Astroid

An astroid is a particular mathematical curve: a hypocycloid with four cusps. Specifically, it is the locus of a point on a circle as it rolls inside a fixed circle with four times the radius.^[1] By double generation, it is also the locus of a point on a circle as it rolls inside a fixed circle with 4/3 times the radius. It can also be defined as the envelope of a line segment with an end point on each of the axes. It is therefore the envelope of the moving bar in the Trammel of Archimedes.

Its modern name comes from the Greek word for "star". It was proposed, originally in the form of "Astrois", by Joseph Johann von Littrow in 1838.^[2][3] The curve had a variety of names, including tetracuspid (still used), cubocycloid, and paracycle. It is nearly identical in form to the evolute of an ellipse.

If the radius of the fixed circle is a then the equation is given by^[4]

${\displaystyle x^{2/3}+y^{2/3}=a^{2/3}.}$

This implies that an astroid is also a superellipse.

Parametric equations are

${\displaystyle x=a{\cos }^{3}t=\frac{a}{4}(3\cos t+\cos 3t),}$

${\displaystyle y=a{\sin }^{3}t=\frac{a}{4}(3\sin t-\sin 3t).}$

The pedal equation with respect to the origin is

${\displaystyle r^2=a^2-3p^2,}$

the Whewell equation is

${\displaystyle s=\frac{3a}{4}\cos 2\phi ,}$

and the Cesàro equation is

${\displaystyle R^2+4s^2=\frac{9a^2}{4}.}$

The polar equation is^[5]

${\displaystyle r=\frac{a}{({\cos }^{2/3}\theta +{\sin }^{2/3}\theta {)}^{3/2}}.}$

The astroid is a real locus of a plane algebraic curve of genus zero. It has the equation^[6]

${\displaystyle (x^2+y^2-a^2{)}^3+27a^2{x}^2{y}^2=0.}$

The astroid is therefore a real algebraic curve of degree six.

The polynomial equation may be derived from Leibniz's equation by elementary algebra:

${\displaystyle x^{2/3}+y^{2/3}=a^{2/3}.}$

Cube both sides:

${\displaystyle x^{6/3}+3x^{4/3}{y}^{2/3}+3x^{2/3}{y}^{4/3}+y^{6/3}=a^{6/3}}$

${\displaystyle x^2+3x^{2/3}{y}^{2/3}(x^{2/3}+y^{2/3})+y^2=a^2}$

${\displaystyle x^2+y^2-a^2=-3x^{2/3}{y}^{2/3}(x^{2/3}+y^{2/3})}$

Cube both sides again:

${\displaystyle (x^2+y^2-a^2{)}^3=-27x^2{y}^2(x^{2/3}+y^{2/3}{)}^3}$

But since:

${\displaystyle x^{2/3}+y^{2/3}=a^{2/3}}$

It follows that

${\displaystyle (x^{2/3}+y^{2/3}{)}^3=a^2.}$

Therefore:

${\displaystyle (x^2+y^2-a^2{)}^3=-27x^2{y}^2{a}^2}$

or

${\displaystyle (x^2+y^2-a^2{)}^3+27x^2{y}^2{a}^2=0.}$

Area enclosed^[7]

${\displaystyle \frac{3}{8}\pi a^2}$

Length of curve

${\displaystyle 6a}$

Volume of the surface of revolution of the enclose area about the x-axis.

${\displaystyle \frac{32}{105}\pi a^3}$

Area of surface of revolution about the x-axis

${\displaystyle \frac{12}{5}\pi a^2}$

The astroid has four cusp singularities in the real plane, the points on the star. It has two more complex cusp singularities at infinity, and four complex double points, for a total of ten singularities.

The dual curve to the astroid is the cruciform curve with equation ${\displaystyle x^2{y}^2=x^2+y^2.}$ The evolute of an astroid is an astroid twice as large.

• Cardioid (epicycloid with one cusp)
• Nephroid (epicycloid with two cusps)
• Deltoid (hypocycloid with three cusps)
• Stoner–Wohlfarth astroid a use of this curve in magnetics.

• J. Dennis Lawrence (1972). A catalog of special plane curves. Dover Publications. pp. 4–5,34–35,173–174. ISBN 0-486-60288-5. 
• Wells D (1991). The Penguin Dictionary of Curious and Interesting Geometry. New York: Penguin Books. pp. 10–11. ISBN 0-14-011813-6. 
• R.C. Yates (1952). "Astroid". A Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards. pp. 1 ff.. 

• Hazewinkel, Michiel, ed. (2001), "Astroid", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=p/a013540 
• Weisstein, Eric W.. "Astroid". http://mathworld.wolfram.com/Astroid.html. 
• "Astroid" at The MacTutor History of Mathematics archive
• "Astroid" at The Encyclopedia of Remarkable Mathematical Forms
• Article on 2dcurves.com
• Visual Dictionary Of Special Plane Curves, Xah Lee
• Bars of an Astroid by Sándor Kabai, The Wolfram Demonstrations Project.

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