Editing Sundial (section)

===Equatorial sundials===
<!-- [[File:Precision sundial in Bütgenbach-Belgium.jpg|left|upright|thumb|Precision sundial in Bütgenbach, Belgium. (Precision = ±30 seconds){{Coord|50.4231|6.2017|type:landmark|format=dms|name=Belgium}}]]  Dup of one in galleryb-->
[[File:Tower-bridge-and-olympic-rings.jpg|upright|thumb|''Timepiece'', [[St Katharine Docks]], London (1973) an equinoctial dial by [[Wendy Taylor]]<ref>{{NHLE|num=1391106|desc=Timepiece Sculpture|grade=II|access-date=10 October 2018}}</ref>]]
[[File:beijing sundial.jpg|upright|thumb|An equatorial sundial in the [[Forbidden City]], Beijing. {{Coord|39.9157|116.3904|type:landmark|format=dms|name=Forbidden City equatorial sundial}} The gnomon points [[true north]] and its angle with horizontal equals the local [[latitude]]. Closer inspection of the [[:File:beijing sundial.jpg|full-size image]] reveals the "spider-web" of date rings and hour-lines.]]

{{anchor|equinoctial sundial}}The distinguishing characteristic of the ''equatorial dial'' (also called the ''equinoctial dial'') is the planar surface that receives the shadow, which is exactly perpendicular to the gnomon's style.<ref>{{harvp|Rohr|1996|pp=46–49}}; {{harvp|Mayall|Mayall|1994|pp= 55–56, 96–98, 138–141}}; {{harvp|Waugh|1973| pp= 29–34}}</ref> This plane is called equatorial, because it is parallel to the equator of the Earth and of the celestial sphere. If the gnomon is fixed and aligned with the Earth's rotational axis, the sun's apparent rotation about the Earth casts a uniformly rotating sheet of shadow from the gnomon; this produces a uniformly rotating line of shadow on the equatorial plane. Since the Earth rotates 360° in 24 hours, the hour-lines on an equatorial dial are all spaced 15° apart (360/24).

:<math> H_E = 15^{\circ}\times t\text{ (hours)} ~.</math>

The uniformity of their spacing makes this type of sundial easy to construct. If the dial plate material is opaque, both sides of the equatorial dial must be marked, since the shadow will be cast from below in winter and from above in summer. With translucent dial plates (e.g. glass) the hour angles need only be marked on the sun-facing side, although the hour numberings (if used) need be made on both sides of the dial, owing to the differing hour schema on the sun-facing and sun-backing sides.

Another major advantage of this dial is that equation of time (EoT) and daylight saving time (DST) corrections can be made by simply rotating the dial plate by the appropriate angle each day. This is because the hour angles are equally spaced around the dial. For this reason, an equatorial dial is often a useful choice when the dial is for public display and it is desirable to have it show the true local time to reasonable accuracy. The EoT correction is made via the relation

: <math> \text{Correction}^{\circ} = \frac{\text{EoT (minutes)} + 60 \times \Delta \text{DST (hours)}}{4} ~.</math>

Near the [[equinox]]es in spring and autumn, the sun moves on a circle that is nearly the same as the equatorial plane; hence, no clear shadow is produced on the equatorial dial at those times of year, a drawback of the design.

A ''nodus'' is sometimes added to equatorial sundials, which allows the sundial to tell the time of year. On any given day, the shadow of the nodus moves on a circle on the equatorial plane, and the radius of the circle measures the [[declination]] of the sun. The ends of the gnomon bar may be used as the nodus, or some feature along its length. An ancient variant of the equatorial sundial has only a nodus (no style) and the concentric circular hour-lines are arranged to resemble a spider-web.<ref>{{cite journal |last=Schaldach |first=K. |year=2004 |title = The arachne of the Amphiareion and the origin of gnomonics in Greece | journal = Journal for the History of Astronomy | volume = 35 | issue = 4 | pages = 435–445 | issn = 0021-8286 | doi=10.1177/002182860403500404| bibcode = 2004JHA....35..435S | s2cid = 122673452 }}</ref>