Discover

(Redirected from Cross-staff)

:''For the plant known as the ocotillo, sometimes called the Jacob's staff, see ocotillo.''
In surveying, the 'Jacob's staff' or 'cross-staff' is a single straight rod or staff, pointed and iron-shod at the bottom, for penetrating the ground. It also has a socket joint at the top, used, instead of a tripod, for supporting a compass.
In navigation, the cross-staff is used to determine the vessel's latitude. The main staff is marked off with a scale of degrees of latitude. The crosspiece (''BC'') slides up and down on the main staff, and has a small hole in each end. The user sights the horizon through the hole on the lower cross arm (''B''), adjusting the cross arm on the main arm until he or she can sight the sun through the hole in the other cross arm (''C''). The latitude can then be read off by comparing the position of the cross arm with the scale on the main staff.

Contents

History

See also

References

History

Named after the Biblical figure Saint James the Great,^[1] the original Jacob's staff was developed as a single pole device in the 1300s that was used in making nautical and astronomical measurements. Its invention has been credited to the Jewish mathematician Levi ben Gerson^[2][3] or to fifteenth century astronomer Georg Purbach^[4] but the latter less likely, since Purbach was not born until 1423. The pole, marked in degrees, was used to determine the altitude of the stars with a sliding wooden panel on the rod. During the Renaissance, the Dutch mathematician and surveyor Metius is known to have developed his own Jacob's staff. Gemma Frisius is also known to have made improvements to this instrument.
As a navigational tool, this instrument was eventually replaced, first by the backstaff or quadrant, neither of which required the user to stare directly into the sun, and later by the octant and the sextant. Thus, in the modern era, the name "Jacob's staff" has been applied simply to the device used to provide support for the compass.

See also

★ Tacheometry

★ Cross of St James

★ Pilgrim's staff

References

1. http://www.georoots.org/term_page.asp?terminit=j
2. "The Mathematics of Levi ben Gershon, the Ralbag"
3. David G. Krehbiel "Jacob's Staff", ''Backsights'', Surveyor's Historical Society
4. "Important Astronomers, their Instruments and Discoveries"