page_title: Jacob's staff
section_title: Construction
hierarchical_section_title: Jacob's staff / In astronomy and navigation / Construction
caption_attribution_description: English: Jacob's staff used in navigation to obtain the altitude of the sun, which gives the latitude (17th century) Français&#160;: Bâton de Jacob utilisé en navigation pour mesurer l'altitude du soleil, ce qui donne la latitude du point d'observation (XVIIè siècle) Norsk bokmål: Jakobsstav Norsk nynorsk: Jakobsstav
context_page_description: The term Jacob's staff, also known as cross-staff, a ballastella, a fore-staff, or a balestilha, is used to refer to several things. 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 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.
context_section_description: In the original form of the cross-staff, the pole or main staff was marked with graduations for length.   The cross-piece (BC in the drawing to the right), also called the transom or transversal, slides up and down on the main staff.  On older instruments, the ends of the transom were cut straight across.  Newer instruments had brass fittings on the ends with holes in the brass for observation.  In marine archaeology, these fittings are often the only components of a cross-staff that survive.
  It was common to provide several transoms, each with a different range of angles it would measure.  Three transoms were common.  In later instruments, separate transoms were switched in favour of a single transom with pegs to indicate the ends.  These pegs mounted in one of several pairs of holes symmetrically located on either side of the transom.  This provided the same capability with fewer parts.  The transom on Frisius' version had a sliding vane on the transom as an end point.
caption: A Jacob's staff, from John Sellers' Practical Navigation (1672)
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