Epicycloid
Lesson notes
Overview
A circle rolling around the outside of another traces looping petals. This topic is rated intermediate and is number 192 of 245 in Mathematics. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows a path traced by a parametric curve as its parameters change. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
An epicycloid is the curve traced by a point on a circle of radius r rolling around the outside of a fixed circle of radius R. When the ratio R/r is a whole number n, the curve has n cusps; for n = 1 it is a cardioid. The name echoes the epicycles of ancient astronomy, where planets were supposed to move on circles carried by other circles. It is also the shape of the cross-section of some gear teeth.
Reading the relationship
The expression "x = (R+r)cos t − r cos((R+r)t/r)" is a compact summary of the idea. It uses the symbols x, R, r, t; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller, and a trigonometric function, so the result repeats as the angle goes round. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
A frequent slip is memorising the rule without the picture. If a result surprises you, sketch the graph or test a simple number by hand before trusting the algebra.
How it connects
It builds on “Gamma Function Preview”, the topic before it in Mathematics. Once you are comfortable, continue with “Hypocycloid and Astroid”.
Why it matters
Mathematical ideas like this one are the language that physics, engineering, computing and statistics are written in.
Where you might use it
- Revising “Epicycloid” before a class, exam or certification
- Checking working on homework and exam questions by watching the graph respond
- Modelling growth, change and uncertainty in finance, data science and statistics
- Building the reasoning behind the algorithms and engineering tools people use every day
Frequently asked questions
What is Epicycloid?
A circle rolling around the outside of another traces looping petals. It is an intermediate topic in Mathematics.
How do I use the Epicycloid simulation?
The panel at the top shows a path traced by a parametric curve as its parameters change. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression x = (R+r)cos t − r cos((R+r)t/r) packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Gamma Function Preview”, the previous topic in Mathematics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.