Mountbatten Medallic History of Great Britain and the Sea Medal: Davis Backstaff Aid to Navigat…

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Mountbatten Medallic History of Great Britain and the Sea Medal: Davis Backstaff Aid to Navigation

AssociatedAssociated with Lord Mountbatten of Burma (Frogmore House, Windsor, England, 1900 - 1979)

Medallist John Pinches, Medallists

DateMay 2004

Object NameMedal

MediumSterling Silver

ClassificationsMedals

Dimensions44mm

AcquisitionPresented in 2004 by Dr Joan M Burrell.

LocationView by Appointment - Aberdeen Treasure Hub

Object numberABDMS072500.10

Keywords

About MeThe backstaff was invented by Captain John Davis (1550-1605) in about 1594 as an improvement on the mariner's quadrant, astrolabe and cross- staff for measuring the sun's meridian altitude in finding the latitude at sea. The earliest versions were first described in Davis's Seaman's Secrets 1595, and the final English form had been evolved by 1604 (it is illu-strated in George Waymouth's The Mathematical Jewell of that year, B. M. Add. Ms. 19,889), when it quickly became popular amongst seamen.

Known as the Davis's Quadrant, or the English Quadrant by continental seamen, it remained basically unchanged until made obsolete by Hadley's invention of the reflecting octant in 1731. However, being cheaper, it was still being used and advertised for sale until late in the 18th century. It was thus an important factor in the history of Britain's expansion at sea for 150 years.

In about 1689 John Flamsteed, the first Astronomer Royal, invented a vane set with a glass lens which, if substituted for the shadow vane, enabled observations to be made when the sun was hazy. Robert Hooke claimed this invention but it was credited to Flamsteed by Perkins, mathematical teacher of Christ's Hospital, in his The Seamans Tutor (1681).

To take a noon observation with a backstaff, the observer turned his back to the sun (hence backstaff,) after placing the three vanes on the instrument. He set the shadow vane to an exact ten or five degree mark on the small arc about ten degrees less than the expected altitude of the sun. He then looked through the peep-hole in the sight vane at the horizon as seen through the slit in the horizon vane and at the same time endeavoured to make the top of the shadow cast by the shadow vane coincide with the slit in the horizon vane. To do this he slid the sight vane either up or down the large arc until the horizon and the shadow were seen together. He repeated these observations until the sun reached its highest altitude, at noon.

By adding the readings on the large and small scales the observer found the comple- ment of the sun's altitude, i.e. 90° - Alt = Zenith Distance. Most backstaff scales were engraved to give zenith distance directly, thus eliminating one step in working out the latitude. The small arc was also, in later examples, so graduated as to allow for the sun's semi-diameter and thus incorporated one more correction normally applied in the calculation.

The Mountbatten Medallic History of Great Britain and the Sea, John Pinches Medallists Ltd.