The Arrow constellation

• A small constellation in the northern hemisphere between Cygnus and Aquila and crossed by the Milky Way
• Any arrowworm of the genus Sagitta

Wikipedia Word definitions in Wikipedia
In optics and especially telescope making , sagitta is a measure of the glass removed to yield an optical curve. It is approximated by the formula $S = \frac{r^2}{2 \times \text{ROC}}$ or $S = \frac{r^2}{4 \times \text{focal length}}$ A good explanation ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sagitta \Sa*git"ta\, n. [L., an arrow.] (Astron.) A small constellation north of Aquila; the Arrow. (Arch.) The keystone of an arch. [R.] --Gwilt. (Geom.) The distance from a point in a curve to the chord; also, the versed sine of an arc; -- so called from ...

Wiktionary Word definitions in Wiktionary
n. 1 The keystone of an arch. 2 (context geometry English) The distance from a point in a curve to the chord; also, the versed sine of an arc; so called from its resemblance to an arrow resting on the bow and string. 3 (context anatomy English) The larger ...