quadrant

A quadrant is a handheld navigation instrument consisting of a quarter-circle (90-degree arc) marked with degree graduations, a sighting vane or telescope, and a plumb bob or weight. It measures the altitude of celestial bodies above the horizon, allowing mariners to determine latitude and establish position when land is out of sight. Before the sextant superseded it in the late eighteenth century, the quadrant was the standard tool for ocean navigation.

The wooden or brass frame forms the quarter-circle, typically between 12 and 18 inches in radius. A movable alidade (sighting arm) rotates along the arc from the center point; as the user aims at a star or the sun, gravity pulls the weighted plumb line against the graduated scale, registering the angle of elevation. Early versions relied on this crude gravity reference; later designs incorporated a horizon glass so the observer could sight both the celestial object and the horizon simultaneously, greatly improving accuracy in rough seas.

The quadrant's 90-degree limitation made it unwieldy for measuring high altitudes; angles above 45 degrees became increasingly difficult to read accurately. More critically, a sextant, covering a 120-degree arc with a double-reflection optical system, could achieve precisions of better than one arcminute, while quadrants rarely exceeded three arcminutes. By the early 1800s, quadrants were relegated to backup instruments or teaching tools.

The term persists in shipyard and navigational traditions. A Davis quadrant, invented in 1594, added a second arc and sighting vanes to improve on the original design and remained in use for two centuries. Modern navigation has eliminated the need for such instruments, but the quadrant remains a historical reference point and appears in maritime museums and period-vessel restorations.