Tools > Compass Tool

Use the Compass tool to construct circles determined by two points — the center point and another point through which the circle passes. This second point is called the radius point, because it determines the radius of the circle.

1.Choose the Compass tool if it’s not already active.

2.Click to locate the center of your circle. (You can click in empty space, on an existing point, on a path object such as a segment or another circle, or on an intersection.)

3.Click again to locate the radius point.

Another way to use the Compass tool is to press the button at the center point, drag, and release at the radius point.

The color and style of a new circle is determined by Sketchpad's Preferences or by the last circle for which you set the color or style.

You can attach either the center point or the radius point to an existing object. To do so, you can click on

•an existing point

•a path object (such as a segment, a line, a circle or an arc)

•an intersection of two path objects

When the Compass tool is in the right place to click on an existing object, that object is highlighted (appearing thicker and in a special color) and identified in the status line.

Be careful when you want to attach the radius point of a circle to an existing object. It’s not enough to position the circle so that the circle appears to go through the desired point. You must actually position the tool itself over the point or object to which you want to attach before clicking or releasing to locate the radius point.

In this illustration, even though the circle appears to pass through the vertices of the triangle, the radius point will not be connected to the triangle. The result is that the radius point will be independent, and dragging either the radius point or any part of the triangle will show that the circle isn’t attached to the vertices.

Radius point not attached to vertex

In this illustration, the tool is positioned at a vertex, and the vertex is highlighted. Thus, that vertex will be the radius point of the circle, and the size of the circle will be linked to the position of the vertex no matter how various parts of the figure are dragged.

Radius point attached to vertex

There are two Construct menu commands that construct circles without using the Compass tool.

•Construct | Circle by Center+Point constructs a circle determined by two selected points. The first point is the center point, and the second is the radius point.

•Construct | Circle by Center+Radius constructs a circle determined by a selected point and a selected distance. The selected point is the center point, and the selected distance determines the radius. (The selected distance can be either a segment or a distance value.)

While you’re constructing a circle, you can hold down the Shift key to make the direction from the center to the radius point horizontal, vertical, or at an angle of 15°, 30°, 45°, 60°, or 75°. Construct the radius point before you release the Shift key.