Sagitta

Sagitta is a term used in several distinct fields, all deriving from the Latin sagitta meaning "arrow." In geometry, sagitta refers to the height of an arc—the maximum distance between the arc itself and the straight line (the chord) that joins the arc’s endpoints. If a circle has radius R and the chord length is c, the sagitta s is given by s = R − sqrt(R^2 − (c/2)^2). The sagitta is used in calculations of arc lengths, radii, and in the design of arches and lenses.

In astronomy, Sagitta is the name of a small, faint constellation in the northern sky, Latin for "the Arrow." It was introduced by the 17th-century astronomer Johannes Hevelius. The constellation lies near Cygnus and Aquila and contains a handful of dim stars.

In biology, Sagitta is a genus of arrow worms (phylum Chaetognatha). They are small, transparent, predatory marine invertebrates that float in the water column as part of the plankton. Sagitta is the type genus of the family Sagittidae and includes several described species.

In other contexts, sagitta appears as a root in terms such as sagittal, which in anatomy denotes a vertical plane dividing the body into left and right parts, and in engineering to denote the height of a curved surface above its chord. Across these uses, the common link is the notion of an arrow-like deviation or height in a curve.