Backstaff

The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon.

It was invented by the English navigator John Davis, who described it in his book Seaman's Secrets in 1594.

It appears that the idea for measuring the sun's altitude using back observations originated with Thomas Harriot.

Davis was a navigator who was quite familiar with the instruments of the day such as the mariner's astrolabe, the quadrant and the cross-staff.

He recognized the inherent drawbacks of each and endeavoured to create a new instrument that could reduce those problems and increase the ease and accuracy of obtaining solar elevations.

By sliding the arc so that the shadow aligned with the horizon, the angle of the sun could be read on the graduated staff.

[3] The arc on the top of the instrument in the previous version was replaced with a shadow vane placed on a transom.

It is easier for a person to place a vane at a specific location than to read the arc at an arbitrary position.

This is due to Vernier acuity, the ability of a person to align two line segments accurately.

Thus an arc with a small radius, marked with relatively few graduations, can be used to place the shadow vane accurately at a specific angle.

On the other hand, moving the sight vane to the location where the line to the horizon meets the shadow requires a large arc.

This is because the position may be at a fraction of a degree and a large arc allows one to read smaller graduations with greater accuracy.

It was useful under conditions where the sky was hazy or lightly overcast; the dim image of the sun was shown more brightly on the horizon vane where a shadow could not be seen.

[5] In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun.

It added an index arm with spirit levels to provide an artificial horizon.

The socket is loosened so that the transom is moved vertically until the shadow of the vane is cast at the yard's 90° setting.

It was a fairly accurate instrument, as the graduations were well spaced compared to a conventional cross-staff.

A late addition to the collection of backstaves in the navigation world, this device was invented by Benjamin Cole in 1748.

The quadrant component is rotated until the horizon and the sun's image or shadow are aligned.

English scientist George Adams created a very similar backstaff at the same time.

Examples of the instrument can be found with a 0° to 90° graduation or with two mirrored 0° to 45° segments centred on the midpoint of the arc.

This serves to ensure the instrument is level while simultaneously measuring the altitude of the sun.

With some versions of this instrument, the sun's declination for each day of the year was marked on the arc.

This permitted the navigator to set the shadow vane to the date and the instrument would read the altitude directly.

Davis quadrant, made in 1765 by Johannes Van Keulen. On display at the Musée national de la Marine in Paris.
Figure 4 – A demi-cross. This instrument was contemporary with the Davis quadrant and popular outside England. It shows its similarity with the first of Davis' versions ( Figure 1 )
Figure 5 – The plough – a transitional instrument with the characteristics of both a cross-staff and a backstaff
Figure 6 – Cross-staff by Thomas Hood. Though called a cross-staff, this instrument uses a shadow vane to cast a shadow on the instrument to measure the altitude of the sun.
Figure 7 – Cole quadrant from 1748.
Figure 8 – Drawing of a cross bow quadrant. The 120° arc was graduated as a 90° arc, resulting in divisions that are slightly greater than their nominal arc size.