Editing Longship (section)

==Navigation and propulsion==
===Navigation===
[[File:Faroe stamp 406 bearing compass.jpg|thumb|Three-dimensional drawing of the Viking sundial (stamp illustration) with a conical vertical gnomon and its shadow, the endpoint of which touches the hyperbola scratched into the horizontal wooden disc]]
During the [[Viking Age]] (900–1200&nbsp;AD) Vikings were the dominant seafarers of the North Atlantic. One of the keys to their success was the ability to navigate skillfully across the open waters.<ref name="Philosophical Transactions 2010">{{Cite journal|title=On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers|journal=Philosophical Transactions: Biological Sciences|volume=366|issue=1565|pages=772–782|language=en|jstor = 41061780|last1 = Horváth|first1 = Gábor|last2=Barta|first2=András|last3=Pomozi|first3=István|last4=Suhai|first4=Bence|last5=Hegedüs|first5=Ramón|last6=Åkesson|first6=Susanne|last7=Meyer-Rochow|first7=Benno|last8=Wehner|first8=Rüdiger|year=2011|doi=10.1098/rstb.2010.0194|pmid=21282181|pmc=3049005}}</ref> The Vikings were experts in judging speed and wind direction, and in knowing the current and when to expect high and low tides. Viking navigational techniques are not well understood, but historians postulate that the Vikings probably had some sort of primitive [[astrolabe]] and used the stars to plot their course.

'''Viking Sundial'''

During an excavation of a [[Viking Age]] farm in southern [[Greenland]] part of a circular disk with carvings was recovered. The discovery of the so-called Viking Sundial suggested a hypothesis that it was used as a compass. Archaeologists found a piece of stone and a fragment of wooden disk both featuring straight and hyperbolic carvings. It turned out that the two items had been parts of [[sundial]]s used by the Vikings as a compass during their sea-crossings along latitude 61 degrees North.<ref name="Philosophical Transactions 2010"/>

Archaeologists have found two devices which they interpret as navigation instruments. Both appear to be [[sundial]]s with [[gnomon]] curves etched on a flat surface. The devices are small enough to be held flat in the hand at {{convert|70|mm|abbr=in}} diameter. A wooden version dated to about 1000&nbsp;AD was found in Greenland. A stone version was also found at [[Vatnahverfi]], Greenland. By looking at the place where the shadow from the rod falls on a carved curve, a navigator is able to sail along a line of latitude. Both gnomon curve devices show the curve for 61° north very prominently. This was the approximate latitude that the Vikings would have sailed along to get to Greenland from Scandinavia. The wooden device also has north marked and had 32 arrow heads around the edge that may be the points of a compass. Other lines are interpreted as the solstice and equinox curves. The device was tested successfully, as a [[Burt's solar compass|sun compass]], during a 1984 reenactment when a longship sailed across the North Atlantic. It was accurate to within ±5°.<ref>Cultural Atlas of the Viking World. Ed. [[James Graham-Campbell|J Graham Campbell]] et al. Andromeda. 1994.</ref>

'''Hypothesis'''

The Danish archaeologist Thorkild Ramskou suggested in 1967 that the "[[Sunstone (medieval)|sun-stones]]" referred to in some sagas might have been natural crystals capable of polarizing skylight. The mineral [[cordierite]] occurring in Norway has the local name "Viking's Compass." Its changes in colour would allow determining the sun's position (azimuth) even through an overcast or foggy horizon.<ref>{{cite web |url=http://www.polarization.com/viking/viking.html |title=The Viking Sunstone |website=Polarization.net |access-date=25 March 2008 |archive-date=26 January 2010 |archive-url=https://web.archive.org/web/20100126022053/http://www.polarization.com/viking/viking.html |url-status=live }}</ref> The sunstones are [[birefringence|doubly refracting]], meaning that objects viewed through them can be seen as double because of positively charged calcium ions and negatively charged carbonate ions. When looking at the sun the stone, it will project two overlapping shadows on the crystal. The opacities of these shadows will vary depending on the sunstone's direction to the sun. When the two projected shapes have exactly the same opacity, it means the stone's long side is facing directly toward the sun. Since the stone uses light polarization, it works the best when the sun is at lower altitudes, or closer to the horizon. It makes sense that Norsemen were able to make use of sunstones, since much of the area they travelled and explored was near polar,<ref>{{Cite journal|title = A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight|journal = Proc. R. Soc. A|date = 8 March 2012|issn = 1364-5021|pages = 671–84|volume = 468|issue = 2139|doi = 10.1098/rspa.2011.0369|first1 = Guy|last1 = Ropars|first2 = Gabriel|last2 = Gorre|first3 = Albert Le|last3 = Floch|first4 = Jay|last4 = Enoch|first5 = Vasudevan|last5 = Lakshminarayanan|bibcode = 2012RSPSA.468..671R|doi-access = | s2cid=67809075 }}</ref> where the sun is very close to the horizon for a good amount of the year.<ref>{{Cite web | url=http://www.atoptics.co.uk/fz767.htm | title=OPOD – Viking Sunstone | website=Atoptics.co.uk | access-date=13 December 2015 | archive-date=17 April 2023 | archive-url=https://web.archive.org/web/20230417091956/https://atoptics.co.uk/fz767.htm | url-status=live }}</ref> For example, in the [[Vinland sagas]] we see long voyages to North America, the majority sailed at over 61 degrees north.<ref name="Philosophical Transactions 2010"/>

