Editing Sundial (section)

===Universal equinoctial ring dial===
{{Main|Astronomical rings}}
[[File:Ringsundial open.jpg|thumb|170px|upright|left|Universal ring dial. The dial is suspended from the cord shown in the upper left; the suspension point on the vertical meridian ring can be changed to match the local latitude. The center bar is twisted until a sunray passes through the small hole and falls on the horizontal equatorial ring. See [[:Commons:File:Ringsundial_open.jpg|Commons annotations]] for labels.]]

A ''universal equinoctial ring dial'' (sometimes called a ''ring dial'' for brevity, although the term is ambiguous), is a portable version of an armillary sundial,<ref>{{harvp|Waugh|1973|p=157}}</ref> or was inspired by the [[mariner's astrolabe]].<ref name=swanick>{{cite thesis |last=Swanick |first=Lois Ann |title=An Analysis Of Navigational Instruments in the Age of Exploration: 15th&nbsp;Century to Mid-17th&nbsp;Century |degree=MA |publisher=[[Texas A&M University]] |date=December 2005}}</ref> It was likely invented by [[William Oughtred]] around 1600 and became common throughout Europe.<ref>{{harvp|Turner|1980|p=25}}</ref>
 
In its simplest form, the style is a thin slit that allows the Sun's rays to fall on the hour-lines of an equatorial ring. As usual, the style is aligned with the Earth's axis; to do this, the user may orient the dial towards [[true north]] and suspend the ring dial vertically from the appropriate point on the meridian ring. Such dials may be made self-aligning with the addition of a more complicated central bar, instead of a simple slit-style. These bars are sometimes an addition to a set of [[Gemma's rings]]. This bar could pivot about its end points and held a perforated slider that was positioned to the month and day according to a scale scribed on the bar. The time was determined by rotating the bar towards the Sun so that the light shining through the hole fell on the equatorial ring. This forced the user to rotate the instrument, which had the effect of aligning the instrument's vertical ring with the meridian.

When not in use, the equatorial and meridian rings can be folded together into a small disk.
 
In 1610, [[Edward Wright (mathematician)|Edward Wright]] created the '''sea ring''', which mounted a universal ring dial over a magnetic compass. This permitted mariners to determine the time and [[magnetic variation]] in a single step.<ref name=may>{{cite book |last=May |first=William Edward |year=1973 |title=A History of Marine Navigation |publisher=G.T. Foulis & Co. |place=Henley-on-Thames, Oxfordshire, UK |isbn=0-85429-143-1}}</ref>