Jacob's staff

Measurement tool
the plant sometimes called the Jacob's staff

The term Jacob's staff is used to refer to several things, also known as cross staff, a ballastella, a fore staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs are much more complicated than that, and usually contain a number of measurement and stabilization features. The two most frequent uses are:
in astronomy and navigation for a simple device to measure angles, later replaced by the more precise sextants;
in surveying (and scientific fields that use surveying techniques, such as geology and ecology) for a vertical rod that penetrates or sits on the ground and supports a compass or other instrument. The simplest use of a Jacob's staff is to make qualitative judgements of the height and angle of an object relative to the user of the staff.

In navigation the instrument is also called a cross staff and was used to determine angles, for instance the angle between the horizon and Polaris or the sun to determine a vessel's latitude, or the angle between the top and bottom of an object to determine the distance to said object if its height is known, or the height of the object if its distance is known, or the horizontal angle between two visible locations to determine one's point on a map. The Jacob's staff, when used for astronomical observations, was also referred to as a radius astronomicus. With the demise of the cross staff, in the modern era the name "Jacob's staff" is applied primarily to the device used to provide support for surveyor's instruments.

The origin of the name of the instrument is not certain. Some refer to the Biblical patriarch Jacob, It may also take its name after its resemblance to Orion, referred to by the name of Jacob on some medieval star charts. Another possible source is the Pilgrim's staff, the symbol of St James (Jacobus in Latin). The name cross staff simply comes from its cruciform shape.

The original Jacob's staff was developed as a single pole device, in the 14th century, that was used in making astronomical measurements. It was first described by the French Jewish mathematician Levi ben Gerson of Provence, in his "Book of the Wars of the Lord" (translated in Latin as well as Hebrew). He used a Hebrew name for the staff that translates to "Revealer of Profundities", while the term "Jacob's staff" was used by his Christian contemporaries. Its invention was likely due to fellow French Jewish astronomer Jacob ben Makir, who also lived in Provence in the same period. is less likely, because Purbach was not born until 1423. (Such attributions may refer to a different instrument with the same name.) Its origins may be traced to the Chaldeans around 400 BCE. Although it has become quite accepted that ben Gerson first described Jacob's staff, the British Sinologist Joseph Needham theorizes that the Song dynasty Chinese scientist Shen Kuo (1031–1095), in his Dream Pool Essays of 1088, described a Jacob's staff. Shen was an antiquarian interested in ancient objects; after he unearthed an ancient crossbow like device from a home's garden in Jiangsu, he realized it had a sight with a graduated scale that could be used to measure the heights of distant mountains, likening it to how mathematicians measure heights by using right angle triangles.

In geology, the Jacob's staff is mainly used to measure stratigraphic thicknesses in the field, especially when bedding is not visible or unclear (e. g., covered outcrop) and when due to the configuration of an outcrop, the apparent and real thicknesses of beds diverge therefore making the use of a tape measure difficult. There is a certain level of error to be expected when using this tool, due to the lack of an exact reference mean for measuring stratigraphic thickness. High precision designs include a laser able to slide vertically along the staff and to rotate on a plane parallel to bedding.