Epicycloids and Hypocycloids

In the book Trigonometric Delights by Eli Maor, there is a chapter on epicycloids and hypocycloids and their many fascinating properties.

These properties are illustrated with both descriptions and diagrams, which is the best a book can do, but require an active imagination to visualize each moving part.  Fortunately in the last two decades since Trigonometric Delights was written, programs for making animated diagrams have become more and more accessible, and I was able to use the free websites www.desmos.com and www.gifsmos.com to make some animated epicycloids and animated hypocycloids to better illustrate these theorems.

Hypocycloids

A hypocycloid can be formed by tracing the path of a point on a small circle that is rolling on the inside of a larger circle, similar to a design that can be made with spirograph. 

Although different hypocycloids can be made by varying the radii of each circle, all of them can be described by parametric equations.  Consider the following diagram, where R is the radius of the large circle and r is the radius of the small circle:

The coordinates of P relative to C are P(r cos Φ, -r sin Φ) and the coordinates of C relative to O are C((R – r) cos θ, (R – r) sin θ) so that the coordinates of P relative to O are:

x = (R – r) cos θ + r cos Φ

y = (R – r) sin θ – r sin Φ

However, from the rolling motion, arc PQ’ = arc QQ’, and since PQ’ = Rθ and QQ’ = r(θ + Φ), that means Rθ = r(θ + Φ), which rearranges to Φ = (R – r)/r · θ.  Substituting Φ into the equations above gives the parametric equations for a hypocycloid:

x = (R – r) cos θ + r cos (R – r)/r · θ

y = (R – r) sin θ – r sin (R – r)/r · θ

If the larger circle is twice as big as the smaller circle, then R = 2r, and the equations become:

x = 2r cos θ

y = 0

which traces a horizontal line segment along the diameter:

If the larger circle is four times as big as the smaller circle, then R = 4r, and the equations become:

x = 3r cos θ + r cos 3θ

y = 3r sin θ – r sin 3θ

which traces an astroid:

Double Generation Theorem of Hypocycloids

In 1725, Daniel Bernoulli discovered the double generation theorem of hypocycloids: a circle of radius r₁ rolling on the inside of a fixed circle of radius R generates the same hypocycloid as does a circle of radius r₂ = R – r₁ rolling inside the same fixed circle.

For example, here is the astroid again, the hypocycloid made by circles with radii R = 1 and r₁ = ¼:

and the following is the same astroid formed by circles with radii R = 1 and r₂ = R – r₁ = 1 – ¼ = ¾, but with the smaller circle moving in the opposite direction: 

and here are both animations at the same time:

Here is the double generation theorem illustrated by circles with radii R = 1, r₁ = 2/5, and r₂ = 3/5: 

and here it is illustrated by circles with radii R = 1, r₁ = 2/7, and r₂ = 3/7:

The double generation theorem of hypocycloids can be proved by algebraic manipulation.  The hypocycloid formed by the circles with radii R and r₂ has the parametric equations:

x = (R – r₂) cos θ₂ + r₂ cos (R – r₂)/r₂ · θ₂

y = (R – r₂) sin θ₂ – r₂ sin (R – r₂)/r₂ · θ₂

Replacing r₂ = R – r₁ and θ₂ = 2πp – (R – r₁)/r₁ · θ₁ (where p is an integer) and then simplifying gives: 

x = (R – r₁) cos θ₁ + r₁ cos (R – r₁)/r₁ · θ₁

y = (R – r₁) sin θ₁ – r₁ sin (R – r₁)/r₁ · θ₁

which are the parametric equations for the hypocycloid formed by the circles with radii R and r₁.

Epicycloid

An epicycloid can be formed by tracing the path of a point on a small circle that is rolling on the outside of a larger circle: 

Using a similar argument as the hypocycloid, but adding r to R instead of subtracting r from R, the parametric equations for the epicycloid can be found to be: 

x = (R + r) cos θ – r cos (R + r)/r · θ

y = (R + r) sin θ – r sin (R + r)/r · θ

An epicycloid can also be formed by tracing the path of a point on a large circle that is rolling around a fixed smaller circle in its interior:

If r is the radius of the large circle and R is the radius of the fixed smaller circle, the parametric equations for this type of epicycloid can be found to be: 

x = r cos (r – R)/r · θ – (r – R) cos θ

y = r sin (r – R)/r · θ – (r – R) sin θ

Double Generation Theorem of Epicycloids

As alluded to in the epicycloid animations above, there is a double generation theorem for epicycloids: a circle of radius r₁ rolling on the outside of a fixed circle of radius R generates the same epicycloid as does a circle of radius r₂ = R + r₁ rolling around the same fixed circle in its interior. 

