Which ancient Greek mathematician officially defined the diameter in his famous book 'Elements' around 300 BCE?
Chapter 1

The math of the middle

Chapter 2

Sizing up the cosmos

Chapter 3

Engineering the everyday

YOUR GOAL

Master of The story of diameter

Ch 1 · The math of the middle
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Pi in the Middle
BETA

sonia Play Pi in the Middle, an exciting educational math game. Master circle geometry and pi concepts while having fun. Play online for free now!

Jul 23

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hue The Reuleaux Heptagon

My diameter's a constant as I roll across the floor, So I slip right through the slot of the vending machine door

Jul 20

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hue The secret ratio of pi

Jul 20

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Every circle in the universe, from coins to planets, shares this exact ratio.

sonia Discover the universal ratio shared by all circles in our universe, from coins to planets. Explore this stunning infographic to learn the secrets of Pi.

Jul 23

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The secret ratio of pi

hue Dividing any circle's outer perimeter by its diameter always yields the magical mathematical constant known as pi.

Jul 20

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Meet in the Middle
BETA

hue Drag lines to meet in the middle to discover how diameter measures a circle.

Jul 20

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Fun Fact

Circle's big line: The word diameter comes from the Greek 'diametros,' which translates to 'measuring through' a shape.

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Jul 20

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Glossary
radius (noun)
A straight line from the center to the circumference of a circle or sphere. In 1570, John Dee translated Euclid's work, defining this line as half of the circle's diameter.
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Jul 20

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Gossip

Which circle-based invention is the greatest of all time?

Jul 20

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Quiz

If you double the diameter of a circle, what happens to its circumference?

Jul 20

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hue Half a line makes a radius

Jul 20

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Every circle in the universe shares this exact same hidden ratio.

sonia Explore the magic of Pi! This educational infographic reveals the hidden constant ratio shared by every single circle in the universe.

Jul 23

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Quiz

From which language does the word 'diameter' originate?

Jul 20

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hue Why the straight line matters

Jul 20

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Half a line makes a radius

hue Cutting a circle's diameter exactly in half gives you the radius, the foundational measurement for drawing perfect curves.

Jul 20

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Quiz

Which of these shapes does NOT have a measurable diameter through its center?

Jul 20

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hue Measuring circles with ancient ropes

Jul 20

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Quiz

If a bicycle wheel has a diameter of 26 inches, what is its radius?

Jul 20

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Quiz

What is the mathematical relationship between a circle's radius and its diameter?

Jul 20

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Quiz

What did ancient Egyptian 'rope-stretchers' use knotted ropes to measure?

Jul 20