bow

A bow, in the context of marine navigation, is a simple altitude-measuring instrument that predates the modern sextant. It consists of a wooden or brass quadrant (a quarter circle) marked with degree graduations along its curved edge, fitted with a sighting vane at the center point and a plumb bob suspended from the vertex. The observer held the instrument vertically and aligned the sighting vane with the sun or a star, then read the altitude where the plumb bob's string crossed the scale.

The bow was used from roughly the 16th through 18th centuries aboard merchant and naval vessels. A navigator would take it on deck at noon to measure the sun's angle above the horizon, a crucial measurement for determining latitude. The instrument typically measured between 6 and 12 inches in radius, making it portable enough for regular use despite its crude construction and the motion of the ship. Unlike a cross-staff, which required the observer to look along a transom, the bow allowed sighting directly at the sun through a small hole in the vane, though this still presented eye safety hazards.

Limitations and why it fell from use

The bow's accuracy was poor, typically limited to within one or two degrees of true altitude. The plumb bob would swing unpredictably in rough seas, and reading the position of a moving string against a curved scale demanded both skill and luck. Metal corrosion from salt spray degraded the graduations, and the instrument offered no magnification, making precise observation difficult. These failings became unacceptable as trade routes lengthened and navigation methods grew more sophisticated.

By the mid-18th century, the sextant replaced the bow almost entirely. The sextant's superior optical design, use of mirrors to bend light into the eye, and ability to measure angles to within seconds of arc made the old bow obsolete. However, bows remained in use aboard some merchant vessels into the 19th century among crews unfamiliar with newer instruments or in regions where repair facilities were scarce.

The name 'bow' derives from the shape: it is simply a bow or arc, a quarter circle. The term reflects a direct descriptive vocabulary common in early maritime English, where instruments were often named for their physical form rather than their function. In this it differs from later instrument naming conventions that reference their inventors or the optical principles they employ.