 When the circles have radii R = 1, r₁ = 1, and r₂ = 2, the epicycloid traced is a cardioid:

And here is an epicycloid traced by circles with radii R = 1, r₁ = 2/5, and r₂ = 7/5:

Just like hypocycloids, the double generation theorem for epicycloids can be proved by algebraic manipulation.  The epicycloid formed by a large circle with a radius of r₂ rotated around a fixed circle with a radius of R has the parametric equations:

x = r₂ cos (r₂ – R)/r₂ · θ₂ – (r₂ – R) cos θ₂

y = r₂ sin (r₂ – R)/r₂ · θ₂ – (r₂ – R) sin θ₂

Replacing r₂ = R + r₁ and θ₂ = (R + r₁)/r₁ · θ₁ and then simplifying gives:

x = (R + r₁) cos θ₁ – r₁ cos (R + r₁)/r₁ · θ₁

y = (R + r₁) sin θ₁ – r₁ sin (R + r₁)/ r₁ · θ₁

which are the parametric equations for the epicycloid formed by a circle with radius r₁ rotating around a fixed circle with a radius of R.

Conclusion

Epicycloids and hypocycloids have many fascinating properties, as described by Eli Maor in his book Trigonometric Delights.  My hope is that these animated diagrams will help people understand these properties better, and give a better appreciation for mathematics in general.

Proving Common Trig Identities with Circle Theorems

I just re-read Trigonometric Delights by Eli Maor, and in the middle of his historical account of mathematics, he gives creative proofs for some common trigonometric identities based solely on Greek circle theorems.

Using the inscribed angle theorem, Ptolemy’s Theorem, and Thales’s Theorem, Maor is able to prove the law of sines, the angle addition and subtraction formulas for sine, the Pythagorean Theorem, and the Pythagorean identity of sine and cosine.  His proofs are outlined below.

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Consider the following triangle that is inscribed in a circle:

By the inscribed angle theorem, ∠AOB = 2∠ACB = 2γ.

Since OM bisects ∠AOB = 2γ, ∠MOB = γ.

Then from ΔOMB,

which rearranges to:

By a similar argument,

and

Therefore,

which proves the law of sines.

If 2r = 1, then a = sin α, b = sin β, and c = sin γ.  In other words, the side of an inscribed triangle in a circle with a unit diameter is equal to the sine of its opposite angle.

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Now consider the following diagram, where AC = 1:

Since 2r = 1, then as shown above BD is equal to the sine of its opposite angle, so BD = sin(α + β).

From ΔABC, cos α = AB/AC = AB/1 = AB and sin α = BC/AC = BC/1 = BC. 

Similarly from ΔADC, AD = cos β and CD = sin β.

But by Ptolemy’s Theorem, 

AC · BD = AB · CD + BC · DA

and after substituting the sine and cosine values, 

1 · sin(α + β) = cos α · sin β + sin α · cos β

which proves the angle addition formula for sine. 

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Similarly, consider the following diagram, where AD = 1:

Now BC is equal to the sine of its opposite angle, so BC = sin(α – β). 

From ΔABD, cos α = AB/AD = AB/1 = AB and sin α = BD/AD = BD/1 = BD. 

Similarly from ΔACD, AC = cos β and CD = sin β.

But by Ptolemy’s Theorem, 

AC · BD = AB · CD + BC · DA

and after substituting the sine and cosine values,

cos β · sin α = cos α · sin β + sin (α – β) · 1

which rearranges to:

sin(α – β) = sin α · cos β – cos α · sin β

which proves the angle subtraction formula for sine.

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Lastly, consider the following diagram of rectangle ABCD inscribed in a circle:

By the properties of a rectangle, AB = CD and AD = BC.

By Thales’s Theorem, AC and BD are diameters, so AC = BD.

By Ptolemy’s Theorem, 

AC · BD = AB · CD + BC · DA

and after substituting AB = CD, AD = BC, and AC = BD,

(AC)² = (AB)² + (BC)²

which proves the Pythagorean Theorem.

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Furthermore, if AC = 1, then cos α = AB/AC = AB/1 = AB and sin α = BC/AC = BC/1 = BC.

Substituting AC = 1, AB = cos α, and BC = sin α into the Pythagorean Theorem gives:

1 = cos²α + sin²α

which proves the Pythagorean identity of sine and cosine.

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Today’s math textbooks also have proofs for these trigonometric identities, but they are typically long and cumbersome.  Maor’s proofs are much more elegant, and also logically flow from a historical perspective.