page_title: Backstaff
section_title: Thomas Hood cross-staff
hierarchical_section_title: Backstaff / Thomas Hood cross-staff
caption_attribution_description: Drawing of a Hood cross-staff
context_page_description: The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back and observed the shadow cast by the upper vane on a horizon vane. It was invented by the English navigator John Davis, who described it in his book Seaman's Secrets in 1594.
context_section_description: Thomas Hood invented this cross-staff in 1590. It could be used for surveying, astronomy or other geometric problems.
  It consists of two components, a transom and a yard.  The transom is the vertical component and is graduated from 0° at the top to 45° at the bottom.  At the top of the transom, a vane is mounted to cast a shadow.  The yard is horizontal and is graduated from 45° to 90°.  The transom and yard are joined by a special fitting (the double socket in Figure 6) that permits independent adjustments of the transom vertically and the yard horizontally.
  It was possible to construct the instrument with the yard at the top of the transom rather than at the bottom.
  Initially, the transom and yard are set so that the two are joined at their respective 45° settings.  The instrument is held so that the yard is horizontal (the navigator can view the horizon along the yard to assist in this).  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.  If the movement of just the transom can accomplish this, the altitude is given by the transom's graduations.  If the sun is too high for this, the yard horizontal opening in the socket is loosened and the yard is moved to allow the shadow to land on the 90° mark.  The yard then yields the altitude.
  It was a fairly accurate instrument, as the graduations were well spaced compared to a conventional cross-staff.  However, it was a bit unwieldy and difficult to handle in wind.
caption: 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.
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