An ingenious navigation method is detailed in ''Viking Navigation Using the Sunstone, Polarized Light and the Horizon Board'' by Leif K. Karlsen.<ref>{{Cite web|url=http://www.oneearthpress.com/pdf/nav_notes.pdf|title=One Earth Press|website=Oneearthpress.com|access-date=14 December 2021|archive-date=12 December 2014|archive-url=https://web.archive.org/web/20141212050835/http://www.oneearthpress.com/pdf/nav_notes.pdf|url-status=dead}}</ref> To derive a course to steer relative to the sun direction, he uses a sun-stone (solarsteinn) made of [[Iceland spar]] (optical calcite or silfurberg), and a "horizon-board." The author constructed the latter from an Icelandic saga source, and describes an experiment performed to determine its accuracy. Karlsen also discusses why on North Atlantic trips the Vikings might have preferred to navigate by the sun rather than by stars, as at high latitudes in summer the days are long and the nights short.

A Viking named Stjerner Oddi compiled a chart showing the direction of sunrise and sunset, which enabled navigators to sail longships from place to place with ease. Almgren, an earlier Viking, told of another method: "All the measurements of angles were made with what was called a 'half wheel' (a kind of half sun-diameter which corresponds to about sixteen minutes of arc). This was something that was known to every skipper at that time, or to the long-voyage pilot or ''kendtmand'' ('man who knows the way') who sometimes went along on voyages&nbsp;... When the sun was in the sky, it was not, therefore, difficult to find the four points of the compass, and determining latitude did not cause any problems either." (Almgren){{citation needed|date=August 2018}}

Birds provided a helpful guide to finding land. A Viking legend states that Vikings used to take caged crows aboard ships and let them loose if they got lost. From their vantage, the crows could easily spot land, which they would instinctively head for, giving the sailors a course to steer.

===Propulsion===

The longships had two methods of propulsion: oars and sail. At sea, the sail enabled longships to travel faster than by oar and to cover long distances overseas with far less manual effort. Sails could be raised or lowered quickly. In a modern facsimile the mast can be lowered in 90 seconds. Oars were used when near the coast or in a river, to gain speed quickly, and when there was an adverse (or insufficient) wind. In combat, the variability of wind power made rowing the chief means of propulsion. The ship was steered by a vertical flat blade with a short round handle, at right angles, mounted over the starboard side of the aft gunwale.

Longships were not fitted with benches. When rowing, the crew sat on sea chests (chests containing their personal possessions) that would otherwise take up space. The chests were made the same size and were the perfect height for a Viking to sit on and row. Longships had hooks for oars to fit into, but smaller oars were also used, with crooks or bends to be used as oarlocks. If there were no holes then a loop of rope kept the oars in place.

An innovation that improved the sail's performance was the [[beitaass]], or stretching pole—a wooden spar stiffening the sail. The windward performance of the ship was poor by modern standards as there was no centreboard, deep keel or leeboard. To assist in tacking the beitaass kept the luff taut. Bracing lines were attached to the luff and led through holes on the forward gunwale. Such holes were often reinforced with short sections of timber about {{convert|500|to|700|mm|ft|abbr=in}} long on the outside of the hull.