sun at the center
Uncolored stops go beyond that question: gravity as curved space-time, and a universe of many galaxies.
How the books answer
Oldest first. Each stop is our own short summary of what the author argued; follow the link to the rung for the whole lesson.
c. 360 BC
Plato, Timaeus and Critias
Earth at the center
In the Timaeus, Plato tells a story of a divine craftsman shaping the world as a living whole, with the earth at the center. The heavenly bodies move in circles, which he thought the most perfect motion. Their regular paths make time and give people a model of order.
c. 350–322 BC
Aristotle, On the Heavens, On Generation and Corruption, Meteorology
Earth at the center
Aristotle gives evidence that the earth is a sphere: its shadow on the moon in an eclipse is always curved, and new stars appear as you travel south. He places the earth, unmoving, at the center, with the heavens made of a special element that moves forever in circles.
“How else would eclipses of the moon show segments shaped as we see them?”
c. AD 150
Claudius Ptolemy, The Almagest
Earth at the center
Ptolemy builds a full mathematical model of the heavens with the earth at the center. To match what astronomers saw, such as planets seeming to loop backward, he adds small circles riding on larger ones. His tables predicted planet positions well enough to be used for some 1,400 years.
c. 1316–1321
Dante Alighieri, The Divine Comedy: Paradiso
Earth at the center
Dante rises through the nine heavens of the medieval picture: the moon, the planets, the sun, the fixed stars and beyond. He uses the astronomy of his day as a ladder of light and joy. The poem ends with the love that moves the sun and the other stars.
“But yet the will roll'd onward, like a wheel In even motion, by the Love impell'd, That moves the sun in heav'n and all the stars.”
1543
Nicolaus Copernicus, On the Revolutions of the Heavenly Spheres
Sun at the center
Copernicus puts the sun near the center and the earth among the planets, turning daily and circling yearly. This explains the planets' backward loops simply, as the earth overtaking or being overtaken. He still used circles and some small epicycles, and his idea took decades to win wide acceptance.
1617–1621 (Books IV–V, 1620–1621)
Johannes Kepler, Epitome of Copernican Astronomy, Books IV and V
Sun at the center
Kepler, working from Tycho Brahe's careful observations, found that planets move in ellipses with the sun at one focus, and faster when they are closer to it. His Epitome teaches the whole sun-centered system with these laws. He also guessed that a force from the sun drives the planets.
1687
Isaac Newton, Mathematical Principles of Natural Philosophy
Sun at the center
Newton shows that one law of gravity explains both an apple's fall and the moon's orbit. From it he derives Kepler's laws, the tides and the paths of comets. The heavens and the earth now follow the same physics.
1916 (English 1920)
Albert Einstein, Relativity: The Special and the General Theory
Einstein's general theory describes gravity as the bending of space and time by mass. It explained a small puzzle in Mercury's orbit and predicted that starlight would bend near the sun, which was observed in 1919. Newton's law remains an excellent approximation for most purposes.
1933
Arthur Eddington, The Expanding Universe
Eddington explains for general readers the new discovery that distant galaxies are moving away from us, so that the universe is expanding. The heavens are far larger than earlier astronomers imagined, and they have a history. He also wrote with wonder about what this meant for our place in it.
For young readers
Look up on a clear night: the stars seem to turn around us, and the sun seems to travel across the sky by day. For a long time, most people thought the earth stood still in the middle of everything.
Then astronomers like Copernicus, Kepler and Galileo showed that the earth spins and travels around the sun. Try it: walk in a circle around a lamp while spinning slowly. The lamp seems to move around you, but you are the one moving!
Talk about it
- Why did it make sense for people long ago to think the sun moved around the earth?
- Aristotle used a lunar eclipse to show the earth is round. How does that work?
- What would it take to change your mind about something everyone believes?
- Dante saw the heavens as full of meaning. How do the stars make you feel?
Where else on the site
- From an Earth-Centered to a Sun-Centered Sky
- The Celestial Sphere
- Kepler's First Law: Ellipses
- Eclipse Seasons
- Planets, Moons and Small Bodies
- Hubble's Law
Every rung tagged with this idea:
These pages are our own short synthesis, written for families. They were inspired by Mortimer J. Adler's Syntopicon, the index of great ideas in volumes 1 and 2 of Great Books of the Western World, but the questions, summaries and groupings are ours, not the Syntopicon's. Short excerpts come from public-domain translations, credited where they appear.