Backstaff
Introduction
The backstaff is a navigational instrument that was widely used during the Age of Exploration by mariners to measure the altitude of the sun and determine latitude at sea. Its development marked a significant advancement in the field of navigation, allowing sailors to navigate more accurately and safely across the world's oceans.
Historical Background
The backstaff was invented by the English navigator John Davis in 1594. It was designed to replace the cross-staff, which required the user to look directly at the sun, often causing eye damage. The backstaff allowed navigators to measure the sun's altitude without looking directly at it, thus protecting their eyesight.
Design and Construction
The backstaff is composed of two main parts: the arc and the shadow vane. The arc is a graduated scale, typically made of wood or brass, marked with degrees of altitude. The shadow vane is a small, movable piece that casts a shadow on the arc. The user aligns the shadow vane with the sun, and the shadow it casts on the arc indicates the sun's altitude.
Usage
To use the backstaff, the navigator would stand with their back to the sun, holding the instrument at arm's length. The shadow vane would be adjusted until its shadow fell on the horizon vane, which was aligned with the horizon. The angle of the sun could then be read directly from the arc. This measurement, combined with the time of day, allowed the navigator to calculate their latitude.
Advantages over Previous Instruments
The backstaff offered several advantages over earlier instruments like the cross-staff and the astrolabe. Firstly, it allowed for safer observations by eliminating the need to look directly at the sun. Secondly, it provided more accurate measurements due to its refined scale and design. Lastly, it was easier to use in rough seas, as it required fewer adjustments and could be operated with one hand.
Evolution and Variants
Over time, the backstaff underwent several modifications to improve its accuracy and ease of use. One notable variant is the Davis quadrant, which featured an additional arc for more precise measurements. Another variant is the octant, which incorporated a double reflection system to further enhance accuracy.
Decline and Legacy
The backstaff remained in use until the mid-18th century, when it was gradually replaced by more advanced instruments like the sextant. Despite its decline, the backstaff played a crucial role in the history of navigation, paving the way for future innovations. Its legacy can be seen in modern navigational tools and techniques that continue to rely on the principles established by early instruments like the backstaff.