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A circle at a center you pick, sized to touch one other object.

Menu: Construct > Circle Tangent to... | Node: Circle Tangent to... | In depth: Circles & arcs

Click the center, then click the object the circle should touch: a circle, an arc, a segment, a ray or a line. Locus works out the radius that makes it tangent and draws it. Move the cursor to choose among the answers, then click to keep one.

You can select first here too. Run the command with a point selected and it takes that as the center; with a target selected it takes that instead and asks for a center; with both, it goes straight to the choose prompt. A seeded pair is checked before it jumps ahead, so if that center and that target have no answer it clears them and says so rather than leaving you choosing among nothing.

A round target gives two circles with that center: the near one, which stops at the target, and the far one, which carries on past it so that the target ends up enclosed, or so the new circle sits inside the target. Both are ghosted while you choose, and the cursor picks.

A straight target gives one answer, the perpendicular distance from the center to the line the object lies on. A segment and a ray are taken as the full line they sit on, so the circle can touch beyond where the segment stops. With one answer there is nothing to choose between.

Some centers have no answer at all, and Locus says so rather than drawing a wrong circle:

• The center sits on a straight target, so there is no distance to be tangent at.
• The center sits on a target circle's own center, which is concentric: every circle with that center either misses or crosses.
• The target has no size, a zero-radius circle or a zero-length segment.

Both tools make tangent circles, and they answer different questions:

• This tool fixes the center and finds the radius. One object to touch.
• Circle: Apollonius fixes three things to touch and finds the center and radius. That is the classical problem, and it can have up to eight answers.

< Previous: Circle: Apollonius | Next: Points: Intersection >

• In the node graph: Circle Tangent to... - build the same circle as a node.
• Circle: Apollonius - the circle that touches three things at once.
• Line: Tangent to Circle - the tangent line from a point.