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A 4-cusped hypocycloid which is sometimes also called a tetracuspid, cubocycloid, or paracycle. The parametric equations of the astroid can be obtained by plugging in or into the equations for a general hypocycloid, giving parametric equations
(1)
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(2)
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(3)
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(4)
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(5)
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(6)
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for .
The polar equation can be obtained by computing
(7)
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and plugging in to to obtain
(8)
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for .
(9)
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A generalization of the curve to
(10)
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gives "squashed" astroids, which are a special case of the superellipse corresponding to parameter .
In pedal coordinates with the pedal point at the center, the equation is
(11)
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and the Cesàro equation is
(12)
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A further generalization to an equation of the form
(13)
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is known as a superellipse.
The arc length, curvature, and tangential angle are
(14)
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(15)
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(16)
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where the formula for holds for .
The perimeter of the entire astroid can be computed from the general hypocycloid formula
(17)
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with ,
(18)
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For a squashed astroid, the perimeter has length
(19)
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The area is given by
(20)
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with ,
(21)
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(22)
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(OEIS A093828).
The evolute of an ellipse is a stretched hypocycloid. The gradient of the tangent from the point with parameter is . The equation of this tangent is
(23)
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(MacTutor Archive). Let cut the x-axis and the y-axis at and , respectively. Then the length is a constant and is equal to .
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The astroid can also be formed as the envelope produced when a line segment is moved with each end on one of a pair of perpendicular axes (e.g., it is the curve enveloped by a ladder sliding against a wall or a garage door with the top corner moving along a vertical track; left figure above). The astroid is therefore a glissette. To see this, note that for a ladder of length , the points of contact with the wall and floor are and , respectively. The equation of the line made by the ladder with its foot at is therefore
(24)
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which can be written
(25)
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The equation of the envelope is given by the simultaneous solution of
(26)
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which is
(27)
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(28)
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Noting that
(29)
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(30)
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allows this to be written implicitly as
(31)
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the equation of the astroid, as promised.
The related problem obtained by having the "garage door" of length with an "extension" of length move up and down a slotted track also gives a surprising answer. In this case, the position of the "extended" end for the foot of the door at horizontal position and angle is given by
(32)
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(33)
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Using
(34)
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then gives
(35)
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(36)
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Solving (◇) for , plugging into (◇) and squaring then gives
(37)
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Rearranging produces the equation
(38)
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the equation of a (quadrant of an) ellipse with semimajor and semiminor axes of lengths and .
the astroid is also the envelope of the family of ellipses
(39)
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illustrated above (Wells 1991).
An attractive arrangement of astroids can be constructed as a set of tangents to circular arcs (Trott 2004, pp. 18-19).