Mathematica Notebook for This Page.
Astroid and its parallels.
Mathematica Notebook for This Page.
The cycloidal curves, including the astroid, were discovered by Roemer (1674) in his search for the best form for gear teeth. Double generation was first noticed by Daniel Bernoulli in 1725. [verbatim, Robert C Yates, 1952]
The astroid seems to have acquired its present name only in 1838, in a book published in Vienna; it went, even after that time, under various other names, such as cubocycloid, paracycle, four-cusp-curve, and so on. The equation x^(2/3) + y^(2/3) == a^(2/3) can, however, be found in Leibniz's correspondence as early as 1715. [verbatim, E.H.Lockwood, 1961]
Astroid is a special case of hypotrochoid. (see Curve Family Index). Astroid is defined as the trace of a point on a circle of radius r rolling inside a fixed circle of radius 4 r or 4/3 r. The latter is known as double generation.
 
Tracing Astroid |
Double Generation |
The two sizes of rolling circles that generate the astroid can be synchronized by a linkage. (this means: the 2 roulette methods trace the curve with the same speed and has a geometric relation) Let A be the center of the fixed circle. Let D be the center of the smaller rolling circle. Let F be a fixed point on this circle (the tracing point). Let G be a point translated from A by the vector DF. G is the center of the large rolling circle, with the same tracing point at F. ADFG is a parallelogram with sides having constant lengths.
The following formulas describe a astroid centered on the origin, and the length from center to one cusp is a, where a is a scaling factor.
Parametric: {Cos[t]^3, Sin[t]^3}, 0 < t ≤ 2 * π. 
parametric plot
Cartesian: (x^2 +y^2 -1)^3 + 27 * x^2 * y^2 == 0. Expanded: -1 + 3*x^2 - 3*x^4 + x^6 + 3*y^2 + 21*x^2*y^2 + 3*x^4*y^2 - 3*y^4 + 3*x^2*y^4 + y^6 == 0.
This equation is centered on origin and a cusp at {1,0}. Replace x by x/a and y by y/a and multiply both sides by a^6 and we obtain the classic equation given with scaling factor a as: (x^2 + y^2 - a^2)^3 + 27*a^2*x^2*y^2 == 0.
another equivalent equation is: x^(2/3) + y^(2/3) == 1. equation plot
The astroid is rich in properties that one can construct the curve, its tangent, and center of tangent circle, and device other mechanical ways to generate the curve.
Let there be a circle c centered on B passing K. We will construct a astroid centered on O with one cusp at K. Let O be the origin, and K be the point {1,0}. Let L be a point on c. Drop a line from L perpendicular to x-axis, let M be their intersection. Similarly drop a line from L perpendicular to y-axis, call the intersection N. Let P be a point on MN such that LP and MN are perpendicular. Now, P is a point on the astroid, and MN is its tangent, LP is its normal. Let D be the intersection of LP and c. Let D' be the reflection of D thru MN. Now, D' is the center of tangent circle at P.
Define the axes of the astroid to be the two perpendicular lines passing its cusps. Property: The length of tangent cut by the axes is constant. A mechanical devise where a fixed bar with endings sliding on two penpendicular tracks is called the Trammel of Archimedes. The envelope of the moving bar is then the astroid. A fixed point on the bar will trace out a ellipse. (see left figure)
Astroid is also the envelope of co-axial ellipses whose sum of major and minor axes is contsant.(see right figure)
Trammel of Archimedes |
Envelope of Ellipses.
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The evolute of a astroid is another astroid. (all epi/hypocycloids' evolute is equal to themselves)
above: Left: points on the curve are connected to their center of tangent circles. Above right: the evolute is drawn as the envelope of normals.
The pedal of a astroid with respect to its center is 4 petalled rose, called a quadrifolium. Astroid's radial is also quadrifolium. (all epi/hypocycloid's pedal and radial are equal, and they are roses.)
  Astroid's Pedal
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Astroid is the catacaustic of deltoid with parallel rays in any direction.
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The orthoptic with respect to its center is r^2 == (1/2)*Cos[2*θ]^2. [Robert C Yates.] Recall that a orthoptic of a curve is the locus of all points where the curve's tangents meet at right angles.
See: Websites on Plane Curves, Printed References On Plane Curves.
Robert Yates: Curves and Their Properties.
The MacTutor History of Mathematics archive↗.
Wikipedia: astroid↗.
© 1995-2008 by Xah Lee.
