For young readers
About ages 8 to 12. Read it together, or on your own.
Some people think that long ago everyone believed the Earth was flat. That isn't so. Educated Greeks knew the Earth is a ball by the time of Aristotle, about 2,350 years ago, and he gave reasons you can check yourself.
His best reason: during an eclipse of the Moon, the Earth's shadow falls on the Moon, and the edge of the shadow is always curved. Only a ball casts a round shadow whichever way it is turned. A flat plate would sometimes cast a thin, straight shadow.
Try this
Shadow test. In a dark room, shine a flashlight on a wall. Hold up a ball, then a paper plate, then a small box. Turn each one slowly. Which one always makes a round shadow? Now you know why Aristotle said the Earth is round.Aristotle also noticed that when travelers go south, new stars appear above the southern horizon, stars that can't be seen from Greece. That only makes sense if the Earth's surface curves. But Aristotle also believed some things that turned out wrong: that the Earth sits still at the center of everything, and that the Sun, Moon and stars are carried around it on great invisible spheres.
For teens and adults
The full lesson.
Who, when and why
Aristotle (384–322 BC) was born in Stagira in northern Greece, the son of a doctor. At seventeen he went to Athens and studied in Plato's Academy for about twenty years. After Plato died he lived on the island of Lesbos and elsewhere, tutored the young Alexander the Great for a few years, and in 335 BC returned to Athens and founded his own school, the Lyceum. On the Heavens is his account of the universe as a whole: the stars and planets, the Earth at the center, and the elements everything is made of.
The problem: earlier thinkers disagreed wildly about the cosmos. Some thought the Earth floated on water, or was a flat drum held up by air. The mathematician Eudoxus had built a model of nested spheres to explain the planets' motions. Aristotle wanted a physical account: what the heavens are made of, why they move in circles, and why the Earth is where it is and shaped as it is.
Two kinds of motion, five elements
- Down and up: earth and water naturally move toward the center of the universe; air and fire naturally move away from it. Each element seeks its natural place.
- Around: the heavens move in circles forever. So they must be made of a fifth element, which does not rise or fall, grow or decay. Later writers called it aether or the quintessence ("fifth essence").
- The Earth sits at the center, at rest, because that is where all the heavy matter collects.
The Earth is a sphere
How else would eclipses of the moon show segments shaped as we see them? As it is, the shapes which the moon itself each month shows are of every kind, straight, gibbous, and concave, but in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth's surface, which is therefore spherical.
Again, our observations of the stars make it evident, not only that the earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon.
He adds that some stars seen in Egypt and Cyprus are never seen farther north, and reports that mathematicians had put the Earth's circumference at 400,000 stades. That is the oldest estimate of the Earth's size we have. A stade was a racecourse length, and Greek stades varied, so we can't convert it exactly; on most estimates it is about one and a half to two times the true size.
Worked example: Eratosthenes measures the Earth
- A full circle is 360°, and $360 \div 7.2 = 50$.
- So the distance from Syene to Alexandria is $\tfrac{1}{50}$ of the way round.
- Circumference $= 50 \times 5000 = 250{,}000$ stades.
Answer: 250,000 stades (reports also give 252,000)
Heavy and light
A given weight moves a given distance in a given time; a weight which is as great and more moves the same distance in a less time, the times being in inverse proportion to the weights. For instance, if one weight is twice another, it will take half as long over a given movement.
Common mistake
What later science kept, and what it changed
- Kept: the spherical Earth and the evidence for it, which every astronomer since has accepted.
- Changed: the unchanging heavens. Tycho Brahe showed in 1572 and 1577 that a new star and a comet were far beyond the Moon. Galileo's telescope (1610) found mountains on the Moon and moons around Jupiter.
- Changed: the Earth at rest in the center. Copernicus (1543) put the Sun at the center, and Newton showed that the same gravity rules the heavens and the Earth, ending Aristotle's split between them.
Going deeper
Context, the big questions, and how later books answered.
Why a different physics for the sky?
On Earth things start and stop, rise and fall, are born and decay. In the sky, as far as any Greek could see, the stars kept the same patterns forever and moved in perfect circles. Aristotle concluded that the heavens are made of different stuff with different laws. It was a reasonable inference from the evidence of his day, and it was the main idea that Galileo and Newton overturned.
Aristotle's model put each planet on a set of nested spheres, following Eudoxus and Callippus, 55 spheres in all. It could not explain why planets change in brightness, which suggested their distance changes. Later astronomers, above all Ptolemy (next on this ladder), used other circles instead, while keeping Aristotle's Earth at rest in the center.
Big questions
- Aristotle reasoned from evidence and got some things right and some wrong. What would have helped him tell which was which?
- Why might people prefer to believe that the heavens are perfect and unchanging?
- Many people today think medieval people believed in a flat Earth. Why do myths about the past spread?
Talk about it together
- Try the shadow test with a ball and a plate. How would you explain Aristotle's argument to a friend?
- Have you seen a lunar eclipse? If one comes up, watch the edge of the shadow together.
- Why might sailors have been some of the first people to suspect the Earth is curved? (Think about ships coming over the horizon.)
- Aristotle thought the sky was made of a different element from the Earth. What would it take to change your mind about something everyone around you believes?
- Is it surprising that a thinker could be right about the shape of the Earth and wrong about whether it moves?