Editing Sundial (section)

===Spherical sundials===
[[File:Zw stelling.jpg|thumb|170px|upright|Equatorial bow sundial in [[Hasselt]], [[Flanders]] in [[Belgium]] {{Coord|50|55|47|N|5|20|31|E|type:landmark|name=Hasselt equatorial bow sundial}}. The rays pass through the narrow slot, forming a uniformly rotating sheet of light that falls on the circular bow. The hour-lines are equally spaced; in this image, the local solar time is roughly 15:00&nbsp;hours {{nobr|( 3 {{sc|p.m.}} ).}} On September&nbsp;10, a small ball, welded into the slot casts a shadow on centre of the hour band.]]

The surface receiving the shadow need not be a plane, but can have any shape, provided that the sundial maker is willing to mark the hour-lines. If the style is aligned with the Earth's rotational axis, a spherical shape is convenient since the hour-lines are equally spaced, as they are on the equatorial dial shown here; the sundial is ''equiangular''. This is the principle behind the armillary sphere and the equatorial bow sundial.<ref>{{harvp|Rohr|1996|pp=114, 1214–125}}; {{harvp|Mayall|Mayall|1994|pp= 60, 126–129, 151–115}}; {{harvp|Waugh|1973| pp= 174–180}}</ref> However, some equiangular sundials – such as the Lambert dial described below – are based on other principles.

In the ''equatorial bow sundial'', the gnomon is a bar, slot or stretched wire parallel to the celestial axis. The face is a semicircle, corresponding to the equator of the sphere, with markings on the inner surface. This pattern, built a couple of meters wide out of temperature-invariant steel [[invar]], was used to keep the trains running on time in France before World War I.{{sfn|Rohr|1996|p=17}}

Among the most precise sundials ever made are two equatorial bows constructed of [[marble]] found in [[Yantra mandir (Jaipur)|Yantra mandir]].<ref>{{harvp|Rohr|1996|pp=118–119}}; {{harvp|Mayall|Mayall|1994|pp=215–216}}</ref> This collection of sundials and other astronomical instruments was built by Maharaja [[Jai Singh II]] at his then-new capital of [[Jaipur]], India between 1727 and 1733. The larger equatorial bow is called the ''Samrat Yantra'' (The Supreme Instrument); standing at 27&nbsp;meters, its shadow moves visibly at 1&nbsp;mm per second, or roughly a hand's breadth (6&nbsp;cm) every minute.