#backstaff

Backstaff, by Thomas Greenough made for Mr Edmond Gross, 1765

Ivory Backstaff, by Thomas Tuttel, c. 1700

Backstaffs find altitude by measuring the angle between the horizon and the sun. Unlike previous instruments, however, navigators relied on the sun’s shadow rather than looking directly at it. Navigators could do this using the key parts of the instrument. The shadow vane on the small arc used to cast a shadow; the horizon vane on the larger arc was where you view the horizon; and the sighting vane was the slit the observer would look through. So how does it work?

Hardwood Backstaff, late 18th century

Backstaffs find altitude by measuring the angle between the horizon and the sun. Unlike previous instruments, however, navigators relied on the sun’s shadow rather than looking directly at it. Navigators could do this using the key parts of the instrument. The shadow vane on the small arc used to cast a shadow; the horizon vane on the larger arc was where you view the horizon; and the sighting vane was the slit the observer would look through. So how does it work?

Navigation using celestical object is much easier with an accurate method of measuring the angle between objects in the sky such as the North star or the Sun, and the horizon. If the object's movements in the sky are known or predicable, navigators can use its present position to determine their latitude. Since the medieval times Arab mariners had used an instrument called a kamal to do this.

Kamàl, reproduction, 1977, by Nautica, Peter Ifland Collection, The Mariners’ Museum. Before leaving, the navigator would tie a knot in the cord, hold the knot between his teeth while holding the wooden piece out vertically. The cord would be pulled taut and upwards to line it with the North Star. They would sail either North or South, and would measure the North Star along the way. When the North Star and horizon lined up along each end of the kamal, the sailor would maintain that latitude

In the 15th century, European sailors adapted the idea in the form of a cross staff. The wooden staff was held against the body, and a bar called cross piece was was moved along the staff until the top end of the bar coincided with the object being measured, and the bottom end alligned woth the horizon. A scale marked on the staff then revealed the angle between the two.

Cross-staff, made 1776 by van Keulen, Amsterdam, Musée national de la marine. How to use an cross-staff, illustrated by the Canadian museum of history

Looking directly at the sun harms the eyes, so English explorer John Davis invented the backstaff 1594 which allowed reading without facing the sun. In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun.

Backstaff,made 1700 by Thomas Tuttell, London, National Maritime Museum. How to use an backstaff, illustrated in Samuel Storing's Mariner's Magazine, 1663

Holding the instrument in front of the body, with the sun at the back, the sailor holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs. Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.