Editing Sundial (section)

===Noon marks===
{{Main|Noon mark}}
[[File:Greenwich Royal Observatory Noon Mark.jpg|thumb|170px|[[Noon mark]] from the [[Greenwich Royal Observatory]]. The analemma is the narrow figure-8 shape, which plots the [[equation of time]] (in degrees, not time, 1°=4 minutes) versus the altitude of the Sun at noon at the sundial's location. The altitude is measured vertically, the equation of time horizontally.]]

The simplest sundials do not give the hours, but rather note the exact moment of 12:00&nbsp;noon.<ref>{{harvp|Waugh|1973| pp=18–28}}</ref> In centuries past, such dials were used to set mechanical clocks, which were sometimes so inaccurate as to lose or gain significant time in a single day. The simplest noon-marks have a shadow that passes a mark. Then, an almanac can translate from local solar time and date to civil time. The civil time is used to set the clock. Some noon-marks include a figure-eight that embodies the [[equation of time]], so that no almanac is needed.

In some U.S. colonial-era houses, a noon-mark might be carved into a floor or windowsill.<ref>{{harvp|Mayall|Mayall|1994|p=26}}</ref> Such marks indicate local noon, and provide a simple and accurate time reference for households to set their clocks. Some Asian countries<!-- Date and location needed--> had post offices set their clocks from a precision noon-mark. These in turn provided the times for the rest of the society. The typical noon-mark sundial was a lens set above an [[analemma]]tic plate. The plate has an engraved figure-eight shape, which corresponds to the [[equation of time]] (described above) versus the solar declination. When the edge of the Sun's image touches the part of the shape for the current month, this indicates that it is 12:00&nbsp;noon.