Editing Sundial (section)

===Horizontal sundials===
{{For|a more detailed description of such a dial|London dial|Whitehurst & Son sundial (1812)}}
[[File:Garden sundial MN 2007.JPG|thumb|upright|left|Horizontal sundial in [[Minnesota]]. June 17, 2007 at 12:21. 44°51′39.3″N, 93°36′58.4″W]]
In the ''horizontal sundial'' (also called a ''garden sundial''), the plane that receives the shadow is aligned horizontally, rather than being perpendicular to the style as in the equatorial dial.<ref>{{harvp|Rohr|1996|pp=49–53}}; {{harvp|Mayall|Mayall|1994|pp= 56–99, 101–143, 138–141}}; {{harvp|Waugh|1973| pp= 35–51}}</ref> Hence, the line of shadow does not rotate uniformly on the dial face; rather, the hour lines are spaced according to the rule.<ref>{{harvp|Rohr|1996|p=52}}; {{harvp|Waugh|1973| p= 45}}</ref>

:<math>\ \tan H_H = \sin L\ \tan \left(\ 15^{\circ} \times t\ \right)\ </math>

Or in other terms:

:<math> \ H_H = \tan^{-1}\left[\ \sin L\ \tan(\ 15^{\circ} \times t\ )\ \right] </math>

where L is the sundial's geographical [[latitude]] (and the angle the gnomon makes with the dial plate), <math>\ H_H\ </math> is the angle between a given hour-line and the noon hour-line (which always points towards [[true north]]) on the plane, and {{mvar|t}} is the number of hours before or after noon. For example, the angle <math>\ H_H\ </math> of the 3&nbsp;{{sc|pm}} hour-line would equal the [[inverse trigonometric function|arctangent]] of {{nobr| [[trigonometric function|sin]] {{mvar|L}} ,}} since tan 45° = 1. When <math>\ L = 90^\circ\ </math> (at the [[North Pole]]), the horizontal sundial becomes an equatorial sundial; the style points straight up (vertically), and the horizontal plane is aligned with the equatorial plane; the hour-line formula becomes <math>\ H_H  = 15^\circ \times t\ ,</math> as for an equatorial dial. A horizontal sundial at the Earth's [[equator]], where <math>\ L = 0^\circ\ ,</math> would require a (raised) horizontal style and would be an example of a polar sundial (see below).

[[File:Kew Gardens 0502.JPG|thumb|Detail of horizontal sundial outside [[Kew Palace]] in London, United Kingdom]]
The chief advantages of the horizontal sundial are that it is easy to read, and the sunlight lights the face throughout the year. All the hour-lines intersect at the point where the gnomon's style crosses the horizontal plane. Since the style is aligned with the Earth's rotational axis, the style points [[true north]] and its angle with the horizontal equals the sundial's geographical latitude {{mvar|L}}&nbsp;. A sundial designed for one [[latitude]] can be adjusted for use at another latitude by tilting its base upwards or downwards by an angle equal to the difference in latitude. For example, a sundial designed for a latitude of 40° can be used at a latitude of 45°, if the sundial plane is tilted upwards by 5°, thus aligning the style with the Earth's rotational axis. {{Citation needed|date=August 2012}}

Many ornamental sundials are designed to be used at 45 degrees north. Some mass-produced garden sundials fail to correctly calculate the ''hourlines'' and so can never be corrected. A local standard [[time zone]] is nominally 15 degrees wide, but may be modified to follow geographic or political boundaries. A sundial can be rotated around its style (which must remain pointed at the celestial pole) to adjust to the local time zone. In most cases, a rotation in the range of 7.5° east to 23° west suffices. This will introduce error in sundials that do not have equal hour angles. To correct for [[daylight saving time]], a face needs two sets of numerals or a correction table. An informal standard is to have numerals in hot colors for summer, and in cool colors for winter.{{Citation needed|date=August 2012}} Since the hour angles are not evenly spaced, the equation of time corrections cannot be made via rotating the dial plate about the gnomon axis. These types of dials usually have an equation of time correction tabulation engraved on their pedestals or close by. Horizontal dials are commonly seen in gardens, churchyards and in public areas.