## Boundary

A division between two regions. Physicists frequently analyze only a part of a larger system, so that they do not need to keep track of everything. This usually simplifies the analysis, but requires an understanding of what happens at the boundary. Interesting computer simulations usually require a boundary.

## Inspiration

In the 1st year of the period Chih-ho, the 5th moon, the day chi-ch'ou, a guest star appeared...

Yang Wei-T'e, Imperial Astronomer of the Sung Dynasty, 1054 A.D.