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The Backstaff (Edited from Wikipedia)

SUMMARY

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 (hence the name) and observed the shadow cast by the upper vane on a horizon vane. It was invented by the English navigator who described it in his book Seaman's Secrets in 1594.

HISTORY AND USAGE

Backstaff is the name given to any instrument that measures the altitude of the sun by the projection of a shadow. Many types of instruments evolved from the cross-staff that can be classified as backstaves. Only the Davis quadrant remains dominant in the instruments. Indeed, the Davis quadrant is essentially synonymous with backstaff. However, Davis was neither the first nor the last to design such an instrument.

Captain John Davis invented a version of the backstaff in 1594. Davis was a navigator who was quite familiar with the instruments of the day such as the mariner's astrolabe, the quadrant and the cross-staff. He recognized the inherent drawbacks of each and endeavored to create a new instrument that could reduce those problems and increase the ease and accuracy of obtaining solar elevations.

One early version of the quadrant staff had an arc affixed to a staff so that it could slide along the staff (the shape is not critical, though the curved shape was chosen). The arc was placed so that it would cast its shadow on the horizon vane. The navigator would look along the staff and observe the horizon through a slit in the horizon vane. By sliding the arc so that the shadow aligned with the horizon, the angle of the sun could be read on the graduated staff. This was a simple quadrant, but it was not as accurate as one might like. The accuracy in the instrument is dependent on the length of the staff, but a long staff made the instrument more unwieldy. The maximum altitude that could be measured with this instrument was 45°.

The instrument that is now identified with Davis has two arcs. The quadrant arc has been split into two parts. The smaller radius arc, with a span of 60°, was mounted above

1 the staff. The longer radius arc, with a span of 30° was mounted below. Both arcs have a common center. At the common center, a slotted horizon vane was mounted. A moveable shadow vane was placed on the upper arc so that its shadow was cast on the horizon vane. A moveable sight vane was mounted on the lower arc.

Thus Davis was able to optimize the construction of the quadrant to have both a small and a large arc, allowing the effective accuracy of a single arc quadrant of large radius without making the entire instrument so large. This form of the instrument became synonymous with the backstaff. It was one of the most widely used forms of the backstaff. Continental European navigators called it the English Quadrant.

In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun. Holding the instrument in front of him, with the sun at his back, he holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs.

Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.

JOHN DAVIS

John Davis or Davis (c. 1550 – 29 December 1605) was one of the chief English navigators of . He led several voyages to discover the and served as pilot and captain on both Dutch and English voyages to the . He discovered the (today a British Overseas Territory) in August 1592.

Davis was born in the parish of Stoke Gabriel in circa 1550, and spent his childhood in Sandridge Barton nearby. It has been suggested that he learned much of his seamanship as a child while plying boats along the river Dart, and went to sea at an early age. His childhood neighbours included Adrian Gilbert and and their half-brother .From early on, he also became friends with .

He began pitching a voyage in search of the Northwest Passage to the queen's secretary in 1583. Two years later, in 1585, the secretary relented and funded the expedition, which traced Frobisher's route to 's east coast, around Cape

2 Farewell, and west towards . In 1586, he returned with four ships, two of which were sent to Greenland's iceberg-calving eastern shore; the other two penetrated the strait which became known for him as far as 67°N before being blocked by the ice cap. Sunshine attempted (and failed) to circumnavigate the island from the east.

The initially amiable approach he took to the – bringing musicians and having the crew dance and play with them – changed after they stole one of his anchors; they were likely irate at having been interrupted during one of their religious ceremonies. His ships were also attacked by Inuit in Hamilton Inlet. A third expedition in 1587 reached Disko Island before being repulsed by unfavorable winds. On his return, he charted the Davis Inlet in the coast of . The log of this trip remained a textbook model for later captains for centuries.

In 1588, he seems to have commanded Black Dog against the . In 1589, he joined the Earl of Cumberland as part of the Azores Voyage of 1589. In 1591 he accompanied on the man's last voyage, which sought to discover the Northwest Passage "upon the back parts of America" (i.e., from the western entrance). After the rest of Cavendish's expedition returned unsuccessful, Davis continued to attempt on his own account the passage of the ; though defeated by foul weather, he probably discovered the Falkland Islands in August 1592 aboard Desire. His crew was forced to kill hundreds of penguins for food on the islands, but the stored meat spoiled in the tropics and only fourteen of his 76 men made it home alive.

Davis's explorations in the Arctic were published by and appeared on his world map. Davis himself published a valuable treatise on practical navigation called The Seaman's Secrets in 1594 and a more theoretical work called The World's Hydrographical Description in 1595. The account of Davis's last voyage was written by Michelborne on his return to England in 1606.

On 28 September 1582, Davis married Mistress Faith Fulford, daughter of Sir John Fulford (the High Sheriff of Devon) and Dorithy Bourchier, the daughter of the Earl of Bath. He had five children: his first son, Gilbert was baptized on 27 March 1583; a daughter Elizabeth who died in infancy; Arthur, born 1586; John, born and died 1587; and Philip.

3 THE NORTHWEST PASSAGE

The Northwest Passage is a sea route connecting the northern Atlantic and Pacific Oceans through the , along the northern coast of North America via waterways through the Canadian Arctic Archipelago. The various islands of the archipelago are separated from one another and from the Canadian mainland by a series of Arctic waterways collectively known as the Northwest Passages or Northwestern Passages.

For centuries explorers sought a navigable passage as a possible trade route. An ice- bound northern route was discovered in 1850 by the Irish explorer Robert McClure, however it was through a more southerly opening in an area explored by the Scotsman in 1854 that Norwegian made the first complete passage in 1903–1906. Until 2009, the Arctic pack ice prevented regular marine shipping throughout most of the year. Arctic sea ice decline has rendered the waterways more navigable.

Between the end of the 15th century and the 20th century, colonial powers from Europe dispatched explorers in an attempt to discover a commercial sea route north and west around North America. The Northwest Passage represented a new route to the established trading nations of Asia.

England called the hypothetical northern route the "Northwest Passage". The desire to establish such a route motivated much of the European exploration of both coasts of North America. When it became apparent that there was no route through the heart of the continent, attention turned to the possibility of a passage through northern waters.

There was a lack of scientific knowledge about conditions; for instance, some people believed that seawater was incapable of freezing. (As late as the mid-18th century, Captain had reported that icebergs had yielded fresh water, seemingly confirming the hypothesis.) Explorers thought that an open water route close to the must exist. The belief that a route lay to the persisted for several centuries and led to numerous expeditions into the Arctic. Many ended in disaster, including that by Sir in 1845.

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