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The other day I started wondering how many circles of the same size can touch the center circle at one point without overlapping. OK I know I am different, but that is just how my mind works. A tangent is a line that “just touches” a curve at a single point without entering the interior of the curve. For a circle, this means the tangent intersects the circle at exactly one point, and the radius drawn to this point is always perpendicular to the tangent line. This point is known as the point of tangency. A tangent is like a kiss because it touches a curve or another shape at exactly one single point without crossing through it. A tangent line meets a curve or a circle at one precise location, representing a momentary meeting before parting ways. When I studied calculus in college, I didn’t know what the subject was about, but I was able to do the problems because I could see the patterns of how they were solved. After I retired from engineering, I decided to study calculus to see what it was about, because I felt that my teachers never explained this subject properly to me. I was always pretty good at math, and I understood that algebra was the study of mathematical symbols and the rules for manipulating them using variables, that geometry was the study of sizes, shapes, positions, and dimensions of shapes that included space, lines, points, and solids, and trigonometry was the study of the relationships between the sides and angles of triangles that when rotated made special wave forms.
Once I got the hang of calculus while I worked as a substitute teacher, I would aske my calculus students what calculus was, because I knew this aspect of the subject was not being taught. Most od the students replied, “It is a really hard math”, and I couldn’t argue with that, but calculus is the study of change. In math change is depicted by curves because they visually represent how one variable changes in relation to another. A curve is also a matter of space, and one way of describing a curve is with a circle. A curve is just a piece of a circle, and a circle is a two dimensional object that is made of an infinite amount of curves which all vary at the same rate. You need to be cautious here and understand that everything is a curve, even a straight line, which is a curve without curvature (OK that is not too confusing).
When circles are tangent to one another, they form configurations called Apollonian gaskets or circle packings that are named after the ancient Greek mathematician Apollonius of Perga, who first studied the problem of mutually tangent circles around 200 BC. In 1643, René Descartes derived Descartes theorem to solve a quadratic equation (which I will not get into), where he found that three mutually tangent circles can have for a fourth circle added, either inside the other circles nestled in the center gap, or becoming the outer circle that wraps around the other three, kissing every circle. In 1936, chemist Frederick Soddy wrote a poem titled “The Kiss Precise” that proved the Kissing Circles Theorem. You can view kissing circles of the same size in two dimensions by placing pennies around a central penny on a flat table where they only touch the central circle and don’t overlap each other. The US stopped making pennies in the end of 2025, but there are still roughly 300 billion pennies in circulation. This could also be done on paper where you draw a circle with a compass. If you pick one circle in the configuration, the points where it kisses the other circles can be connected by straight lines. These line segments form chords of that specific circle. Mathematicians use these chords to calculate the angles, coordinates, or centers of the kissing circles.
Before I go, I thought I should explain why exactly six circles can be positioned around a central circle, because it is easy to see that is the case by looking at the pennies, and if math can be explained, then it is not torture. When a circle is touching another circle at one point, the distance between the centers is two times the radius. When a third circle is added that touches both of the other two, the distances between its center to each of the other centers is also clearly two times the radius. The triangle formed, with each center at a vertex, is an equilateral triangle. Each angle of an equilateral triangle is 60 degrees. A circle is 360 degrees around its circumference, which means you divide this 360 degrees by the 60 degrees (360/60=6) and you get six of these triangles (which corresponds to 6 circles) that go the whole way around. For all of my readers that love poetry, I have placed the poem below:
For pairs of lips to kiss maybe
Involves no trigonometry.
This not so when four circles kiss
Each one the other three.
To bring this off the four must be
As three in one or one in three.
If one in three, beyond a doubt
Each gets three kisses from without.
If three in one, then is that one
Thrice kissed internally.
Four circles to the kissing come.
The smaller are the benter.
The bend is just the inverse of
The distance form the center.
Though their intrigue left Euclid dumb
There’s now no need for rule of thumb.
Since zero bend’s a dead straight line
And concave bends have minus sign,
The sum of the squares of all four bends
Is half the square of their sum.
To spy out spherical affairs
An oscular surveyor
Might find the task laborious,
The sphere is much the gayer,
And now besides the pair of pairs
A fifth sphere in the kissing shares.
Yet, signs and zero as before,
For each to kiss the other four
The square of the sum of all five bends
Is thrice the sum of their squares.
Written for Di’s Three Things Challenge MM556 where today the words are: Cautious, Chord and Compass.




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