gnomon

The geometric interpretation, according to Euclid

And in any parallelogrammic area let any one whatever of the parallelograms about its diameter with the two complements be called a gnomon.

The space included between the boundary lines of two similar parallelograms, the one within the other, with an angle in common; as, the gnomon bcdefg of the parallelograms ac and af. The parallelogram bf is the complement of the parallelogram df.

• Make a parallelogram.
• Take any point on either diagonal of the parallelogram.
• Make a smaller parallelogram, using this point and either corner of the original parallelogram which is an endpoint of the diagonal, with edges parallel to the edges of the original.
• Subtract the smaller from the larger, and the L-shaped figure remaining is a gnomon.

Gno"mon (?), n. [L. gnomon, Gr. one that knows, the index of a sundial. See Gnome.]

1. Dialing

The style or pin, which by its shadow, shows the hour of the day. It is usually set parallel to the earth's axis.

2. Astron.

A style or column erected perpendicularly to the horizon, formerly used in astronomocal observations. Its principal use was to find the altitude of the sun by measuring the length of its shadow.

3. Geom.

The space included between the boundary lines of two similar parallelograms, the one within the other, with an angle in common; as, the gnomon bcdefg of the parallelograms ac and af. The parallelogram bf is the complement of the parallelogram df.

4.

The index of the hour circle of a globe.

© Webster 1913.