Backstaff

A backstaff is a specialized navigational instrument used to measure the altitude of celestial bodies. Altitude is the measurement of how high an object, such as the Sun or the Moon, sits in the sky. This measurement was vital for sailors to determine their location while traveling across the ocean. The instrument is named for its unique method of observation. Instead of looking directly at the Sun, the navigator keeps the Sun to their back. This technique protects the eyes and uses shadows to find the correct angle.

The mechanism of a backstaff relies on the projection of a shadow. To use the device, a navigator first positions a shadow vane at an anticipated altitude. They hold the instrument in front of them with the Sun behind them. The goal is to cast a shadow from the shadow vane onto a horizon vane. This horizon vane contains a small slit that allows the user to see the sea line. The navigator then moves a sight vane until they can see the horizon through the slit. They must adjust the vanes until the shadow aligns perfectly with the view of the horizon. The final altitude is calculated as the sum of the angles read from two different arcs.

Over time, the design of these instruments evolved through several distinct stages. Early versions were simple quadrants where an arc slid along a staff. These were limited because they could only measure a maximum altitude of 45 degrees. Later, John Davis improved the design by replacing the sliding arc with a shadow vane on a transom. The most famous version, the Davis quadrant, emerged by the mid-17th century. This model split the quadrant arc into two parts: a small radius arc and a large radius arc. The small arc allowed for the precise placement of the shadow vane. The large arc allowed for much more accurate readings of the final angle.

The history of the backstaff is tied to the work of several key figures. The English navigator John Davis is credited with inventing a major version in 1594. He detailed his improvements in his book, Seaman's Secrets. Davis aimed to fix the drawbacks of older tools like the mariner's astrolabe and the cross-staff. Other inventors also contributed to this field of navigation. Thomas Harriot is believed to have originated the idea of using back observations. Edmund Gunter invented the cross bow quadrant, also called the mariner's bow, around 1623. Much later, in 1748, Benjamin Cole invented the Cole quadrant, which remained useful even after newer tools appeared.

Precision was a major focus for these inventors, leading to various specialized parts. The Davis quadrant utilized Vernier acuity, which is the human ability to align two line segments accurately. By using a small arc for positioning and a large arc for reading, the instrument achieved high accuracy without being too large. Some versions even used a Flamsteed glass. This was a lens placed on the vane that projected a bright image of the Sun onto the horizon vane. This was especially helpful when the sky was hazy or lightly overcast. In these conditions, a shadow might be hard to see, but a projected image remained visible.

Several other interesting types of instruments existed alongside the Davis quadrant. The demi-cross was a popular instrument outside of England that used a vertical transom. The plough was a transitional tool that shared characteristics with both the cross-staff and the backstaff.

There was also the Almucantar staff, which was used specifically for measuring the Sun at low altitudes. Even the cross-staff was eventually modified to allow for back observations. These variations show how many different ways people tried to solve the problem of solar measurement. Each tool offered a different balance of accuracy and ease of use.

The development of the backstaff connects to the broader history of mathematics and physics. It demonstrates how humans learned to use the properties of light and geometry to navigate the globe. The instrument relies on the fact that light travels in straight lines to create predictable shadows. This allowed sailors to turn a visual observation into a mathematical measurement. By mastering these tools, navigators could travel with much more confidence across the vast, open oceans. The evolution from simple quadrants to complex devices like the Cole quadrant shows a constant drive for better precision in science.