Editing Sundial (section)

===Polar dials===
[[File:Sundial - Melbourne Planetarium.jpg|thumb|Polar sundial at [[Scienceworks (Melbourne)|Melbourne Planetarium]]]]
[[File:Reloj de sol polar en Donramiro (Lalín, España).jpg|thumb|Monumental polar sundial in [[Lalín]] ([[Spain]])]]
In ''polar dials'', the shadow-receiving plane is aligned ''parallel'' to the gnomon-style.<ref>{{harvp|Rohr|1996|p=72}}; {{harvp|Mayall|Mayall|1994|pp= 58, 107–112}}; {{harvp|Waugh|1973| pp= 70–73}}</ref>
Thus, the shadow slides sideways over the surface, moving perpendicularly to itself as the Sun rotates about the style. As with the gnomon, the hour-lines are all aligned with the Earth's rotational axis. When the Sun's rays are nearly parallel to the plane, the shadow moves very quickly and the hour lines are spaced far apart. The direct East- and West-facing dials are examples of a polar dial. However, the face of a polar dial need not be vertical; it need only be parallel to the gnomon. Thus, a plane inclined at the angle of latitude (relative to horizontal) under the similarly inclined gnomon will be a polar dial. The perpendicular spacing {{mvar|X}} of the hour-lines in the plane is described by the formula

:<math> X = H\ \tan(\ 15^{\circ} \times t\ )\ </math>

where {{mvar|H}} is the height of the style above the plane, and {{mvar|t}} is the time (in hours) before or after the center-time for the polar dial. The center time is the time when the style's shadow falls directly down on the plane; for an East-facing dial, the center time will be 6&nbsp;{{sc|a.m.}}, for a West-facing dial, this will be 6&nbsp;{{sc|p.m.}}, and for the inclined dial described above, it will be noon. When {{mvar|t}} approaches ±6 hours away from the center time, the spacing {{mvar|X}} diverges to [[Extended real number line|+∞]]; this occurs when the Sun's rays become parallel to the plane.