Nested Circles

By the end of this lab you'll be able to:

• Use circle() to draw circles of different sizes
• Use goto() to reposition the turtle between shapes
• Understand why drawing order matters when shapes overlap

Twenty concentric rings alternating cyan and navy pulse outward from the center like a ripple frozen in time.

Welcome to Nested Circles!

In this lab you'll draw circles from biggest to smallest, each one covering the last. The rings you see are the slivers of color peeking out from behind smaller circles. Let's code it together!

How the Rings Work

monty.circle(radius) draws a circle, but the turtle starts at the bottom of the circle, not the center. To keep all circles centered on the same point, we use monty.goto(0, -radius) before each circle — placing the turtle radius pixels below the center.

We draw from largest to smallest so that each new circle neatly covers the middle of the previous one, leaving only a thin ring of the earlier color visible at the edge.

Sample Code

import turtle
monty = turtle.Turtle()
monty.speed(0)
monty.hideturtle()

colors = ['navy', 'cyan']

for i in range(19, -1, -1):
    radius = 10 + i * 15
    monty.color(colors[i % 2])
    monty.penup()
    monty.goto(0, -radius)
    monty.pendown()
    monty.begin_fill()
    monty.circle(radius)
    monty.end_fill()

What Do You Think Will Happen?

The loop counts down: i = 19, 18, 17 … 1, 0. The very last circle drawn (i = 0) sits right in the center — will it be navy or cyan? Make your guess — then click Run to find out!

Try It Now

▶ Run ↻ Reset

i = 0 → 0 % 2 = 0 → colors[0] → navy. The bullseye center is always navy. Were you right?

How It Works

range(19, -1, -1) counts down from 19 to 0 — the three arguments are start, stop (exclusive), and step. A step of -1 counts backwards.

Each iteration computes radius = 10 + i * 15, so the radii are 295, 280, 265 … 25, 10 pixels. colors[i % 2] picks navy when i is even and cyan when i is odd, creating the alternating rings.

The critical line is monty.goto(0, -radius) — without it, every circle would start from wherever the last one ended, scattering circles randomly across the canvas.

Explanation Table

Line	What it does
range(19, -1, -1)	Count down from 19 to 0 (inclusive)
radius = 10 + i * 15	Each ring is 15 pixels wider than the next inner one
colors[i % 2]	Alternates between index 0 (navy) and index 1 (cyan)
monty.goto(0, -radius)	Move to the bottom of the circle so it is centered at the origin
monty.circle(radius)	Draw a full circle with the given radius

Learning Check

Your Turn — Fix the Centering

The program below draws 20 circles but they all start from the same spot — a messy spiral! Add one line inside the loop (after monty.penup()) to center each circle at the origin.

▶ Run ↻ Reset

The missing line is monty.goto(0, -radius) — it repositions the turtle at the bottom of each circle so the center is always at the origin.

Experiments

1. Flip the loop direction. Change range(19, -1, -1) to range(20). Now you draw smallest-first. You'll know it worked when the rings look the same but Monty draws them inside-out.

2. Change the ring gap. Replace i * 15 with i * 10. Rings are closer together. Try i * 25 for wider rings. You'll know it worked when the number of visible rings changes.

3. Try three colors. Change colors to ['navy', 'cyan', 'white'] and use i % 3. You'll know it worked when you see a three-tone bullseye pattern.

4. Make an ellipse. Replace monty.circle(radius) with monty.circle(radius, steps=60) — turtle's circle() can approximate an ellipse by stretching with monty.shapesize() first. You'll know it worked when the rings are no longer perfectly round.

Great Work!

You used counting-down loops, goto() for precise positioning, and draw order to build a perfect bullseye — all core skills for any drawing program. Up next: Starburst — radiating lines from a single center point.