Editing Sundial (section)

==Movable-gnomon sundials==
Sundials can be designed with a gnomon that is placed in a different position each day throughout the year. In other words, the position of the gnomon relative to the centre of the hour lines varies. The gnomon need not be aligned with the celestial poles and may even be perfectly vertical (the analemmatic dial). These dials, when combined with fixed-gnomon sundials, allow the user to determine [[true north]] with no other aid; the two sundials are correctly aligned if and only if they both show the same time. {{citation needed|date=June 2013}}

===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>

===Analemmatic sundials===
{{Main|Analemmatic sundial}}
[[File:Zonnewijzerherkenrode.jpg|upright|thumb|170px|Analemmatic sundial on a [[meridian (geography)|meridian]] line in the garden of the abbey of Herkenrode in [[Hasselt]] ([[Flanders]] in [[Belgium]])]]

'''Analemmatic sundials''' are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. There are no hour lines on the dial and the time of day is read on the ellipse. The gnomon is not fixed and must change position daily to accurately indicate time of day.
Analemmatic sundials are sometimes designed with a human as the gnomon. Human gnomon analemmatic sundials are not practical at lower latitudes where a human shadow is quite short during the summer months. A 66&nbsp;inch tall person casts a 4&nbsp;inch shadow at 27° latitude on the summer solstice.<ref>{{cite report |title=Analemmatic sundials: How to build one and why they work |first1=C.J. |last1=Budd |first2=C.J. |last2=Sangwin}}</ref>

===Foster-Lambert dials===
The Foster-Lambert dial is another movable-gnomon sundial.<ref>{{harvp|Mayall|Mayall|1994|pp= 190–192}}</ref> In contrast to the elliptical analemmatic dial, the Lambert dial is circular with evenly spaced hour lines, making it an ''equiangular sundial'', similar to the equatorial, spherical, cylindrical and conical dials described above. The gnomon of a Foster-Lambert dial is neither vertical nor aligned with the Earth's rotational axis; rather, it is tilted northwards by an angle α = 45° - (Φ/2), where Φ is the geographical [[latitude]]. Thus, a Foster-Lambert dial located at latitude 40° would have a gnomon tilted away from vertical by 25° in a northerly direction. To read the correct time, the gnomon must also be moved northwards by a distance

:<math>
Y = R \tan \alpha   \tan \delta \,
</math>

where ''R'' is the radius of the Foster-Lambert dial and δ again indicates the Sun's declination for that time of year.