Spherical Sketchpad

There is now a new version of this program called Spherical Easel. Spherical Easel has some additional features that may make it more attractive.

Here is a tool, modeled on The Geometer's Sketchpad, for studying geometry on the sphere. You may define points and lines and relationships between them by clicking and releasing the mouse inside of existing points. When a point is subsequently moved, the relationships between the diagram's elements are preserved.

Detailed documentation will soon be available. For now, please note the following features:

To perform menu operations, you should first select the menu item and then follow the instructions given at the bottom of the diagram.
In general, mouse operations such as clicking and dragging operate on points on the front portion of the sphere. To access points on the back of the sphere, hold down the Control key. Features on the back of the sphere are drawn in a lighter color (the contrast can be adjusted using Edit -> Contrast).
When drawing a line segment, holding the Shift key fixes the great circle on which the segment is drawn. In particular, this gives one method for drawing segments whose length is greater than pi. Another method is to apply Construct -> Toggle Line Segment to an existing line segment. A third way is move one endpoint of a line segment carefully through the antipode of the other endpoint.
Generally speaking, only point may be moved. However, if you connect a point to its constructed antipode with a line segment and then enter the move mode, you may swing the line segment around its antipodal endpoints. These are the only type of line segments that can be moved.
When something has been selected, it blinks.

Spherical Sketchpad is created by David Austin and Will Dickinson. It is still very much a work in progress. Please let us know if you find any problems or if you have comments.

Enjoy Spherical Sketchpad!